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Synthesis of cuprate superconductors* 



CNR Rao, R Nagarajart and R Vliayaraghavan CD 

Solid State and Structural Chemistry Unit and CSIft Centra of Excellence in 
Chemistry, Indian Institute or Science. Bangalore 560012, India 



Received 28 August 1992, in final form 19 October 1992 



> 

Q 

Abstract* There has been unprecedenmd activity pertaining lo the synthesis and Q 

characterization of superconducting cupf ates in the last few years. A variety of mf- 

syntheilc strategies has been employed to prepare pure monophasic euprates of \-J 
different families with good superconducting properties. Besides me traditional 

ceramic method, other methods such as copreciptiatlon and precursor methods, me VI/ 
sol-gel method, the alkali fhix method and the combustion method have been 

$fntribyetffe*thesyntt^ \ / 

conditions such as high oxygen or hydrostatic pressure and tow oxygen fugacity are Q 
employed in the synthesis, in this review, we discuss the synthesis of the various 

types of cuprate superconductors and point out the advantages and disadvantages {J 

o< fh& different methods. We have provided the necessary preparative details, •»< 
presenting me crucial information in tabular form wherever necessary. 



1. Introduction 

Since the discovery of high-T c supcrconduciiyity in the 
L^-Ba^GiHO system [11 a variety of etipifate super- 
conductors with ?> going up to 128 K have been syn- 
thesized and characterized [2, 3]. No other class of 
materials has been worked on so widely and intensely in 
recent years as have the cuprate superconductors. 
Several methods of synthesis have been employed for 
preparing the euprates* with the objective of obtaining 
pure monophasic products with good superconducting 
characteristics [3, 4]/The most common method of syn- 
. thesis of cuprate superconductors is the traditional 
ceramic method which has been employed for the prep- 
aration of a krge variety of oxide materials £5]- 
Although the ceramic method has yielded many of the 
euprates yt^Jtix^d^W'- characteristics, different syn- 
thetic sira||^ %w become necessary in order to 
controJ such as the cation composition, oxygen 

sto^cMpi^^^iion oxidation states and carrier con* 
-ceat^jQj^^ra^^ noteworthy amongst these 
methods ^bpiymkal or solution routes which permit 
better mixing of the constituent cations m order to 
reduce the diffiision distances in the solid state £5, 6J 
Such methods include copredpiialion, use of precur- 
sors, the sol-gel method and the use of alkali fluxes. The 
combustion method or self-propagating high- 
temperature synthesis (SHS) has also been employed: In 
this review, we will discuss the preparation of cuprate 
superconductors by the different methods, mentioning 

* Contribution No $74 from the Solid State and Structural Chemistry 
Unit 



the special features of each method and the conditions 
employed for the synthesis. In table X we give a list of 
the cuprate superconductors discussed in tigs review 
along with their structural patain^ters and approxipare 
T c values. Preparative conditions such as reaction tern* 
pcrature, oxygen pressure, hydrostatic pressure and 
annealing conditions are specified b t^ disc^ 
given in tabular form where necessary. It is hoped that 
this review will be found useful by practitioners of the 
subject as welt as those freshly embarking on the syn- 
thesis of these materials. 



Z Ceramic method 

The most common method of synthesizing inorganic 
solids is by the reaction of the component materials at 
elevated temperatures. If all the etm*^ soMs, 
the method is called the ceramic method [33, If one of 
the constituents is volatile or sensitive to the ateo- 
sphere, the reaction is carried out in sealed evacuated 
capsules, f^atinum, silica or alumina containers are gen* 
eraily used for the synthesis of metal oxides. The stort- 
ing materials are metal oxides, carbphates, or other salts* 
which are mixed, homogenized and heated at a #ven 
temperature sufficiently long for the reaction to be 
completed. A knowledge of thi pii^e diagram is useful 
in fixing the composition and condition in such a syn- 
thesis. 

The ceramic method generally requires relatively high 
temperatures (up to 2300 K) which are generally 
attained by resistance heating Electric arc and skull 



0953-2048/93/010001 •+ 22 507.50 © 1993 IOP Publishing Lid 



1 



Table 1* Structur 



rameters and approximate T c values of cupralWopfcrcontJuctors 



Cuprate 



Structure 



r e (K) 

(max. value) 



1 La 2 CuO^, 

2 La,^,Sr.{BajCu0 4 

3 tajCa^.S^Cu^Os 

4 YBa a Cu 3 0 7 

5 YBa a Cu«O s 

6 Y 2 Ba A Cu 7 0 >& 

7 Bi 2 Sf 2 CuO e 

8 Bi 3 CaSr 2 Cu 2 O fi 

10 BijSfjftn, m ,Ce J 2 Cu 2 O to 

11 Tl a Ba 2 Cu0 6 

12 Tl a CaBa 2 Cu a 0 a 

13 TI 3 Ca 2 Ba 3 Cu 3 O to 

14 T1(BaLa)Cu0 5 

15 Tl (Sri.a)Cu0 5 

*S fn M Pb 05 )Sr a CuO 9 

17 TICaBaaCuaO^ 

13 (Tlo.ePba^CaSraCu^ 

19 TISr^Yo^Gao^CujO^ 

20 TICa 3 Ba^Cu 3 09 

22 TlBa^Ln^.CeJaCu^O* 

23 PbjSrjLfto sCa^ B Cu 3 0, 

24 Pb 3 (Sr, La) 2 Cu 3 O e 

25 (Pb, Cu)Sr 2 {Ln, Ca)Cu 2 0 7 

26 (Pb. Cu)(Sr, £u){Eu. CeKSu^O, 

27 Nd^/^Cu0 4 

28 Ca^ySr^OuOa 

29 Sr,_>W,CoO a 



Bmafr ; a = 5.355. b = 5.401, c ~ 13.15 A 39 

14/mmm; a =» 3.779. c « 13.23 A 35 

14/mmm ; a = 3.825. c » 19.42 A 60 

Pmmm; a « 3.821, t> ~ 3.885. c * 11.676 A 93 

Ammm; a = 3.84, b « 3.87, c « 27.24 A ^ 80 

Ammm; a « 3,851. fc * 3.869. c «■ 5029 A 93 

Amaa.a =- 5.362, £ « 5.374, c » 24.622 A 10 

A2aa ; a = 5.409, b = 5.420. c = 30.93 A 92 

A2aa; a - 5.39. 6 - 5.40, c - 37 A 110 

P4/mmm; a * 3.888. c = 17.28 A 25 

A2aa ; a - 5.468, 6 - 5.472, 92 

c - 23.238 A; 14/mmm; a « 3.866, c » 23.239 A 

14/mmm ; a - 3.855, c - 29.318 A 1 19 

14/mmm ; a = 3.85, c » 35.9 A 128 

P4/mmm ; a « 3.83, c * 9.55 A 40 

P4/mmm; a ^3.7, c ~ 9 A 40 

P4/mmm ; a - 3.738. c » 9.01 A 40 

P4/mmm ; a » 3.856, c ■« 12.754 A 103 

P4/mmm ; a » 3.80. c - 12.05 A 90 

P4/mmm; a = 3.80, c ~ 12.10 A 90 

P4/mmm ; a * 3.853, c' « 15.913 A 1 10 

P4/mmm ; a « 3.81, c « 15.23 A 120 

I4ytamm;a - 3.8, 29.5 A 40 

Cmmm ; a = 5.435, b - 5.463, c « 15.817 A 70 

P22,2; a « 5 333. 6 • 5.421, c » 12.609 A 32 

P4/mmm; a « 3.820, c « 11.826 A 50 

14/mmm ■ a = 3.837, c * 29.01 A 25 

14/mmm: a ■ » 335. c » 12.07 A 30 

P4/mmm; a * &902.C ~ &35 A 110 

P4/mmm; a » 3.942. c = 3.^3 A 40 



techniques give temperatures up to 3300 K while high- 
power OOj lasers give temperatures up to 4300 K. The 
main disadvantages of the ceramic method are the 
following: 

(1) The starting mixtures are inhomogcneous at the 
atomic levd, 

(ii) When ho melt is formed during the reaction, the 
entire reaction has to occur in the solid state, first by a 
phase boundary reaction at tike points of contact 
between the components ami later by the diffusion of 
the constituents through the product phase. With the 
progress of the reaction, diffusion paths become longer 
and the reaction rate slower; the reaction can be 
speeded up to some extent by intermittent grinding 
between heating cycles. 

(iii) There is no simple way of monitoring the 
progress of the reaction. It is by trial and error that one 
decides on the appropriate conditions required for the 
completion of the reaction. Because of this difficulty, 
with the ceramic method one often ends up with mix- 
tures of reactants and products. Separation of the 
desired products from such mixtures is difficult, if not 
impossible. 

(iv) Frequently it becomes difficult to obtain a com- 
positionally homogeneous product even where the reac- 
tion proceeds nearly to completion. 

Despite the above limitations, the ceramic method is 
widely used for the synthesis of a large variety of inor- 
ganic solids. In the case of the cuprate superconductors, 



the ceramic method involves mixing and grinding tht 
component oxides, carbonates or other salts, and 
heating the mixture; generally in pellet form, at tb* 
desired temperature. A common variation of the 
method is to heat a mixture of nitrates obtained by 
digesting the metal oxides/carbonates in concentrated 
HNO3 and evaporating the solution to dryness 
Heating is carried out in air or in an appropriate atmo* 
sphere, controlling the pflu^prcs^re of oxygen whan 
necessary. In the case of thallium cuprates, because o 
the volatility and poisonous nature of the thaUiun 
oxide vapour, reactions are carried out in sealed tubes 
In come of the earlier preparations* the thallium cup- 
rates were synthesized in open furnaces. This is 
however, not recommended. A successful synthesis b) 
the ceramic method depends on several factors wtuct 
indude the nature of the starting materials (the choice 
of oxides, carbonates)^ the homogeneity of the mixturt 
of powders, the rate of heating as well as the reactior 
temperature and duration. 

11. La 2 Cu0 4 -reUted 214 ceprates 

Synthesis of alkaBne^arUndoped I&i~JA x Cu& 4 
(M = Ca, Sr and Ba) of KjNiF* structure with super- 
conducting transition temperatures up to 35 K is 
readily achieved by the ceramic method. Typically, the 
synthesis is carried out by reacting stoichiometric quan- 
tities of the oxides andj/or carbonates around 1300 K is 



. oxygen airoc^piieic ;a*. ^4*- $** e ? T *H ^^SfJUiS ^?R^ 

starting 

fliate^^s f° r the synthesis [H-13]. By starting with 
QietaJ nitrates, one obtains a more homogeneous start- 
ing mixture; since the hydrated metal nitrates have low 
melting points leading to a uniform melt in the initial 
stage of the reaction. Furthermore, nitrates provide an 
oxidative atmosphere, which is required to obtain the 
necessary oxygen content. 

Stoichiometric La 2 Cu0 4 is an antiferromagnetic 
insulator. La 2 Cu0 4 prepared under high oxygen pres- 
sures, however, shows superconductivity (7^ ~ 35 K) 
since the oxygen excess introduces holes just as the alk- 
aline earth dopants [14-16]. La 3 Cu0 4 ^ {5 up to 0.05) 
has been synthesized by annealing La 2 CuO« under an 
oxygen pressure of 3 kbar at 870 K [14, 15] or 23 kbar 
at 1070 K [16]. Qxygen plasma has also been used to 
increase the oxygen content 

The next homologue of La 2 Cu0 4 containing two 
Cu-O layers, La x ? Sr 0 A CaCu 2 0 6 (T c - 60 KX has been 
synthesized by using high oxygen pressures [17], The 
synthesis involves heating the sample at an oxygen pres- 
sure of around 20 bar at 1240 K. The material prepared 
at ambient oxygen pressures (in air) is an insulator. 
Several other high-oxygen-pressure preparations have 
been reported on the n = 2 member of the 
La^+jCu^G^^j homologous series by making use of 
commercially available high-pressure furnaces [18, 19]. 
In table 2, we have summarized the preparative condi- 
tions for 214 and related cuprate superconductors. 

YBa a Cs 3 0 7 and ©titer 123 cuprates 

Superconducting YBa 2 Cu 3 0 7 - 3 with the orthorhombic 
structure can be easily prepared by the ceramic method. 
Most of the investigations of the 123 compound, 
YBa 2 Cu 3 0 7 _ 3 have been carried out on the materials 
prepared by reacting Y 2 0 3 and GuO with BaC0 3 [20, 
21], It is noteworthy that Rao et al [21] obtained 
monophasic YBa 2 Cu 3 0 7 as the x =1.0 member of the 
Y 3 _ JK Ba 3 >,Gu 6 O u series. In the method employed for 
preparing YBa 2 Cu 3 0 7 , stoichiometric quantities of 
high^purity Y 2 Q 3 , BaCOj and GuO are ground thor- 
oughly and heated initially in powder form around 
1223 K for a period of 24 h. Following the calcination 
step, the powder is ground, pelletized and sintered at 
the same temperature for another 24 h. Finally, anneal- 
ing is carried out in an atmosphere of oxygen around 
773 K for 24 h to obtain the orthorhombic 
YBa 2 Cu 3 0 7 _j phase showing 90 K superconductivity. 
Oxygen annealing has to be carried out below the 
orthorhombic tetragonal transition temperature (—960 
K); tetragonal Y6a 2 0u 3 0 7 _> (0.6 < £ < 1.0) is not 
superconducting. Intermittent grinding is necessary to 
obtain monophasic, homogeneous powders. This kind 
of complex heating schedule often gives rise to micro- 
scopic compositional inhomogeneities. Furthermore* 
CQ 2 released from the decomposition of BaC0 3 can 
react with YBa 2 Cu 3 0 7 -3 to form non-superconducting 



rities or side products in the^rep^ratipn ^ 
are BaCu0 2 , Y 2 BaCu0 5 and Y 2 Cu 2 0 5 The 
ternary phase diagram given in figure 1 illustrates the 
complexites of this cuprate system. 

Using BaO a as the starting material has two advan- 
tages. It has a lower decomposition temperature than 
BaC0 3 and the 123 compound is therefore formed at 
relatively low temperatures. Ba0 2 acts as an internal 
oxygen source and the duration of annealing in an 
oxygen atmosphere is reduced to a considerable extent 
Sharp superconducting transitions are observed in 
samples of YBa 2 Cu 3 0 7 _a made using BaQ 2 . Slight 
excess of copper in the ceramic method is reported to 
give cuprates with sharper transitions [25]. Preparation 
of YBa 2 Cu 3 0 7 _j is accomplished in a shorter period 
if one employs metal nitrates as the starting materials 
[13, 23 J In table % we present the conditions employed 
for preparing 123 cuprates by the ceramic method. 

Other rare-earth cuprates of the 123 type, 
LnBa 2 Cu 3 0 7 ^ where Ln = La, Nd, Srn, Eu, Gd, Dy, 
Ho, Er and Tm (all with % values around 90 K) have 
also been prepared by the ceramic method [26, 27]. 
Oxygen annealing of these cuprates should also be 
carried out below the orthorhomic-tetragonal tran- 
sition temperature [3]: La, 754 K; Nd, 837 K; Gd, 
915 K; Er, 973 K; Yb, 976 K etc, Nearly 30% of Y ean 
be substituted by Ca in YBa 2 Cii 3 0 7 „a, retaining the 
basic crystal structure [28]; the % decreases with the 
increase in calcium content Both La and Sr can be sub- 
stituted at the Ba site in Vto^tt' 3 0 7V # [29^311 With 
La, monophasic products are obtained for 0 < x ^ 1.0 
in YBa 2 .,La J( Cu3p 7 ^ 5 , the T c decreasing with increase 
in jc. In the case of Sr substitution, monophasic 
products are obtained for 0^x^1:25 in 
YBa 2 „ it Sr J( Cu 3 0 7 .^; high T c is retained up to x = 1.0. 
Ceramic methods have also been used to prepare 
YBa 2 Cu 3 „ x M x 0 7 ^ a solid solutions, where M generally 
stands for a transition element of the first series. In most 



GuO 




BoO &Q t Y 2 0 7 SpYjPji YO i^ 



Figure 1. Phase diagram of the Y 2 0 3 -BaO-CuO system at 
1220 K (from [24]). 



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^tm/iiv wiip increasing level 01 suosnt i ISA !*}. 

2& YB»aCa 4 O t (124), Y a Ba 4 Cu 7 O l5 (247) and related 
cuprates 

The first bulk synthesis of YBa 2 Cu 4 0 8 was reported by 
Karpinski ei al [34] who heated the mixture of oxides 
at 1313 K, under an oxygen pressure of 400 bar. Syn- 
thesis of YBa 2 Cu 4 0 8 by the conventional ceramic 
method without the use of high oxygen pressure suf- 
fered from some limitations due to kinetic factors. Cava 
ei al [35] found that additives such as alkali carbonates 
enhance the reaction rate. The procedure involves two 
steps. In the first step Y 2 0 3 , Ba(N0 3 ) a and CuO are 
mixed in the stoichiometric ratio and heated at 1023 K 
for 16-24 h in an oxygen atmosphere. In the second 
step, the pre-reacted powder is ground with an approx- 
imately equal volume of either NajC0 3 or K 2 C0 3 
powder and pellets of the resulting mixture are heated 
at 1073 K in flowing oxygen for 3 days. After the reac- 
tion, the product is washed with water to remove the 
excess alkali carbonate and dried by gentle heating in 
air. The product after this step has YBa 2 Cu 4 0 8 as the 
majority phase (T e , 77 K) with little BaCu0 2 impurity. 
Other reaction rate enhancers such as NaNOj, KN0 3 , 
dilute HNOj and Na 2 0 2 have also been used suc- 
cessfully (in small quantities) to prepare YBa 2 Cu 4 O s 
[36-38]. The 124 cuprate can also be prepared without 
the addition of a rate enhancer by the solid state reac- 
tion of Y 2 0 3> BaCu0 2 and CuO at 1088 K in flowing 
oxygen [3$]. Synthesis of YBa 2 Cu 4 O s from the solid 
state reaction between YBa 2 Cu 3 0 7 and CuO in flowing 
oxygen has also been reported [39]. The synthesis of 
YBa 2 Cu 4 O s by the ceramic method generally takes a 
long time and requires repeated grinding and pellet- 
izing 

Other rare-earth 124 cuprates, LnBa 2 Cu 4 O g with 
Ln .« Eu* Gd, Dy» Ho and Er have been prepared by 
the ceramic method under an oxygen pressure of 1 atm 
[36, 40], The 7~ of these cuprates decreases with the 
increasing ionic radius of the rare earth. Calcium can be 
substituted at the Y site up to 10% in YBa 2 Cu 4 0 8 , and 
the 7; increases from 79 K to 87 K in such substituted 
YBa 2 Cu 4 O s [41]. Lanthanum can be substituted for 
barium in YBa 2 Cu 4 0 8 [42]. Single phases of 
YBa 2 „ JLa x Cu 4 0 8 have been obtained for 0 x < 0 4 
with the T c decreasing with increase in x. 

Extensive studies have been carried out on the syn- 
thesis of YBa 2 Cu 4 G 8 under high oxygen pressures [43, 
^41 The P-T phase diagram of 124, 123 and 247 cup- 
rates is shown in figure 2, High-oxygen pressure synthe- 
sis essentially involves the solid state reaction followed 
by sintering under high oxygen pressures. The typical 
sintering temperature and the pressure at which synthe- 
sis of YBaiCu^Oa has been carried out are 1200 K arid 
^20 atm of oxygen (for 8 h). By the use of high oxygen 
Pressures [45], it is possible to prepare 1 24 compounds 
w Hh other rare earths such as Nd and Sm, which is 
otherwise not possible under ambient pressures. 




7 e 9 *o 

?0*7T<K> 

Figure 2. Phase diagram of the 124, 247 and 123 cuprates 
<trom[43]). 



A variety of substitutions has been carried out at 
the Y, Ba and Cu sites in YBa 2 Cu 4 O g under high 
oxygen pressures. Yttrium can be substituted up to 10% 
by Ca in YBa,Cu 4 0 8 giving a T c of -90 K [46] ; 20% 
Ba has been substituted by Sr without affecting the 7* 
[47]. Single-phase iron-substituted YBa 3 Cu 4 ^Fe»0 9 
(0 ^ x *S 0.05) has been prepared at an oxygen pressure 
of 200 bar [48]; the 7; falls monotonieally with increas- 
ing iron concentration. 

Bordet et al [49] first reported the preparation of 
Y 2 Ba 4 Cu 7 0 1J under oxygen pressures of 100-200 bar. 
it was soon realized that Y 2 Ba 4 Cu 7 O l5 can be synthe- 
sized by the ceramic method under an oxygen pressure 
of 1 atm by a procedure similar to that employed for 
YBajCu 4 O e , except for the difference in the sintering 
temperature [3(6]. There is a narrow stability regjjpn 
between 1123 K and 1143 K for the 247 cuprate to be 
synthesized under 1 atm oxygen pressure. The best 
sintering temperature at which the 247 cuprate is 
formed is 1 133 K. Other rare-earth 247 cuprates, 
Ln 2 Ba 4 Cu 7 O l5 {Ln - Dy, Er), can also be prepared by 
this method [36, 38]. About 5% of Y can be replaced by 
Ca in Y 2 Ba 4 Cu 7 Q 15 and the T c increases to 94 K [42]. 
Substitution of La at the Ba site is limited to —10% in 
Y 2 Ba 4 Cu 7 0 J5 where the T e decreases continuously with 
increasing lanthanum content [42]. 

Synthesis of 247 cuprates by the high-pressure 
oxygen method is generally carried out at 1203 K at an 
oxygen pressure of around 19 bar (for 8 h). This step is 
followed by slow cooling (typically 5 °€ min" , ) to room 
temperature at the same pressure [50]. Other rare-earth 
247 compounds, Ln 2 Ba 4 Cu 7 0 15 (Ln = Eu, Gd, Dy, Ho 




w *« t* nay cn ar 

and Er), have been prepared in the oxygen pressure 
range of 14-35 bar [50]. Preparative conditions for the 
1 24 and 247 cuprates are given in table, 2. 

2.4. Bismuth cuprates 

Although the ceramic method is widely employed for 
the synthesis of superconducting bismuth cuprates of 
the type BMCa. St^iCm/)*****. it « generally diffi- 
cult to obtain monophask compositions, due to various 
factors [51, 52], Both thermodynamic and kinetic 
factors are clearly involved in determining the ease of 
formation as well as phasic purity of these cuprates. The 
n « 1 member (220!) of the formula Bi 2 Sr 2 Cu0 6 
appears to be stable around 1083 K and the n = 2 
member, Bi,(Ca, Sr) a Cu 2 0 8 (2122) around 1113 !C The 
n « 3 member, Bi 2 (Ca, Sr) 4 €u 3 O l0 (2223). can be 
obtained close to the melting point (1123 K) after 
heating for several days or even weeks. Of all the 
members of the ByCa, SrUi<X°2»**t« family, the 
n = 2 member (2122) seems to be most stable. Bi 2 0 3> 
which is often used as one of the starting materials, 
melts at around 1103 K. Increasing the reaction tem- 
perature therefore leads to preferential loss of volatile 
Bi 2 0 3 . This results in micro-inbomogeneities and the 
presence of the unreacted oxides in the final product 
Since these materials contain so many cations, partial 
reaction between various pairs of oxides leading to the 
formation of impurity phases in the final product 
cannot easily be avoided. A noteworthy structural 
feature of all these bismuth cuprates is the presence of 
superlattice modulation ; the modulation has nothing to 
do with superconductivity. 

Most of the above problems have been overcome by 
employing the matrix reaction method [53, 54). This 
method reduces the number of reacting components 
and gives better products. In this method, synthesis is 
carried out by reacting the oxide matrix made from 
CaC0 3 , SrC0 3 and CuO with Bi 2 Q 3 in the tem- 
perature range of S083-1123 K in air for a minimum 
period of AS k Quenching the samples in air from the 
sintering temperature or heating in a nitrogen atmo- 
sphere improves the superconducting properties of 
bismuth cuprates. The matrix reaction method yields 
monophask n « 2(2122) and n « 3 (2223) compositions 
showing T t values of 85 K and 110 K respectively [55, 
563. Partial melting for a short period (~5 nun) also 
favours the rapid formation of the n = 2 (2122) and the 
n = 3 (2223) members. 

The n =* 1 member, Bi 2 Sr a Cu0 6 , showing T c in the 
range 7-22 K is a rather complicated system and has 
two structurally different phases near the stoichiometric 
composition [51, $7-603- Many workers have varied 
the Bi/Sr ratio and obtained single-phase materials with 
a 7; of 10 K at a composition which is strontium defi- 
cient, Bi^jSr^CuOy [60, 61]. This cuprate is best pre^ 
pared by reacting the oxides and/or carbonates of the 
constituent metals at 1123 K in air for extended periods 
of time. In figure 3 we show the phase diagram of the 
BHSr-Cu-O system. The phase diagram of the 




V0 Sf 2 CM<>3 S*Cu0 2 S^jOigO, CuO 



Figure x Phase diagram of the Bfc-Sr-Cu-JO system at 
11 10 K In air (from [603). 

Bi 2 0 3 -SrG-CaOCuO system at a constant Ci 
content is shown in figure d 

Substitution of a small amount of lead for btsmi 
results in good superconducting samples of n » 2(2K 
and » « 3 (2223) members* A number of workers ha 
therefore preferred to synthesize both n - 2 (2122) a 
n = 3 {2223) members with substitution of lead up 
25% in place of bismuth [5& t 63H56]* They arc obtain 
either by direct reaction of oxides and/or carbonates 
the cations or by the matrix reaction method 

Other than the matrix reaction method, mdt que 
ching (glass route) [67, 683 and a semi-wet method [6 
have been employed for the synthesis of superconduc 
ihg bismuth cuprates. In the melt quenching metho 
the mixture of starting materials (in the form of oxid 
and/or carbonates) is melted in a platinum or alumif 
crucible around 1473 K for a short period in air ar 
then quenched in Kquid nitrogen. The quench* 
specimens are given an annealing treatment aroun 
1 103 K in air to obtain the sur^rconducting crystalBi 
cuprates. This method has been shown to produt 
both w 2 (2122) and lead-doped « « 3 (2223) membei 



CaO 



SrO 8i Of45 
Bgur*4» Sedkm throupn the phase diagram of the 
Bl a 03--SK>-CaO--CuO system at a constant CuO content of 
28.6 mol% (from [62]). 




Sgnu-wei mcuiou involves inc spue uc reaction 
between two precursors which areflbpreti^itaied 

separate^- For example, in the preparation of 
BiLePbo^SraCajCojO^o, a precipitate of Pb, Sr and 
Ca (as carbonates) and one of Bi and Cu (as oxalates) 
are reacted at 1138 K in air for a minimum period of 
72 h. The duration of the reaction for the formation of 
2223 phase is drastically reduced by this method. 

The starting composition of the reactant materials 
plays an important role in the synthesis of these cup- 
rates. For example, strontium deficiency in the n *» 1 
(2201) member favours monophasic compositions [59, 
61}. Strontium deficiency also helps in obtaining a 
phase-pure n « 2 (2122) member [70]. Starting with a 
4:3:3:4 stoichiometry of Bi:Ca:Sr:Cu t it has been 
possible to obtain a monophasic 2122 member [54, 71]. 
The n =» 3 (2223) phase, on the other hand, is either 
obtained through the substitution of Bi by Pb (up to 
25%) or by taking an excess of Ca and/or Cu [63-66, 
72]. The problem of balancing between phasic purity 
and high 7* of the cuprate gives rise to some difficulty in 
the synthesis of these cuprates. The coexistence of some 
of the members of the homologous series, especially in 
the form of polytypic intergrowths of different layered 
sequences, is also a problem. This problem is also 
encountered with thallium cuprates [73, 74]. 

The « = 4 phase, Bi K jPbo.3Ca 3 Sr 2 Cu40 w , which 
was observed in an electron micrograph along with 
n - 3 phase as an intergrowth, was synthesized in bulk 
by Rao et al [75] (with a small proportion of the n - 3 
phase) by the ceramic method. The n = 4 phase has a 
slightly lower T e (103 K\ than the n ■« 3 phase. This 
cuprate has also been prepared by Lpsch et al [75]. 

A variety of substitutions has been carried out in 
superconducting bismuth cuprates employing the 
ceramic method [58, 76-79]; some of them are note- 
worthy. For example, the simultaneous substitution of 
Bi by Pb and Sr by La in Bi 2 Sr 2 Cu0 6 results in a 
modulation-free superconductor of the formula 
BipbSr^Uj^CuOe with T c increased to 24 K [77], 
Similarly, co-substitution of Bi by Pb and Ca by Y in 
the n = 2 member (2122) gives a modulation-free super- 
conductor, BiPbY 0 5 Ca 0 3 Sr 3 Cu 2 0 8 with a T t of 85 K 
P7]. ftare-earth substitution for Ca in Bi 2 CaSr 2 Cu 2 0 8 
causes the T c to go up to 100 K without the intro- 
duction of the n = 3 phase [58^ 78]. As mentioned 
earlier, the n » 3 phase is stabilized by the partial sub- 
stitution of lead in place of bismuth [63-65]. Another 
significant discovery is the iodine intercalation of the 
&-2122 superconductor [80]. Intercalation does not 
greatly affect the superconducting properties of the 
material; clearly, superconductivity is confined to the 
tridimensional Cu0 2 sheets in these materials. 

Synthesis of a new series of superconducting eup- 
!^* of *be general formula Bi^Sr^Ln, _,CeJ 2 Cu 2 O l0 
fl P^ as « with Lh-Sm, Eu, Gd) containing a 

^onte-like (Ln, ^Ce^O* layer between the two 
F8n 2 S ^ eels ^ as been possible by the ceramic method 

J- Partial substitution of bismuth by lead increases 



with other rare eaMs [i^fc - - 

As mentioned earlier^ne does, not start with an 
exact stoichiometric composition to obtain the desired 
final product in the case of superconducting bismuth 
cuprates* Although structural studies (see for example 
[84]) indicate the presence of bismuth atoms over stron- 
tium and calcium sites as well* it is not possible to pre- 
scribe an exact initial composition to obtain the desired 
final stoichiometry. For example, starting from a 
nominal composition of (Bto^Pbo.aJSrCaCuaQ^ , one 
ends up with the formation of the n « 3 (2223) member 
[65]. Therefore, for the purpose erf characterizing the 
various members of the superconducting bismuth cup- 
rates, one starts with some arbitrary composition and 
varies the synthetic conditions suitably to obtain the 
desired final product in pure form. The actual composi- 
tions of the final cuprate are quite unexpected (e.g. 
Bi, ie3 Pb 0 5 ©Sr 2 ^Ca^CuaO,) as found from analyti- 
cal electron microscopy [85]. In table 3 we have sum- 
marized the preparative conditions of all the members 
of Bi j(Ca, SrU jCu^b**** femily. 



IS. Thallium curates 

The conventional ceramic method employed for the 
synthesis of 214, 123 and bismuth cuprates has to be 
modified in the case of thallium cuprates of the 
TljCa^jBa^O^, TlCa^jBaiC^O^j and 
nCa n .jSr 2 Cu s 0 2l> ^3 families due to the toxicity and 
volatility of thallium oxide; In the early days, the reac- 
tion was carried out in an open furnace in air or oxygen 
atmosphere at high temperatures (1150-1180 JC) for 
5-10 min [86, 87]. In a typical procedure, the mixture 
of reactants in the form of a pellet was quickly intro- 
duced into the furnace maintained at the desired tem- 
perature. Since melt-solid reactions take place faster 
than solid-solid reactions, the product was formed 
quickly by this method [87]. Although this method 
requires a very short duration of heating, it results in 
the loss of thallium, leading to the danger of inhaling 
thallium oxide vapour. Some workers have taken 
certain precautions not to release the TI 2 G 3 vapour into 
the open laboratory, but the method is still not recom- 
mended. Furthermore; the formation of the desired 
phase is not ensured under the open reaction condi- 
tions. Synthesis of thallium cuprates has therefore been 
carried out in closed containers (sealed tubes) by most 
workers. By this method, both polycrystalline samples 
and single crystals can be prepared, since the reaction is 
carried out over longer periods. Better control of stoi- 
chiometry, homogeneity of phases and the total avoid- 
ance of the inhalation of toxic thallium oxide vapours 
are some of the advantages of carrying put sealed tube 
reactions. 

Closed reaction conditions have been achieved in 
different ways. The reactant mixture is sealed in gold 
[88] or silver tubes [89] or in a platinum [90] or nickel 



7 



o. N ;H Hao m at 




fable 3. Preparative conditions for the synthesis of bismuth cuprates by the coramic method. 



Conditions* 



Starting composition 


iemp. \K) 


Time 


KfOOUCt 




nei* 


□ 1 ^\^U^\J t 


1103 


2d 


2201 major phase 


20 


[51] 




1123 


1 d 


2201 maior nhase 


9 


[571 


Ri *5r /"!« *f~} 


1123 


2 d 


Single phase 


10 


f59, 611 




1150 


1 d 


o ■* *y *^ |«***^%5 


24 


L' * J 




5 d 




65 


T611 


at f a Or f*Ni r\ ** 

Bi^a^Sr^Cu^ 


1103 


3 d 




30 




1108 


2d 


2122 single phase 


85 


[713 


Bi^Sr, ^CaCu^ 


1113 


3d 


2122 single phase 


65 


[70] 


BiPbSr a Y a ,Cao_ 5 Cu a O a 


1200 


1 d 


2122 single phase 


65 


[77] 


Bi , ^^o^Ca^SfaCUaQ, D 


1140 


5d 


2223 major phase 


120 


[55] 


Bh sP^Ca^ST , ^C^O, to 


1100 


4d 


2223 major phase 


105 


[64] 




1153 


10 d 


2223 single phase 


110 


[72] 


Bi c 7 Pb c jSrCaCu, ^O, 


1153 


5d 


2223 major phase 


105 


[65] 


B*CaSrCu 3 0, 


1143 


5d 


2223 major phase 


120 


[65] 


Bi 3 .iPb a ..C8«Sr 4 CM,0 JI 


1133 


5d 


2223 major phase 


108 


[64] 


BtaGd^^Ceo^SraOu^Co 


1273 


10 h 


2222 single phase 


30 


[81] 



* All the preparations carried out th air. 
to Obtained by matrix reaction method. 



alloy (Inconel) container £91] closed tightly with a silver 
ltd. Alternatively, tfie rcactant mixture is taken in the 
form Sot a pellet, wrapped in a platinum [92] or gold 
[93] foil and then sealed in a quartz tube. This method 
has the advantage of carrying out the reaction under a 
vacuum. Some workers place the reactant pellet in an 
alumina crucible [94] which is then sealed in a quartz 
ampoule. Thallium-excess starting compositions have 
been employed by a few workers to compensate for the 
thallium loss during the reaction [95]. 

In the preparation of the thallium cuprates, the 
matrix miction method is often employed. Here; a 
mixed oxide containing all the metal ions other than the 
volatile thallium oxide is first prepared by reacting the 
corresponding oxides and/or carbonates around 1280 K 
for 24 h in air [89,961. The freshly prepared mixed 
oxide is then taken with a calculated quantity of Tl^O, 
and heated at appropriate temperatures in a sealed 
tube. This method is desirable when a carbonate is used 
as the starting material. Some of the thallium cuprates 
have been prepared by a modified matrix method [97] 
wherein a thallium-containing precursor such as 
Ba 2 Tl,0 5 is prepared first and then reacted with other 
components under closed conditions. Thallium- 
containing precursors are less volatile than TljO* 
so that the Joss of thallium is minimized during the 
preparation. 

Thermodynamic and kinetic factors associated with 
the synthesis of thallium cuprates are complex due to 
the existence of various phases which are structurally 
related and which can therefore intergrow with one 
another. Is fact, one of the common defects that occurs 
in the thallium cuprates is the presence of random inter- 
growths between the various layered phases [98]. Fur- 
thermore, many of the thallium, lead and bismuth 
superconductors are metastable phases which are 
entropy stabilized £99]. The temperature of the reac- 



tion, the sintering time and the starting composition are 
therefore all crucial to obtaining monophasic products 
(table 4X 

The effect of the starting composition is best illus- 
trated by the formation of the n = 3 phase of the bilayer 
thallium cuprates (H 2 CajBa 3 Cu30io> Synthesis of this 
compound starting from the stoichiometric mixture of 
the oxides corresponding to the ideal composition often 
yields the n -« 2 member of the family. It was found that 
starting with compositions rich in Ca and/or Gu 
(namely TlCa 3 BaOi,O y , Tljea^BaiGujO^ yielded a 
nearly pure n = 3 phase [90, 98, J00]. The actual com- 
position is, however, dose to TJi^BajGa^jCujO,. In 
the case of TJCaBa 2 Cu a O, (1 1 22) starting from a ston 
chsometric mixture of oxides corresponding to the ideal 
stoichiometry always yielded a mixture of M22 and 
2122 phases, the relative proportion of the two being 
dependent on the conditions. It has been demonstrated 
recently [201] that thallium-deficient comjpositions cor- 
responding to Tl, .jCaBaiCujOy (S = 0.0 to 0.3) yield 
better monophasic 1 122 materials. 

The thallium content of the material not only deter- 
mines the number of Tt-O layers but controls the bote 
concentration. As mentioned earlier, one of the good 
starting compositions to obtain TTj^Ba^ji^ 
(2223) is TlCa 3 BaCu 3 D, (1313) which bears Bitte rela- 
tion to the composition of the final product. Another 
example is the formation of the ji = 4 phase, 
TlCa 3 Ba 2 Cu 4 0, (1324). Detailed studies [102] have 
shown that the 2223 phase formed initially transforms 
to the 1223 phase with an increase in the duration of 
beating. After prolonged sintering, the 1324 phase is 
formed at the expense of the 1223 phase. Similar trans- 
formations have also been observed in the formation 
process of TlCa 4 Baa€^ 3 C) > with five Gu-O layers [103], 

The Sr analogue of TICa* _ i Ba 2 Cu*0 2 , + 3 cannot be 
prepared in pure form. However, they are stabilized by 



Oondrtlpns 



Starting composition 


femo iK\ 


^Prirne 


Gas 


nyvyyi 






TJ 2 Ba 2 CuO d 


1143 


3h 


Seated gold tubes 


2201 single phase 


84 


[831 


Tl 2 CaBa 3 Cu 2 O t 


1173 


6h 


Sealed gold tubes 


2122 single phase 


98 


[831 


1150 


3h 


Sealed silica ampoule 


2122 single phase 


95 


[983 




1150 


0.5 h 


Sealed silica ampoule 


2122 single phase 


95 


[98] 


TLCa->Ba„Cu*0 


1150 


0.5 h 


Seated sitica ampoule 


2122 single phase 


95 


[983 


TUCa,Ba,Cu*Oi A 


1173 


6h 


Seated gold tubes 


2223 major phase 


105 


urn 


1123 
1103 


20 min 
12 h 


Sealed silica ampoule 


2223 major phase 


106 


[95] 


TlCa^BaCiKO. 


1153 


3h 


Seated sitica ampoules 


2223 major phase 


125 


[1003 


TIjCaBajCu^O, 


1153 


3h 


Seated sitica ampoules 


2223 major phase 


108 


[1003 


TIBa^Lao.aCuOj 


1163 


3h 


Seated silica ampoules 


1021 Single phase 


40 


[111] 


TISrLaCuOj 


1170 


2h 


Sealed silica ampoules 


1021 single phase 


40 


[1W] 


TlSr^tNd&jCUsO, 


1170 


2h 


Sealed silica ampoules 


1122 major phase 


80 


[110] 


7JCaBa 2 Cu 2 0 7 


1170 


3h 


Sealed silica ampoules 


1122 major phase -f 


90 


[101] 








2122 impurity 




[101] 


Tl* -CaBa-iGuoO-. 

ml «Pb« «>Ca5r,Cu.>0T 

TKGao 5 Y 0 ^Sf a Cu 2 0 7 


1170 


3 h 


Sealed silver tubes 


1122 major phase 


90 


1170 


3h 


Sealed silica ampoules 


1122 single phase 


90 


[104] 


1170 


3h 


Sealed silver tubes 


1122 single phase 


90 


[92] 


TICa 2 8a 2 Cu 3 0 9 


1163 


6h 


Sealed sitica ampoules 


1223 single phase 


115 






1198 


3-12 h 


Sealed gold tubes 


1223 single phase 


122 


[105] 




1170 


2h 


Sealed silica ampoules 


1223 major phase 


60 


[110] 



partly substituting Tl by Pb (or Bi) or Ca by yttrium or 
a trivaient rare earth [92, 104-107} Thus, 
Tl 0 5 Pb 0 . 5 Ca li „ 1 Sr 2 Cu J( O 2j(+ 3 shows a T c of -90 K for 
n = 2 and -120 K for -it » 3- TlCa^Yo^SrjCujO, 
also shows a T t of 90 1C These cuprates in the Tl/Pb- 
Ca/Ln-Sr-Ct*-0 systems are prepared in a manner 
similar to the Tl-Ca-Ba-Cu-O system except that 
SrCD 3 is used in place of BaC0 3 or Ba0 2 . Sr 4 Tl 2 0 7 
has also been used as a starting material in some 
instances [97]. The n = 1 member, TlM 2 Cu0 5 (M = Sr 
or Ba) is also stabilized by the substitution of Pb or Bi 
for Tl or a trivaient rare earth for Sr or Ba [108-11 1]. 
All these compounds showing a 7; of 40 K have been 
prepared by the matrix reaction method. 

Single thallium layer cuprates of the general formula 
ll 1+ ^2-x^jCu 2 O v with A « Sr, Ba; Ln = Pr (Nd, 
€e) as well as Tl 0 , 5 Pb <> .5(Ln l - x Ce J( ) 2 Sr 2 Cu 2 O 9 
(Ln = Pr, Gd) with a fluorite-type Ln 2 0 2 layer have 
been prepared by the ceramic method [112, 113], The 
as-prepared materials are semiconductors. It has been 
shown by Liu et al [114] that annealing TlBa 2 (Eu t 
Ce) 2 Cu 2 0* (1222 phase) under an oxygen pressure of 
t^-toindt^es superconductivity with a 7^ of —40 K. 

As in the case erf bismuth cuprates, the final com* 
position of thallium cuprates is unlikely to reflect the 
composition of the starting mixture. Structural studies 
[99, 115) have shown that there is cation disorder 
between Tl and Ca/Sr sites. Therefore, in order to 
obtain a superconducting composition corresponding to 
a particular copper content, one has to start with 
various arbitrary compositions and vary the synthesis 
conditions. The actual composition of the final product 
!? n quite unexpected (eg. T^ ^Ba^aj ^Cu 3 O y or 
T1 i.86Ba 201 GuO y ) as shown by analytical electron 
microscopy [85]. In table 4 we have listed the pre* 



parative conditions employed for the synthesis of thal- 
lium cuprates by the ceramic method. 

2.6. Lead cuprates 

The conditions for the synthesis of superconducting 
lead cuprates are more stringent than for the other 
copper oxide superconductors. Direct synthesis of 
members of the PbjSr^Ln, Cap^Og^ (Ln ■ * Y or 
rare earth) family by the reaction of the component 
metal oxides or carbonates in air or oxygen at tem- 
peratures below 1173 K is riot possible because of the 
high stability of SrPbQ 3 -related perovskite oxides. Pref- 
erential loss of the more volatile PbO leads to micro- 
inhomogeneities. Furthermore, Pb in these compounds 
is in the 2 + state while part of the Cu is in the 1 + 
state. Synthesis has therefore to be carried out under 
mildly reducing conditions, typically in an atmosphere 
of N 2 containing 1% 6 2 . The most common method 
that has been employed for the: synthesis of these lead 
cuprates is the matrix reaction method [116]. For 
Pb 2 Sr 2 (Ln, Ca)Cu 3 Q,u> (Ln = Y or rare earth), a 
mixed oxide containing ail the metaL ions except Pb is 
made by reacting SrC6 3 , Ln 2 Q 3 or Y 2 Q 3 ,CaC0 3 and 
CuO in the appropriate ratios around 1223 K in air for 
16 h. The mixed oxide is then taken with an appropri- 
ate amount of PbO, ground thoroughly, pelletized and 
heated in the 1 133-1 198 K range in a flowing stream of 
nitrogen containing 1% 0 2 for periods between I and 
16 h> Generally, short reaction times and quenching the 
product from the sintering temperatures into liquid 
nitrogen in the same atmosphere gives better-quality 
samples. Even though this is the common method for 
preparing Pb 2 Sr 2 (Ln, Cs^C^O^ it is riot Always 
easy to obtain samples exhibiting good, reproducible 



9 




superconducting properties. The lead cuprates from the 
method described above generally show broad tran- 
sitions in the R-T curves with negative temperature 
coefficients of resistance above 7^ . 

Studies of the dependence of T c on the calcium con- 
centration in the Pb^Y^Ca^CtijOa^ system [117] 
have shown that heating the samples near the melting 
point between 1198 and 1228 K for 2 h and post- 
annealing in flowing nitrogen gas at a temperature 
between 673 and 773 K improves the superconducting 
properties of the samples dramatically. Direct one-step 
synthesis has been achieved [118] by reacting the metal 
oxides in sealed gold tubes around 1223 K. An alterna- 
tive route to the direct synthesis from metal oxides 
and/or carbonates has also been demonstrated [119]. 
Superconductivity near 70 K has been reported in 
Ca-free Pb 2 Sr 2 LnCu 3 0 8+ ^ (Ln - Y or rare earth) 
employing the vacuum annealing procedure [120]* Sub- 
stitution of Pb by Bi in Pb^rjY 0 .3Ca M Cu 3 O 8 , M has 
also been carried out by the ceramk method £1213* 
About 30% of Pb can be substituted by Et, and such a 
substitution increases the 7; up to 100 K. The n ~ 0 
member of the P^S^Ca, -.^^<frj+,0«+ series 
(namely Pb 2 fSrLa)Cu 3 0 6 ^ i ) has been prepared 
successfully by this matrix reaction method [122], 

Unlike the 221 3-type lead cuprates, superconducting 
1212-type lead cuprates of the formula 
(Pbo. 5 Cu 0 , 5 )& 2 (Yo 4 Ca 0 5 )Cu 2 0 7 ^ are synthesized in 
an oxidizing atmosphere. Several authors have reported 
direct synthesis as well as reactions under dosed con- 
ditions [123-127], In the direct synthesis of these cup- 
rates, care is taken to prevent the loss of Pb by 
wrapping pellets in gold or platinum foil [127]. Rouil- 
lon et at [125, 126] have reported the synthesis of 1212 
lead cuprates by the direct reaction of the component 
oxides in evacuated silica ampoules. This method has 




the advantage of adjusting the oxygen partial pressure 
required for the synthesis. Both 221 3-type and 1212- 
type lead cuprates have been prepared using the nitrates 
of the metal ions as the starting materials [128], 
Although this procedure yields 2213 or 1212 phases in a 
single step, the product obtained always has impurities 
such as Y 2 G 3 , CuO eta 

A superconducting lead cuprate of the formula (Pb, 
CuXEu, Ce) 2 (Sr, Eu) 3 Cu 2 C) 9 (1222 phase) containing a 
ftuorite layer has been prepared by the direct reaction of 
the component metal oxides at 1273 K in oxygen atmo- 
sphere [129]. 

High-pressure ceramic synthesis has been employed 
to prepare lead cuprates of the 1212 type [13d 131]. In 
order to prepare Pb 0 . 3 Cu 0 , 5 Sr 3 Y 0iJ Cao.5Cu 2 0 7 -*» 
sintering is carried out at 1213 K for 15 h under an 
oxygen pressure of 100 bar followed by fast cooling to 
373 K. The samples obtained from high-pressure 
oxygen treatment show higher %s than those processed 
at 1 bar pressure of oxygen Substitution of Y by otter 
rare earths has been possible by this high-oxygicn-pres' 
sure method [131]. All the rare-earth substituted com- 
pounds are superconducting with 7^s in the 50-70 K 
range. The T c decreases with increase in the size of the 
rare earth. In table 5 we summarize the conditions for 
the synthesis of the various lead cuprates by the ceramic 
method 

2.7 Ekctroo-doped superconductors 

All the cuprates discussed till sow are bole supercon- 
ductors. Synthesis of electron-doped cuprate supercon- 
ductors of the type LnwM^CuO*., (Ln ~ No\ Pr, 
Sin, Eu: M = Cc> ThX possessing the T structure, is 
generally achieved by the ceramic method £132^134]. 
The conditions of synthesis are more stringent since the 



Table 5. Conditions tor the synthesis of lead cuprates by the ceramic method. 



Conditions 



Compound 



Starting materials 


Temp.OQ 


Time 


Gas 


Comments 




Rcf. 


PbO + S^Y^Ca^ 


1143 


1*16 h 


+ 1%G» 




T8 


t««3 


Cu,0, matrix 












[«»] 


PbO.Pt?O a ,Ca0 3 . 


1223 


12-48 h 




Sealed ©ok* tubes 


78f 


SrO,.Y a O,.CuO 


1673 












PbO t SrCO a .Y,0,. 


15 h 


air 








CaCOj.CuO 


1173 


2rt 


air 










1073 


1-5 h 






78 


im 


PtK>.La a O,, 


1083 


6h 




2202 major phase + 


26 


[122] 


Sr a CuO*. CoO 








PbjLaCu^O, impurity 






PbO.SrCOj.UjO*, 


1073 


5h 


air 








CuO 


1273 


2h 


O a 




25 


[»23] 


PbO.SrCOs.YaO,, 


1123 


10 h 


air 








Ca0O 3 .CuO 


1273 


1 h 




1212 major phase* 


50 










Sr 5 Pb3Cu0 13 impurity 






PbO + Sr a Y^C^ 


1243 


3n 




1212 ma|or phase* 


47 


[127] 


Gu^O,, matrix 
PbO. Pt>O a ,Sr a CuO». 








Sr s Pb,CuO t? Impurrty 




im 


1108-1223 


1-10 h 




Evacuated silica tubes 


100 


Y a O, . Cb0 3 . Co,0, CuO 










80 


['28] 


Pt>0 3 .PbO.SrO a . 


1108-1223 


M0h 




Evacuated since tubes 


SrCuOa.YaO,. CaO.CuO 














PtO.S/COj.Et^O,. 


1123 


10 h 


air 


Single phase 


25 




CeO a .CuO 


1323 


Ih 




1222 







Pb&sCu^JSrLaCud* 



(Pb^Sr^, 
(Y^Cao^^O, 

tPfco^Ce^sJSr, 
(Y^Ca^CUaO, 

tPbo^Cu^^tSr , . 75 Eu 0 ^) 
(Eu^Ceo^CUaO, 



10 



material, ;by ^aKing .sure that * extra electron 

content olf the capote. For this reason samples after 
calcination and sintering at J 323 K in air (for 24 h) are 
annealed in a reducing atmosphere (typically Ar, N 2 or 
dilute Bj) at 1173 K to achieve superconductivity. 
Samples prepared in this manner show a negative tem- 
perature coefficient of resistance above T c in the R-T 
curves; the resistivity drop at T c is also not sharp. An 
alternative synthetic route involves the reaction of 
pre-reacted NdCe0 3 3 material with the required 
amounts of Nd 2 0 3 and CuO at 1253 K for a minimum 
period of 48 h in flowing oxygen [135]. The samples are 
then rapidly quenched from 1253 K in an argon atmo- 
sphere to achieve superconductivity. This procedure 
eliminates the slow diffusion of Ce throughout the 
Nd 3 Cu0 4 -a host and gives uniform concentrations of 
cerium and oxygen. Samples obtained from this route 
show a sharp transition at 21 K. 

Superconductivity with a 7* of 25 K is induced by 
doping fluorine for oxygen in Nd 2 Cu0 4 . This has been 
accomplished by taking NdF 3 as one of the initial reac- 
tants [136]. Substitution of either Ga or In for copper 
in non-superconducting Ndj-^Ce^CuO^-^ also induces 
superconductivity [137, 138]. 

2& InftiiiteJayer cuprates 

Discovery of superconductivity in cu prates containing 
infinite Cu0 2 layers has been of great importance in 
understanding the phenomenon. Very high pressures 
have been employed for obtaining the infinite-layer cup- 
rates. Both hole-doped (eg. Ca^^Sr^GuO^) and 
electron-doped (Sr 2 .^Nd^CuO^) infinite-layer cuprate 
superconductors with a maximum T c of 110 K have 
been reported [139-142]. Infinite-layered cuprates of 
the type (Ba, Sr)Cu0 2 , (Ga, Sr)Cu0 2 are synthesized in 
an oxidizing atmosphere under high hydrostatic pres- 
sure [139, 140, 142]. Electron-doped Sr a86 Nd ai4 Cu0 2 
is also prepared under high hydrostatic pressures [141]. 
Metal nitrates are generally used as the starting 
materials since carbonates of Ba, Sr and Ca have high 
decomposition temperatures. After decomposing the 
metal nitrates at around 873-1 123 K in air, the product 
is subjected to high pressure to obtain the supercon- 
<*ucting phases. Sr 0 96 Nd 0 , A Cu0 2 , which supereonduc- 
ts at 40 jC> is made under a hydrostatic pressure of 
25 kbar at 1273 K. Superconducting (Ca, SrJCuO, is 
prepared at 1273 K under 6 GPa pressure. Defidency of 
Sr and Ga as well as the oxidizing atmosphere make 
this phase superconducting, and the oxidizing atmo- 
sphere is provided by heating a capsule containing 
KGIO* along with the sample. This cuprate has a 7; 
(onset) of 1 10 K. 

3 » Coprecfpitation and precursor methods 

^oprecipttation involves the separation of a solid con- 
taining various ionic species chemically bound to one 



neous cojjrrapatioiu 

•of cry-st^ine or a^^gBi^^^io 'of 
well defined stoic&ometry with' respeci to the metal 
ions is obtained only when the following conditions are 
satisfied. 

(i) The precipitating agent is a multivalent organic 
compound which can coordinate with more than one 
metal ion, and the precipitation rate is fast 

(ii) The solid precipitating out of the solution should 
be really insoluble in the mother liquor. 

The anions generally preferred for coprecipitation of 
oxidic materials are carbonates, oxalates, citrates etc. 
The same is true of high- 7; cuprates. The precipitates in 
some instances could be genuine precursors or solid 
solutions [5, 6]. It is well known that precursor solid 
solutions drastically bring down diffusion distances for 
the cations and facilitate reactions in the solid state. We 
shall not distinguish precursor solid solutions, precipi- 
tated from solutions from other precursors in this 
discussion. 

The precipitates (carbonate, oxalate etc) are heated 
at appropriate temperatures in a suitable atmosphere to 
obtain the desired cupraite. Some of the advantages of 
the coprecipitation technique over the ceramic method 
are an homogeneous distribution of components* a 
decrease in the reaction temperatures and of the dura- 
tion of annealing, a higher density and a lower particle 
size of the final product. The major drawback of this 
route is the control over the stoichiometry of the final 
product. 

3.L La^^CuO* 

La, Sr and Cu in La 2 -jjSr^GuO^ are readily coprecipi^ 
tated as carbonates [11, 12, 143], For this purpose the 
required quantities of the various metal, nitrates are dis- 
solved together in distilled water. Alternatively, the cor- 
responding oxides are dissolved in nitric acid to give a 
nitrate solution and the pH of the solution is adjusted 
ip 7-8 by the addition of KOH solution. A solution of 
K 2 C0 3 of appropriate strength is then slowly added 
under stirring to give a light blue precipitate which is 
thoroughly washed. The precipitate is dried at 420 K 
and calcined at 1070 K for 8 h in air. The resulting 
black powder is ground and pelJetized and sintered at 
1270 K for 16 h in air to obtain monophasic 
La, 85 Sr 013 CuO4, superconducting at 35 K. 

Instead of as carbonate, the metal ions are also 
readily precipitated as oxalate by the addition of either 
oxalic acid or potassium oxalate to the solution of 
metal nitrates [11, 12, 144, 145]. The precipitated 
oxalate is then decomposed to obtain the cuprate. This 
method has certain disadvantages: 

(i) La 3 + in the presence of an alkali metal oxalate 
first yields lanthanum oxalate which further reacts with 
the precipitating agent to give a double salt, Control of 
stoichiometry therefore becomes difficult, leading to 
multiphasic products, 



11 




(ii) The relative solubilities of some of the oxalates 
also pose difficulties. For example, SrC 2 C) 4 is nearly 
four times more soluble than SrC0 3 . 

3.2- YBa 2 Cu 3 0 7 

YBa 2 Cu 3 0 7 and related 123 compounds can be 
obtained via coprecipitation of the component metals 
(from a nitrate solution) as a formate [146, 147], acetate 
[148], oxalate [12, 149-156], hyponitrite [157] or 
hydroxycarbonate [158, 159]. Some of these precipi- 
tates could be genuine precursor compounds as is 
indeed the case with the hyponitrite. 

In oxalate coprecipitation [12, 149-152], oxalic acid 
solution of appropriate concentration is added to an 
aqueous solution of mixture of nitrates of Y, Ba and Cu 
and the pH of the solution is adjusted to 7.5 (by dilute 
NH 3 ). The pale green slurry thus formed is digested for 
1 h, filtered and dried. The oxalate is converted to 
orthorhombic YBa 2 Cu 3 O w by heating at 1053 K in 
air for 5 days followed by oxygenation at 723 K. This 
procedure, even though successful in making supercon- 
ducting YBa 2 Cu 3 0 7 - j in small particulate form, often 
results in undesirable stokhiometry because of the mod- 
erate solubility of barium oxalate. Furthermore, rare- 
earth ions in the presence of ammonium oxalate give a 
double salt with the excess oxalate which competes with 
the precipitation of copper and barium oxalates; These 
difficulties can be overcome either by taking a known 
excess (wt%) of barium and copper or by using tri- 
ethylammonium oxalate as the precipitant in aqeuous 
ethanol medium [153-155]. The alcoholic medium 
decreases the solubility of barium oxalate and the pH of 
the solution is controlled in situ. 

A better method of homogeneous coprecipitation of 
oxalates is that of Liu et dl [156] using urea and oxalic 
acid Urea, on heating, is hydrolysed liberating CQ 2 
and NH 3 v and thus gradually adjusting the pH 
throughout the solution. TheCQ 2 liberated controls the 
bumping of the solution during digestion. The oxalate 
coprecipitation route is widely described in the liter- 
ature. The reactive powders obtained by the oxalate 
coprecipitation method decrease the sintering tem- 
perature. The formation of BaC0 3 in the intermediate 
calcinating step makes it difficult to obtain 
YBa 2 Cu 3 0 7 _j in pure form. 

Complete avoidance of the formation of BaC0 3 
during the synthesis is possible using the hyponitrite 
precursor [157], The hyponitrite precursor is obtained 
froia a nitrate solution of Y, Ba and Cu ions by the 
addition of an aqueous Na 2 N 2 0 2 solution. The precipi- 
tate is converted Into superconducting YBa 2 Cu 3 0 7 -a 
by heating at around 973 K in an argon atmosphere, 
followed by oxygen annealing at 673 K. Although this 
route provides a convenient means of obtaining the 123 
cuprate at much lower temperatures than with other 
methods, there is a possibility of contamination of alkali 
metal ions during the course of the precipitation. 

YBa 2 Cu 3 0 7 can also be prepared by the hydroxy- 
carbonate method [158, 159], Here, KOH and K 2 C0 3 



are employed to precipitate copper as the hydroxide 
and Y and Ba as the carbonates in the pH range of 7-8. 
By employing NaOH and Na 2 C0 3t complete precipi- 
tation as hydroxycarbonate is attained at a pH of — 13. 
The product from the aboye two procedures is homo- 
geneous, showing sharp onset of superconductivity at 
92 K. The possibility of contamination by alkali metal 
ions cannot, however, be avoided. 



33, Vfl8 s Cu^Oa- 

YBa 2 Cu*O e can be prepared by the oxalate route [160] 
wherein the solution of Y, Ba and Cu nitrates in water 
is added dropwise into oxalic acid-triethytamine solu- 
tion under stirring. Complete precipitation of Y, Ba and 
Cu with the desired stoichiometry of 1:2:4 is achieved 
in the pH range of 9.3-1 1.3. The precipitated oxalates 
are filtered and dried in air at 393 K. The solid obtained 
is then heated in the form of pellets at 1078 IC in 
flowing oxygen for 2-4 days. The product after quen- 
ching in air shows the 124 phase as the major product 
with a T c of 79 tC. 

An alternative coprecipitation route for the synthesis 
of YBa 2 Cu 4 0 8 is the method of Chen et al [161] in 
which the aqueous nitrate solution of the constituent 
metal ions is mixed with 8-hydroxyquinolirie-tri- 
ethylamine solution. The precipitated oxine is filtered, 
washed, dried and sintered at 1088 K in oxygen for 3 
days to yield phase-pure YBa 2 Cu^0 8 showing a T € of 
80 K. EthyJenediaminetetraacettcacid [161] as well as 
carbonate routes [162] have also been employed for the 
preparation of YBa 2 Cu 4 O a . Coprecipitation using tri- 
ethylammonium oxalate has been exploited for substi- 
tuting Sr in place of Ba in YBa 2 Cu 4 0 8 [163]. 



3,4. Bismuth cuprates 

Very few coprecipitation studies have been carried out 
on the preparation of bismuth cuprates. One reason 
may be that despite the good sample homogeneity gen- 
erally obtained through solution methods, the chemistry 
pf bismuth cuprates is rather complex. It is not that 
easy to find compounds of all the constituent metal ions 
soluble in a common solvent ; controlling the stoichiom- 
etry in these cuprates is also difficult in the coprecipita- 
tion procedure. Furthermore* bismuth nitrate, which is 
often used as one of the starting materials, depomposes 
in cold water to a basic nitrate precipitate as given by 

Bi(N0 3 ) 3 (s)->Bi 3 * + 3N0 3 ~ 

Bi 3> + 3N0 3 + 2H 2 G*± Bi(OH) 2 N0 3 (s) + 2H+ . 

This problem can be overcome to some extent by pre- 
paring the nitrate solution of bismuth in nitric acid or 
by starting with bismuth acetate instead of the nitrate. 

Bidentate iigands such as the oxalate are found to 
r^act more rapidly than multidentate ligands such as 
citric acid [164-174] in the coprecipitation process. 
Complexes of oxalic acid are also more stable than 



Ithe stdi&iometry because d£ the Telafc^>solubiKty of 
BiCjO^orSrCaCV 

A straightforward oxalate coprecipitation is 
achieved by dissolving the acetates of Bi, Ca, Sr and Cu 
in glacial acetic acid and then adding excess oxalic acid 
to the solution [264]. The oxalate precipitate is dried 
and decomposed at around 1073 K in air and processed 
in the 1 103-1 123 K range for periods ranging from 24 b 
to 4 days, depending on the starting composition. The 
n * 2 (2122) member obtained by this procedure shows 
zero resistance at §3 EC. In another procedure reported 
by Zhang et al [1651 hxst the Sr/Ga/Cu nitrate solu- 
tions are mixed in the required molar ratio. Into this 
solution is poured a solution of bismuth nitrate pre- 
pared in nitric acid along with oxalic acid The com- 
plete precipitation occurs at a pH of around 5 (attained 
by the addition of aqueous NaOH). This process 
involves the possibility of contamination of sodium 
ions; this has been circumvented by using N(CHj) 4 GH 
to adjust the pH of the solution [166] and complete 
precipitation of the oxalates occurs at a pH of 12 AH 
these procedures, however, produce mixed-phase 
samples* 

For the preparation of the monophasic lead-doped 
n = 3 member (2223). oxalate coprecipitation has been 
found effective [167-174]. In the procedure reported by 
Chiang et al [171], the molar ratio of the chelating 
agent (oxalic add) and the nitrate anions (from the 
metal nitrate solutions) is fixed at 0.5 and the pH» 
adjusted by NH 4 OH solution, at which complete pre- 
cipitation occurs is 6.7. The product from this method, 
Bi,^Pb a6 Sr 2 Ca 2 Cu30 y> after sintering at 11 33 & in air 
for 72 h, shows a T t of 1 10 K. 

Coprecipitation as oxalates to prepare the lead- 
doped a - 3 member (2223) has been achieved from an 
ethylene glycol medium using triethylammonium 
oxalate and oxalic acid [172]. A more easily controlled 
and reproducible oxalate coprecipitation procedure 
appears to be that of Shea et al [173] where in a mixture 
of triethylamine and oxalic add is employed. The 
advantage of using triethylamine is that it has a higher 
basicity and a lower completing ability towards Cuff I) 
than has ammonia. Control of the stoichiometry of 
the final product is therefore better obtained with this 
procedure; precipitation occurs in the pH range 1.5-22 
The coprecipitated oxalates sintered at 1133 K in air for 
a minimum period of 72 h give monophasic 
Bii;*Pb a6 Sr 2 Ca 2 Cu 3 O l0 with a T c of 110 K. It is pos- 
sible to avoid adjusting the pH in the coprecipitation of 
oxalates [174]. The procedure involves copredpitatshg 
the oxalates from dilute acetate solutions instead of 
from nitrate solutions. The oxalates are then converted 
to nearly phase-pure Bii, d Pbo. 4 Sr 2 Ca 2 Cu 3 O lo °* 
106 K) by sintering at 1 123 K in air for 160 h. 

Carbonate corprecipitation has also been carried 
out for the synthesis of superconducting bismuth cup- 
fates [175, 176], but the method does not yield mono- 
phasic products. 




Coprecapji&tton oq m p^oasea ccpraies crom 
aqueous solutions 'as'oxW^S;^- ^M^^^iJ ^ * fey "i&a. ' sollu-r 
bibty of thallium oxalate. However, Bernhard md 
Gritzner [177] have fotmd that cranp&te copf&qpka* 
tion as oxalates can be cchkved by starting with thal- 
lium acetate in glacial acetic add medium In the 
procedure reported for the preparation of the » = 3 
member (2223X stoichiometric amounts of thallium 
acetate, CaC0 3 , BaC0 3 and copper acetate are dis- 
solved in water containing gladai acetic add. The solu- 
tion containing all the cations is then added to a 
solution of oxalic add (excess) under stirring. The pre- 
dpitate, after digestion for 1 h, is filtered, washed and 
dried. The oxalates are heated in the form of pellets 
(wrapped in gold foil) at around 1173 & for 6 min m an 
oxygen atmosphere. The product alter annealing in the 
same atmosphere shows 2223 as the major phase with a 
T c of 118 tC 



3A Leadl cuprases 

Carbonate coprecipitation is found to be satisfactory 
for the synthesis of representative members of super- 
conducting lead cuprates [128] of 2213 and 1212 
types, namely Vb^S^Yo^Ca^C^jO q+q and 
Pb 0i 3Sr a5 $r 2 Y 0 . 5 Ca 0i5 Cu 2 O7.i> Copredpitattoa as 
carbonates has been achieved by adding the nitrate 
solution of the constituent metal ions to an aqueous 
solution of sodium carbonate (in excess) under constant 
stirring. The carbonate predpitate thus obtained is 
washed and dried The decomposed powder is heated in 
the form of pellets around 1153 K in a suitable atmo- 
sphere. Pb 2 Sr 2 Ca^5Y 0 .5Cu3O 8+ a obtained by this 
method after heating for 4 h in nitrogen containing 1% 
0 2 showed 2213 as the major phase (% *~ 74 K) with 
impurities such as Y 2 0 3 » CuO. The 1212 phase 
obtained after heating in oxygen at 1153 K for 12 h 
showed a broad transition with a T c (onset) of 100 K . 
This method has the advantage of single heating rather 
than the multistep procedures required in the other 
methods. 



4. SoJ-9<H process • 

The sol-gel process is employed in order to get homo- 
geneous mixing of cations on an atomic scale so that 
the solid state reaction occurs to compkikm in a short 
time and at the lowest possible temperature. The term 
sol often refers to a suspension or dispersion of discrete 
colloidal panicles, while a gel represents a colloidal or 
polymeric solid containing a fluid component which has 
the internal network structure wherein both the solid 
and the fluid components are highly dispersed. In the 
sol-gel process a concentrated sol of the reactant oxides 
or hydroxides is converted to a semi-rigid gel by remo- 
ving the solvent The dry gel is heated at an appropriate 



13 



U N H Rao et as 




temperature to obtain the product Most of the reac- 
tions in the sol-gel jprocess occur via hydrolysis and 
polyeondensation. 

Two different routes for the sol-gel process are 
usually described in the literature for the synthesis or 
high-7^ cuprate superconductors: 

(I) Via molecular precursors (e,g. metal alkoxides) in 
organic medium; 

(ii) Via ionic precursors in aqueous medium (citrate 
gel process^ 

The purity, microstructure and physical properties 
of the product are controlled by varying the precursor, 
solvent, pH, Siring temperatures and atmosphere of heat 
treatment. 

Superconducting 214 compounds are prepared both 
by means of organometailic precursor [178] and by the 
citrate gel process [11]. Lanthanum 2,4-pentane 
dionate, barium 2,4-pentane dionate and copper (II) 
ethyl hexanoate are mixed at room temperature in the 
appropriate ratios in methoxyethanol medium to obtain 
the organometailic precursor. After vigorous stirring at 
room temperature, the precursor get is converted to 
monophasic La,, 9 Ba 0 ^CuO* (7. 23 R) by firing at 
873 Kin oxygen. 

In the citrate gd process, a mixture of citric acid and 
ethylene glycol is added to the solution containing the 
required quantities of metal nitrates. The resulting solu- 
tion is vigorously stirred and heated around 393 K. 
During this process, oxides of nitrogen evolve,, resulting 
in a viscous gel The gel is decomposed at 673 K in air 
and the resulting black powder is then given the neces- 
sary heat treatment to obtain the superconducting 
oxide. 

In the case of YI& 3 0i 3 0 7 _^ the aikoxide precursors 
are both very expensive and difficult to obtain. In addi- 
tion, the solubility of copper a&oxides is very low in 
organic solvents and yttrium alkoxides are readily 
hydrolysed even by a trace of water. Despite these diffi- 
culties, superconducting YBajCto»0 7 .j has been pre- 
pared using ai&ojiides [157, 179-1811 A simple reaction 
involving Y^OCifMe^ , ^OCHMe 2 ) 2 and Cu(NBu a } 
in THF in an argon atmosphere gives the 
organometailic precursor £157]. The precursor powder, 
after removal of the solvent, is sintered at 973 K in 
Mowing argon to obtain tetragonal ¥Ba a Cu 3 07-i . Fol- 
lowing oxygenation at 673 the product shows a % of 
85 Kl Superconducting properties have been improved 
by using »-butorides of Y, Ba am! Cu in butanol 
solvent [179]. 

Alternatively, methoxyethoxides of yttrium, barium 
and copper have been used as precursors in 
methoxyethanol-methylelhylketone-toluerie solvent 
mixture to prepare YBa a Cu 3 0 7 .^ [180]. In some of the 
preparations, Cu^O^ (soluble in ethanol) or copper 




acetylaoetbnate (soluble in toluene) is used along with 
the aikoxides of yttrium and barium to overcome the 
problem of low solubility of copper aikoxides [182, 
183]. Organometailic precursors involving propionates 
[153] and neodeconates [184] have also been used for 
preparing YBa 2 Cu^0 7 ^. 

Modified sol-gel methods which do not involve the 
metal aikoxide precursors have been employed by many 
workers. Thus, Nagano and Greenblatt [185] have 
employed metal nitrates dissolved in ethylene glycol 
After refluxing around 353 K under vigorous stirring, a 
bluish green colloidal gd is obtained. The gel is con* 
verted into orthorhombic YBa 2 Cu 5 0 7 ^ by heating to 
1223 K in flowing oxygen. Precipitating all the three 
ions as hydroxtdes also results in fine colloidal particles 
of the starting materials [186-1 88]. The precipitation is 
genially carried out by the addition of NH^OH [186], 
NfCH^OH [187] or BafOH) 2 [188] to a solution of 
metal nitrates (pH range 7-8). These hydroxides arc 
decomposed around 1223 K in oxygen to give 
YBa z Cuj0 7 showing a T e of 93 K. 

YBa 2 Cu 3 0 T . a has bsen prepared by the citrate gel 
process [189-193]. In this method 1 g univalent of 
citric acid is added to each gram equivalent of the 
metal The pH of the solution is adjusted to around 6 
(either by NH^OIHl or by ethylenediamine). Evaporation 
of the solvent (water) around iC* results in a viscous 
dark blue geL The gel is decomposed and the powder 
sintered in the form of pellets m 1173 K in oxygen to 
obtain orthorhomlac YBa^jO? (T t = 93 KX By 
this method, ultrafine homogeneous powders (pailkle 
size ^0*3 fan) are obtained. The crucial step in this 
process is the adjustment of the pH which controls the 
stoichiometry of the final product This limitation has 
been overcome by dispersing the citrate metal ton com* 
piexes in a solvent mixture of ethylene glycol and water 
[194.195], 

Problems such as the formation of BaGO^ during 
the calcination step, filtration and contamination of 
alkali metal ions in the final product are avoided in the 
sol-gel process. Furthermore, perfect bom^m^ is 
obtained before calcination. The sol-gel process (e.g. 
citrate process) has the advantage over the other 
methods in that the gel can be used for making thick 
and thin superconducting films, fibres etc which have 
technological importance [179, 185* 184 196-19fc|. 

AX YBo^Ci^CPo 

The sol-gel r&eafrod offers a good alternative to the 
ceramic method for the synthesis, of superconducting 
YBa z Cu 4 0 9 . The following procedure has beeti used 
to prepare YBa 2 Oi^0 0 at 1 atm oxygen pressure 
[199]. Appropriate quantities of Y{n — OG *H*h .* 
Ba(s - OC 4 H*) 2 and Cu(s - OBu^ in butanof-xylene 
mixture are refluxed in an argon atmosphere at 343 K 
for a period of 30 h* The fine powder after the vigorous 
reaction is Creed from the solvent and dried. The powder 
is heated in the form of peOets at 1033 K in Sowing 
oxygen to obtain superconducting YBa 2 Cu 4 O 0 , 



used as the source of copper in -ibfrpr&^jre [209]. 

In the modified citrate gel procBF to prepare 
y&a 2 Cu^Q 0 [201, 202], 1 g equivalent of citric add is 
added for each gram equivalent of the metal and the pH 
of the solution is adjusted to ~5.5 by the addition of 
ethylenediamine. The resulting clear solution is evapo- 
rated to yield a viscous purple gel. The decomposed gel 
is sintered in flowing oxygen for 3-5 days at 1088 K to 
obtain nearly monophasic YBa 2 Gu 4 0 8 (7; .» 66 Kj. 
Kakihana et al [203] have reported the preparation of 
YBa 2 Cu^O 0 using a precursor obtained from citrate 
metal ion complexes uniformly dispersed in a solvent 
mixture of ethylene glycol and water. This method 
yields phase-pure YBa 2 Cu 4 O s (T c ~ 79 K) and elimi- 
nates the need to adjust the pR 

4A BSsimstfe oiprates 

There have been very few reports erf the preparation of 
bismuth-based cuprate superconductors by the alkoxy 
sol-gel method [204]. Some of the difficulties arise 
because the relevant bismuth/lead alkoxides are not 
readily available; it is also not easy to get a common 
organic solvent to dissolve the various metal alkoxides 
simultaneously. Dhalle et al [204] have, however, 
attempted to synthesize the lead-doped n - 3 member 
(2223) using organometallic precursors involving propi- 
onates. The starting materials were taken in the form of 
nitrates and converted into propionates by the addition 
of an excess of 100% propyl alcohol. This step was fol- 
lowed by the addition of ammonium hydroxide and eth- 
ylene glycol to increase the alkoxy anion concentration, 
thus in turn increasing the viscosity of the solution. AH 
the solutions were mixed together and dried at 353 K. 
The resin after calcination al 1123 K in air and sinter- 
ing at 1 1 1-8 K gave a mixture of the n « 3 and n = 2 
members. 

A simple sol-gel method involving the addition of 
dilute ammonia to an aqueous solution containing 
nitrates of Bi, Sr and acetates of Ga, Cu and Pb (until 
the pH of the solution reached around 5.5) has also 
been employed to prepare bismuth cuprates [205, 206]. 
The blue solution after concentrating at around 343 It 
gives a viscous gel. The gel is decomposed and the 
powder sintered at around 1128 K in air. The product 
from this procedure is multiphasic showing a T Q of 
104 K. The simplicity of the method and the formation 
of the n = 3 phase in a short time makes it somewhat 
superior to the conventional ceramic route. The modi- 
fied citrate gel process has been employed to prepare 
the n = 2 member (2212) in pure form with a T c of 78 K 
[193]. 

43. Lead cuprates 

The modified citrate gel process has been successfully 
employed by Mahesh et al [207] for the synthesis of 
lead cuprates of the 2213 or 1212 type. In a typical pro- 
cedure, a mixture of citric acid and ethylene glycol in 



reorder to ££t a vis«)us gel 
The gel after decomposiflP is heat^ in the form of 



is concentrated at 373 



pellets in the temperature range of 1073-1173 K either 
in N, containing 1% O a or in an oxygen atmosphere. 
Pb 2 Sr 2 Y 0 5 Cao. 3 Cu 3 0 0 ^ obtained from this process 
shows a sharp superconducting transition at 70 K. The 
1212 cuprate also shows a sharp transition at 60 K. 
This process is superior to the ceramic procedure for 
synthesizing superconducting lead cuprates. 

S. AUCcqII flua motlfood 

Strong alkaline media, either in the form of solid car- 
bonate fluxes, molten hydroxides or highly concentrated 
alkali solutions can be employed for the synthesis of 
high-7; cuprate superconductors. The alkali flux 
method takes advantage of both the moderate tern- 
peratures of the molten media (453-673 K) as well as of 
the acid-base characteristics of molten hydroxides to 
simultaneously precipitate oxides or oxide precursors 
such as hydroxides or peroxides of the constituent 
metals. The method stabilizes higher oxidation states of 
the metal by providing an oxidizing atmosphere. 

Employing fused alkali hydroxides, Ham et al [208] 
have synthesized superconducting La 2 - a M x Cu0 4 
(M » K or Na or vacancy) at relatively low tem- 
peratures (470-570 K). In this method, stoichiometric 
quantities of La 2 O y and CuO are added to a molten 
mixture containing KOH and NaOH (in an approx- 
imately 1 : 1 ratio) in a Teflon crucible and heated at 
around 570 K in air for 100 h. The 1 : 1 mixture Of 
KOH and NaOH melts at 440 K and since the alkali 
hydroxides generally contain some water, the melt is 
acidic and can readily dissolve oxides such as La 2 Q 3 
and CuO. The black crystals obtained from the reaction 
(after washing away the excess hydroxide with water) 
show a T c of 35 K. Since the reaction is carried out in 
alkali hydroxides, incorporation of Na* or K + ions for 
La 3 * in the lattice of La 2 CuO* cannot be ruled out It 
should be noted that superconducting alkali-doped 
La 2 CuO* is normally prepared at higher temperatures 
in sealed gold tubes [209]. Recently, alkaline hypo- 
bromite oxidation has been employed to obtain 
La 2 CuO A+ , with a T c 6f 44 K [210]. 

Superconducting YBa*tti s Q 7 (T c ~ 88 K) has also 
been prepared using the fused eutectic of sodium and 
potassium hydroxides in a similar manner to that 
described above [211). The problem of contamination 
of alkali metals in the preparation of YBa 2 Cu30 7 has 
been overcome by using the Ba(OH) 2 flux [211], The 
procedure involves heating a mixture containing stoi- 
chiometric amounts of Y(NQ3) 3 -6H 2 (\ Ba(OH) 2 and 
Cu(N0 3 ) 2 . 3H 2 0 in an open ceramic crucible at around 
1 023 K in air for a short time (about 10 min) and then 
slowly cooling the melt to room temperature. Since 
Ba(OH) 2 has two hydration states, one melting at 
351 K and the other at 681 K, the lower-melting 
hydrate acts as the solvent for the nitrates of copper 

15 



CNR Rao ei ai 



and yttrium while the high-inching hydrate serves as the 
medium for intimate mixing of the reactants. The pre- 
cipitate obtained from the melt, after washing with 
water, is sintered in air at around 2 1 73 K followed by 
oxygenation at 773 K. This method yields an ortbor- 
hombic YBa 2 Cu 3 0 7 phase (with little CuO impurity) 
showing a 7^ of 92 K. 

The flux method eliminates the need for mechanical 
grinding and introduction of carbon-containing anions, 
which is often encountered in the solution routes. Fur- 
thermore, the method is efficient and cost-effective. 

€. Combustion method 

Although many of the solution routes discussed earlier 
yield homogeneous products, the processes involved are 
quite complex. Combustion synthesis or self* 
propagating high- temperature synthesis [%m\ first 
developed by Merzhanov and Borovinskaya [2121 pro- 
vides a simple and rapid means of preparing inorganic 
materials, many of which are technologically important. 
Combustion synthesis is based on the principle that the 
heat energy liberated by many exothermic non-catalytic 
solid-solid or solid-gas reactions can self-propagate 
throughout the sample at a certain rate. This process 
can therefore occur in a narrow zone which separates 
the starting substances and reaction products. 

Self-propagating combustion has been employed 
recently in this laboratory to synthesize members of 
almost all families of cuprate superconductors {except 
for the thallium cu prates) [213], The method involves 
the addition of an appropriate fuel to a solution con- 
taining the metal nitrates in the proper stoichiometry. 
The ratio of the metal nitrates to the fuel is such that 
when the solution is dried at around 423 K, the solid 
residue undergoes flash combustion, giving an ash con- 
taining the mixture of oxides in the form of very fine 
particles (particle size 03-0.5 nm). The ash is then given 
proper heat treatment under the desired atmosphere to 
obtain the cuprate. The small particle size of the ash 
facilitates the reaction between the metal oxides due to 
smaller diffusion distances between the cations. Fuels 
such as urea [213, 214], glycine [213, 215] and tetra- 
formal triazine (TFTA) [216] are generally employed 
for synthesizing cuprate superconductors. Uitraftne par- 
ticles of copper metal can also act as an internal fuel 
wherein the combustion is Initiated by flashing a laser 
beam for a short rime [217} Some of the cuprate super- 
conductors which have been prepared [213] by this 
route include La^^CaO* (T 9 .**35 K% YBa 2 Cu 3 0 7 
(T c = 90 KX YBa 2 Cu 4 0 8 (T c = 80 K\ Bi 2 CaSr 2 Cu 2 d 8 
(T c » 85 K\ Pb 2 Sr 2 Y 05 Ca* :5 Cu 3 O 8 (T e = 60K) and 
Nd 2 „,Ce*Cu0 4 (T e ~ 30 K). 

7. Other methods 

In addition to the various synthetic methods discussed 
hitherto, a few other methods such as spray drying 
[218-221], freeze drying [186, 222, 223], use of metallic 
precursors [224, 225] and electrochemical methods 



[226, 227] have also been employed for the preparation 
of cuprate superconductors in bulk form. In spray 
drying, a solution containing the metallic constituents, 
usually in the form of nitrates, is sprayed in the form 
of fine droplets into a hot chamber. The solvent 
evaporates instantaneously, leaving behind an 
intimate mixture of the rcactants which on heating at 
the desired temperature in a suitable atmosphere yields 
the cuprate. Some of the superconducting cuprates pre- 
pared by this method include YBa 2 Cu 3 0 7 (T c = 91 K) 
[218]. YBa 2 Cu 4 0 8 (T C = 81K) [219] and 
Bi|.«Pb 0 , 4 Sr 2 Ca a Cu 3 O 10 (T e « 101 K) (220, 221]. In 
freeze drying, the reactants (in a common solvent) arc 
frozen by immersing in liquid nitrogen. The solvent is 
removed at low pressures to obtain the initial reactants 
in fine powder form, arid these are then processed at an 
appropriate temperature. For example, YBa 2 Cu 3 0? 
(i; = 87 K) [186], YBa 2 <X0 8 (% = 79 K) [222] and 
Bi 16 Pb 0il Sr l . 6 Ca i C03O T (r c » 101 K) [223] have been 
prepared by this method 

Metallic precursors have been used in the prepl- 
an* tion of 123 and 247 cuprates [224* 225]. For 
example, oxidizing an Er-Ba^Cu alloy around 1170 K 
gives superconducting ErBa 2 Cu 3 0 7 with a T 9 of 87 K 
[224]. Similarly Yb 2 Ba 4 Cu 8 O l5 has been obtained by 
heating an alloy composition of YbBa 2 Cu 3 (with 33 
wt% of silver) under 1 atra oxygen at 1173 K [225], 

Making use of electrochemical oxidation, 
La 2 CuO A+# with a 7; of 44 K has been prepared at 
room temperature, which is otherwise possible only by 
use of high oxygen pressures [226, 227]; 



8. Oxygen non-atotchlometry 

Oxygen stoichiometry plays a crucial role in determin- 
ing the superconducting properties of many of the cup- 
rates. Thus, stoichiometric La 3 €u0 4 is an insulator, 
while an oxygen-excess material prepared under high 
oxygen pressures shows superconductivity with a T t of 
35 K [15]. The same holds for the next member of the 
homologous family, La^^^Sr^CaCujO^ which is super* 
conducting only when there is an oxygen excess [17]. 
The excess oxygen donates holes in these two systems. 
In the case of YBa 2 Cu 3 0 7 -* oxygen can be easily 
removed giving rise to tetragonal non-superconducting 
YBa 2 Cu 3 C> 6 . The YBa 2 Cu 3 0 6 material can be pre- 
pared by heating YBa 2 Cu 3 O r in an argon atmosphere 
at 973 K for extended periods of time [228]. The varia- 
tion of ^ with oxygen stoichiometry, d, is well known 
[229, 230]. When 6 reaches 0.5, there is an intergrowth 
of YBa 2 Cu 3 0 6 arid YBa 2 Cu 3 0 7 and at this composi- 
tiorv the material shows a T t of 45 K. The i « 0.5 com- 
position is obtained by quenching S^tO material, 
heated in a nitrogen atmosphere at 743 K [231]. Simi- 
larly, by quenching YBa 2 Cu 3 0 7 at 783 K in air, 
YBa 2 Cu 3 0 4 . 7 (showing a T € of -60 K) is prepared 
P31J The T c of 90 K is found only when £<0JL 
YBa 2 Cu 3 0 6 is readily oxidized back to YBa 2 Cu l 0 7 . It 
may be noted that this oxidation-reduction process in 



Cuprate 



(appro*,} Methods of synthesis' 



ta a ,Sr,(Ba)Cu0 4 

La a CtiOw 
YBa s Cu 3 0 7 * 

YBa 2 Cu < ,G 8 t ' 

Bi 3 CaSr 2 Ca 2 0» 

BjaCaaSr^CujO.o 

TiCaBa 2 Cu 2 0 0W c 

TJGa 2 Ba 3 Oi30» +4 c 

Ti a 8a a CuO e c 

Tl a CaBa 2 Cu 3 O t 

Tl a Ca a Ba 3 Cu30 10 

Pb a Sr 2 Ca,_,y,Ci%0 8 



NoV,Ce,CuO« 

Ga, _,3r,CuO» 
Sr^,Nd.Cu0 2 



35 
60 
40 
90 

80 

90 

110 
90 

115 
90 

110 
125 
90 
70 



45 
30 



40-110 
40-110 



Ceramic*, sot-gel. combustion; copreclpltatldn 
Ceramic (high 0* pressure)* 

Ceramic (high 03 pressure)* alkaJMhix, hypobromHe* 
Ceramic (annealing in OJ* sol-«er*. c©|««clpHailon\ 
combusti on 

Ceramic {hiah O a pressure), ceramic (with Ha a O a )* 

sol-a* 1 *. coprecipltatton* 
Ceramic (air-quench)* sqHpl*. combustion, 

mett (glass) route* 
Ceramte*, soH^K mett route 
Ceramic (sealed Ag/Au tube)* 
Ceramic (seated Ag/Au tube)* 
Ceramic (sealed Ag/Au tube)* 
Ceramic (sealed Ag/Au tube)* 
Ceramic (sealed Ag/Au tube)* 
Ceramic (sealed Ag/Au tube)* 
Ceramic {tow O a partial pressure).* 

sol-gel* (low D 2 partial 

pressure) 
Ceramic (flowing Q a J* 
Ceramic (low Oa partial pressure)* 
Coprecfpitation (low pj partial pressure)* 
Ceramic {hf oh pressures)* 

Ceramic (high pressures)* 



■» Recommended methods ar e bleated by asterisks. ■ ■ « , ^^.k. 

b Other rare-earth compounds of this type are also prepared by similar methods. Oxygen annealing is done betow the 

orthorhombSo-ietragonjJ transition. 

c Sr analogues of these compounds with different sabslftutions at CaandTi sites are prepared by a similar procedure. 



YBa 2 Cu 3 0 7 _* is of topochemicai character. The other 
analogous rare-earth 123 cuprates ajso behave in a 
similar way with respect to the variation of S with T c 
[232], 

While YBa 2 Cu 4 0 8 has high oxygen stability, 
Y,Ba*Cu 7 0|s-* shows a wide range of oxygen stoichi- 
ometry (0 ^ 5 ^.1) [233]. The maximum T t of 90 K is 
achieved when £ is close to zero, and when S reaches 
unity the material shows a 7^ of 30 K; there is no struc- 
tural phase transition accompanying the variation in 
oxygen stoichiometry. Usually, both yttrium 124 arid 
247 cuprates and their rare-earth analogues, prepared 
by the ceramic method under 1 atm oxygen pressure, 
show £ dose to zero. 

Bismuth cuprates of the type Bi 3 (Ca, Sr),*, 
Cu*Q*.*4o are best prepared by quenching the 
samples in air or by annealing in a nitrogen atmosphere 
at appropriate temperatures [53, 234]. Heating the 
samples in an oxygen atmosphere is no good, possibly 
because the extra oxygen may add on to the K-O 
layers. In the case of the lead-doped **=3 member 
(2223\ preparing the samples under low partial pres- 
sures of oxygen is found to increase the volume fraction 
or the superconducting phase [235, 236]- The n - 1 
member, Bi 2 Sr 2 Cu0 6+ , shows metallic behaviour when 
there is excess oxygen [237], By annealing in a reducing 
atmosphere {Ar or N 2 )> the excess oxygen can be 
removed to induce superconductivity. 

Oxygen stoichiometry has a dramatic influence on 
the superconducting properties of thallium cuprates [94, 
108, 109, 238-246]. For example, thallium cuprates of 
the TICa^Ba-jCu^Oj,^ family, derivatives of the 



HCa^jS^Cu.O^+j family and n ? Ba 2 Cu0 6 often 
have excess oxygen when prepared in sealed tubes. By 
annealing these samples in a reducing atmosphere (Ar, 
dilute Hy, or vacuum) at appropriate temperatures, 
the excess oxygen is removed to induce superconduc- 
tivity in some cases [108, 109, 238]. Annealing at low 
oxygen partial pressures or in a reducing atmosphere 
also increases the T c of some of the superconducting 
thallium cuprates to higher values by decreasing the 
oxygen content [94, 239-246]. These variations are 
clearly related to the hole concentration where the 
number of holes decreases by removing excess oxygen, 
thereby giving the optimal concentration required for 
maximal T e [247]. 

In lead cuprates of the PbjSr^Lii, Ca)Oi 3 0 84# 
(2213) type, increasing the oxygen content of the 
material by annealing in an oxygen atmosphere oxidizes 
the Pb 2 * and Cu 1 * without affecting the CuO* sheets, 
which governs the ^^conductivity in this material 
[243]. Though this system shows a wide range of 
oxygen stoichiometry (associated with a structural 
phase transition from ortborhombic to tetragonal 
symmetry), maximum 7; is observed for any given com- 
position where in 6 is close to zero [249]. Samples with 
d 0 are therefore prepared by annealing in a nitrogen 
atmosphere containing little oxygen. The lead 1212 cup- 
rates, on the other hand, are best prepared in a Sowing 
oxygen atmosphere. The samples obtained after the 
oxygen treatment are often not superconducting since 
there is an oxygen excess. The samples are quenched in 
air at around 1073 K in order to achieve superconduc- 
tivity [250]. 



17 



C N ft Rao era/ 



Superconducting properties of the electron-doped 
superconductors, Nd^Ce^CuO*-,, are sensitive 10 
the oxygen content. The as-prepared samples which are 
semiconducting have oxygen content greater than four 
Samples with oxygen content less than four are 
obtained by annealing in a reducing atmosphere {N* , 
Ar or dilute H 3 ) at around 1173 K. Maintaining the 
oxygen stoichiometry at less than four is essential for 
having an oxidation state of Cu less than 24 in this 
material [2513- 

9. Concluding remarks 

In the earlier sections we presented details of the pre- 
parative methods for the synthesis of various families of 
cuprate superconductors. In addition, we also examined 
the advantages and disadvantages of the different 
methods. Since more than one method of synthesis has 
been employed for preparing any given cuprate, it 
becomes necessary to make the right choice of method 
in any given situation. In order to assist in making such 
a choice, we have tabulated in table 6 the important 
preparative methods employed to synthesize some of 
the representative cuprates, where the recommended 
methods are also indicated. 

Acknowledgment 

The authors thank the various agencies, expeciaily the 
National Superconductivity Research Board, University 
Grants Commission and the US National Science 
Foundation for support of the research related to 
cuprate superconductors. 



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