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0 Publication number: 0 275 343 

A1 



@ EUROPEAN PATENT APPLICATION 



© Application number: 87100961.9 


(sy int. CL 4 : H01 L 39/1 2 


© Date of filing: 23.01.87 




@ Date of publication of application: 


0 Applicant: International Business Machines 


27.07.88 Bulletin 88/30 


Corporation 




Old Orchard Road 


© Designated Contracting States: 


Armonk, N.Y. 10504(US) 


AT BE CH DE ES FR GB GR IT U LU NL SE 






© Inventor: Bednorz, Johannes Georg, Or. 




Sonnenbergstrasse 47 




CH-8134 Adliswil(CH) 




Inventor: MUller, Carl Alexander, Prof.Dr.. 




Haldenstrasse 54 




CH-8908 Hedfngen(CH) 




inventor: Takashige, Masaakl, Or. 




Rotfarbweg 1 




CH-8803 RUschllkon(CH) 




0 Representative: Rudack, QUnter O., Dlpl.-lng. 




IBM Corporation Siumerstrasse 4 




CH-8803 RUschlikon(CH) 



0 New superconductive compounds of the K2NIF4 structural type having a high transition 
temperature, and method for fabricating same. 



Europaisches Patentamt 
European Patent Office 
Office europ6en des brevets 



0 The superconductive compounds are oxides of 
the general formula RE2.KAExTM.O4-y . wherein RE is 
a rare earth, AE is a member of the group of alkaline 
earths or a combination of at least two member of 
that group, and TM is a transition metal, and wherein 
x < 0.3 and 0.1 £ y S0.5. The method for making 
these compounds involves the steps of coprecipitat- 
ing aqueous solutions of the respective nitrates of 
the constituents and adding the coprecipitate to ox- 
alic acid, decomposing the precipitate and causing a 
^ solid-state reaction at a temperature between 500 
^and 1200°C for between one and eight hours, for- 
COming pellets of the powdered product at high pres- 
sure, sintering the pellets at a temperature between 
500 and 1000°C for between one half and three 
M hours, and subjecting the pellets to an additional 
£j annealing treatment at a temperature between 500 
and 1200°C for between one half and five hours in a 
© protected atmosphere permitting the adjustment of 
the oxygen content of the final product. 

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Xerox Copy Centre 



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NEW SUPERCONDUCTIVE COMPOUNDS OF THE feNJF* STRUCTURAL TYPE HAVING A HIGH TRANSITION 
TEMPERATURE, AND METHOD FOR FABRICATING SAME 



Field of the Invention 

The invention relates to a new class of super- 
conductors, in particular to components of the 
KzNiF* type of structure having superconductor 
properties below a relatively high transition tem- 
perature, and to a method for manufacturing those 
compounds. 



Background of the Invention 

Superconductivity is usually defined as the 
complete loss of electrical resistance of a material 
at a well-defined temperature. It is known to occur 
in many materials: About a quarter of the elements 
and over 1000 alloys and components have been 
found to be superconductors. Superconductivity is 
considered a property of the metallic state of the 
material, in that ail known superconductors are 
metallic under the conditions that cause them to 
superconduct. A few normally non-metallic materi- 
als, for example, become superconductive under 
very high pressure, the pressure converting them 
to metals before they become superconductors. 

Superconductors are very attractive for the 
generation and energy-saving transport of electrical 
power over long distances, as materials for forming 
the coils of strong magnets for use in plasma and 
nuclear physics, in nuclear resonance medical di- 
agnosis, and in connection with the magnetic levita- 
tion of fast trains. Power generation by thermonu- 
clear fusion, for example, will require very large 
magnetic fields which can only be provided by 
superconducting magnets. Certainly, superconduc- 
tors will also find application in computers and 
high-speed signal processing and data communica- 
tion. 

While the advantages of superconductors are 
quite obvious, the common disadvantage of all 
superconductive materials so far known lies in their 
very low transition temperature (usually called the 
critical temperature T c ) which is typically on the 
order of a few degrees Kelvin. The element with 
the highest T c is niobium (9.2 K), and the highest 
known T c is about 23 K for NBsGe at ambient 
pressure. 

Accordingly, most known superconductors re- 
quire liquid helium for cooling and this, in turn, 
requires an elaborate technology and as a matter 
of principle involves a considerable investment in 
cost and energy. 

It is, therefore, an object of the present inven- 



tion to propose compositions for high-T c supercon- 
ductors and a manufacturing method for producing 
compounds which exhibit such a high critical tem- 
perature that cooling with liquid helium is obviated 

5 so as to considerably reduce the cost involved and 
to save energy. 

The present invention proposes to use com- 
pounds having a layer-type structure of the kind 
known from potassium nickel fluoride teNiF*. This 

io structure is in particular present in oxides of the 
general composition RE2TM.O4, wherein RE stands 
for the rare earths (lanthanides) and TM stands for 
the so-called transition metals. It is a characteristic 
of the present invention that in the compounds in 

75 question the RE portion is partially substituted by 
one member of the alkaline earth group of metals, 
or by a combination of the members of this alkaline 
earth group, and that the oxygen content is at a 
deficit. 

20 For example, one such compound that meets 
the description given above is lanthanum copper 
oxide La^uO* in which the lanthanum -which be- 
longs to the IIIB group of elements-is in part substi- 
tuted by one member of the neighboring HA group 

25 of elements, viz. by one of the alkaline earth metals 
(or by a combination of the members of the IIA 
group), e.g., by barium. Also, the oxygen content of 
the compound is incomplete such that the com- 
pound will have the general composition La 2 . 

30 x Ba x CuOd. y . wherein x £ 0.3 and y < 0.5. 

Another example for a compound meeting the 
general formula given above is lanthanum nickel 
oxide wherein the lanthanum is partially substituted 
by strontium, yielding the general formula Lag. 

35 xSr x Ni0 4 .y . Still another example is cerium nickel 
oxide wherein the cerium is partially substituted by 
calcium, resulting in Ce2. x Ca x Ni04. y . 

The following description will mainly refer to 
barium as a partial replacement for the lanthanum 

40 in a LasCuOi compound because it is the Ba-La- 
Cu-0 system which is, at least at present, the best 
understood system of ail possible. Some com- 
pounds of the Ba-La-Cu-0 system have been de- 
scribed by C. Michel and B. Raveau in Rev. Chim. 

45 Min. 21 (1984) 407. and by C. Michel. L. Er-Rakho 
and B. Raveau in Mat. Res. Bull., Vol. 20. (1985) 
667-671 . They did. however, not find nor try to find, 
superconductivity. 

Experiments conducted in connection with the 

50 present invention have revealed that high-T c super- 
conductivity is present in compounds where the 
rare earth is partially replaced by any one or more 
of the other members of the same MA group of 
elements, i.e. the other alkaline earth metals. Ac- 



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tually, the T c of LazCuO^y with Sr 2 is higher and is 
superconductivity-induced diamagnetism larger 
than that found with Ba 2 and Ca 2 . 

As a matter of fact, only a small number of 
oxides is known to exhibit superconductivity, 
among them the Li-Ti-0 system with onsets of 
superconductivity as high as 13,7 K, as reported by 
D.C. Johnston. H. Prakash, W.H. Zachariasen and 
R. Visvanathan in Mat. Res. Bull. 8 (1973) 777. 
Other known superconductive oxides include Nb- 
doped SrTi03 and BaPbi. x Bi x 03 , reported respec- 
tively by A. Baratoff and G. Binnig in Physics 108B 
(1981) 1335, and by A.W. Sleight, J.L Gillson and 
F.E. Bierstedt in Solid State Commun. 17 (1975) 
27. 

The X-ray analysis conducted by Johnston et 
al. revealed the presence in their Li-Ti-0 system of 
three different crystal lographic phases, one of 
them, with a spinel structure, showing the high 
critical temperature. The Ba-La-Cu-0 system, too, 
exhibits a number of crystallographic phases, 
namely with mixed-valent copper constituents 
which have itinerant electronic states between non- 
Jahn-Teller Cu 3 and Jahn-Teller Cu 2 ions. 

This applies likewise to systems where nickel 
is used in place of copper, with Ni 3 being the 
Jahn-Teller constituent, and Ni 2 being the non- 
Jahn-Teller constituent. 

The existence of Jahn-Teller polarons is con- 
ducting crystals was postulated theoretically by 
K.H. Hoeck, H. Nickisch and H. Thomas in Helv. 
Phys. Acta 56 (1983) 237. Polarons have large 
electron-phonon interactions and. therefore, are fa- 
vorable to the occurrance of superconductivity at 
high critical temperatures. 

Generally, the Ba-La-Cu-0 system, when sub- 
jected to X-ray analysis reveales three individual 
crystallographic phases, viz. 

- a first layer-type perovskite-like phase, related to 
the K2MF4 structure, with the general composition 
La 2 . x Ba x Cu04. y . with X<*1 and y*0; 

- a second, non-conducting CuO phase; and 

- a third, nearly cubic perovskite phase of the 
general composition Lai. x Ba x Cu0 3 ^ which appears 
to be independent of the exact starting composi- 
tion, 

as has been reported in the paper by J.G. 8ednorz 
and K.A. Muller in Z, Phys. B • Condensed Matter 
64 (1986) 189-193. Of these three phases the first 
one appears to be responsible for the high-T c 
superconductivity, the critical temperature showing 
a dependence on the barium concentration in that 
phase. Obviously, the Ba 2 substitution causes a 
mixed-valent state of Cu 2 and Cu 3 to preserve 
charge neutrality. It is assumed that the oxygen 
deficiency, y, is the same in the doped and un- 
doped crystallites. 

Both LazCuOi and LaCu03 are metallic conduc- 



tors at high temperatures in the absence of barium. 
Actually, both are metals like LaNi03. Despite their 
metallic character, the Ba-La-Cu-0 type materials 
are ceramics, as are the other compounds of the 

5 REzTM.O* type, and their manufacture more or less 
follows the known principles of ceramic fabrication. 
The preparation of a Ba-La-Cu-0 compound, for 
example, in accordance with the present invention 
typically involves the following manufacturing 

10 steps: 

• Preparing aqueous solutions of the respective 
nitrates of barium, lanthanum and copper and 
coprecipitation therof in their appropriate ratios. 

- Adding the coprecipitate to oxalic acid and for- 
/5 ming an intimate mixture of the respective oxalates. 

- Decomposing the precipitate and causing a solid- 
state reaction by heating the precipitate to a tem- 
perature between 500 and 1200°C for one to eight 
hours. 

20 - Pressing the resulting product at a pressure of 
about 4 kbar to form pellets. 

- Re-heating the pellets to a temperature between 
500 and 900°C for one half to three hours for 
sintering. 

25 It will be evident to those skilled in the art that 
if the partial substitution of the lanthanum by stron- 
tium or calcium is desired, the particular nitrate 
thereof will have to be used in place of the barium 
nitrate of the example described above. Also, if the 

30 copper of this example is to be replaced by an- 
other transition metal, the nitrate thereof will obvi- 
ously have to be employed. 

Experiments have shown that the partial con- 
tents of the individual compounds in the starting 

35 composition play an important role in the formation 
of the phases present in the final product. While, as 
mentioned above, the final Ba-La-Cu-0 system ob- 
tained generally contains the said three phases, 
with the second phase being present only to a very 

40 small amount, the partial substitution of lanthanum 
by strontium or calcium (and perhaps beryllium) 
will result in only one phase existing in the final 
La 2 - x Sr x Cu04. y or La 2 .xCa x Cu04.y. respectively, pro- 
vided x < 0.3. 

45 With a ratio of 1:1 for the respective (Ba. La) 
and Cu contents, one may expect the said three 
phases to occur in the final product. Setting aside 
the said second phase, i.e. the CuO phase, whose 
amount is negligible, the relative volume amounts 

50 of the other two phases are dependent on the 
barium contents in the La 2 . x BaxCu04. y complex. At 
the 1:1 ratio and with an x = 0.02, the onset of a 
localization transition is observed, i.e.. the resistiv- 
ity increases with decreasing temperature, and 

55 there is no superconductivity. 

With x = 0.1 at the same 1:1 ratio, there is a 
resistivity drop at the very high critical temperature 
Of 35 K. 



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With a (Ba.la) versus Cu ratio of 2:1 in the 
starting composition, the composition of the 
LaaCuOi:Ba phase, which was assumed to be re- 
sponsible for the serconductivity, is imitated, with 
the result that now only two phases are present, 
the CuO phase not existing. With a barium content 
of x = 0.15, the resistivity drop occurs at T c = 26 
K. 

The method for preparing the Ba-La-Cu-0 
complex involves two heat treatments for the 
precipitate at an elevated temperature for several 
hours. In the experiments carried out in connection 
with the present invention it was found that best 
results were obtained at 900°C for a decomposition 
and reaction period of 5 hours, and again at 900°C 
for a sintering period of one hour. These values 
apply to a ratio 1:1 composition as well as to a 2:1 
composition. 

For the ratio 2:1 composition, a somewhat 
higher temperature is permissible owing to the 
melting point of the composition being higher in the 
absence of excess copper oxide. Yet it is not 
possible by high-temperature treatment to obtain a 
one-phase compound. 

Measurements of the dc conductivity were con- 
ducted between 300 and 4.2 K. For barium-doped 
samples, for example, with x < 0.3, at current 
densities of 0.5 A/cm 2 , a high-temperature metallic 
behavior with an increase in resistivity at low tem- 
peratures was found. At still lower temperatures, a 
sharp drop in resistivity (>90%) occurred which for 
higher current densities became partially sup- 
pressed. This characteristic drop was studied as a 
function of the annealing conditions, i.e. tempera- 
ture and oxygen partial pressure. For samples an- 
nealed in air, the transition from itinerant to lo- 
calized behavior was not found to be very pro- 
nounced, annealing in a slightly reducing atmo- 
sphere, however, led to an increase in resistivity 
and a more pronounced localization effect At the 
same time, the onset of the resistivity drop was 
shifted towards higher values of the critical tem- 
perature. Longer annealing times, however, com- 
pletely destroy the superconductivity. 

Cooling the samples from room temperature, 
the resistivity data first show a metal-like decrease. 
At low temperatures, a change to an increase oc- 
curs in the case of Ca compounds and for the Ba- 
substituted samples. This increase is followed by a 
resistivity drop, showing the onset of superconduc- 
tivity at 22±2 K and 33i2 K for the Ca and Ba 
compounds, respectively. In the Sr compound, the 
resistivity remains metallic down to the resistivity 
drop at 40±1 K. The presence of localization ef- 
fects, however, depends strongly on alkaline-earth 
ion concentration and sample preparation, that is to 
say, annealing conditions and also on the density 
which have to be optimized. All samples with low 



concentrations of Ca. Sr, and Ba show a strong 
tendency to localization before the resistivity drop 
occur. 

Apparently, the onset of the superconductivity. 

5 i.e the value of the critical temperature T c , is de- 
pendent, among other parameters, on the oxygen 
content of the final compound. It seems that a 
certain oxygen deficiency is necessary for the ma- 
terial to have a high-T c behavior. In accordance 

10 with the present invention, the method described 
above for making the La^uO^Ba complex is com- 
plemented by an annealing step during which the 
oxygen content of the final product can be ad- 
justed. Of course, what was said in connection with 

is the formation of the La2Cu04:Ba compound, like- 
wise applies to other compounds of the general 
formula REaTM.O^AE, such as, e.g. NdaNiO*:Sr. 

In the cases where a heat treatment for de- 
composition and reaction and/or for sintering was 

20 performed at a relatively low temperature, i.e. at no 
more than 950°C, the final product is subjected to 
an annealing step at about 900°C for about one 
hour in a reducing atmosphere. It is assumed that 
the net effect of this annealing step is a removal of 

25 oxygen atoms from certain locations in the matrix 
of the RE2TM.04 complex, thus creating a distortion 
in its crystalline structure. The O2 partial pressure 
for annealing in this case may be between 10 1 
and 10 5 bar. 

30 In those cases where a relatively high tempera- 
ture (i.e. above 950°C) was employed for the heat 
treatment, it might be advantageous to perform the 
annealing step in a slightly oxidizing atmosphere. 
This would make up for an assumed exaggerated 

35 removal of oxygen atoms from the system owing to 
the high temperature and resulting in a too severe 
distortion of the system's crystalline structure. 

Resistivity and susceptibility measurements, as 
a function of temperature, of Sr 2 and Ca 2 -doped 

40 U&CuO^y ceramics show the same general ten- 
dency as the Ba 2 -doped samples: A drop in re- 
sistivity p(T), and a crossover to diamagnetism at a 
slightly lower temperature. The samples containing 
Sr 2 actually yielded a higher onset than those 

45 containing Ba 2 and Ca 2 . Furthermore, the dia- 
magnetic susceptibility is about three times as 
large as for the Ba samples. As the ionic radius of 
Sr 2 nearly matches the one of La 3 . it seems that 
the size effect does not cause the occurrence of 

50 superconductivity. On the contrary, it is rather ad- 
verse, as the data on Ba 2 and Ca 2 indicate. 

The highest T c 's for each of the dopant ions 
investigated occur for those concentrations where, 
at room temperature, the Re 2 .xTM x 0 4 .y structure is 

55 close to the orthorhombic-tetragonal structural 
phase transition which may be related to the sub- 
stantial electron-phonon interaction enhanced by 
the substitution. The alkaline-earth substitution of 



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the rare earth metal is clearly important, and quite 
likely creates TM ions with no e g Jahn-Teller or- 
bitals. Therefore, the absence of these J.-T. or- 
bitals, that is. J.-T. holes near the Fermi energy 
probably plays an important role for the T c en- 
hancement. 



Claims 

1) Superconductive compound of the RE2TM.O4 
type having a transition temperature above 28 K, 
wherein the rare earth (RE) is partially substituted 
by one or more members of the alkaline earth 
groups of elements (AE). and wherein the oxygen 
content is adjusted such that the resulting crystal 
structure is distorted and comprises a phase of the 
general composition RE2.xAExTM.O4-y . wherein TM 
represents a transition metal, and x < 0.3 and y < 
0.5. 

2) Compound in accordance with claim 1, 
wherein the rare earth (RE) is lanthanum and the 
transition metal (TM) is copper. 

3) Compound in accordance with claim 1, 
wherein the rare earth is cerium and the transition 
metal is nickel. 

4) Compound in accordance with claim 1, 
wherein the rare earth is lanthanum and the transi- 
tion metal is nickel. 

5) Compound in accordance with claim t, 
wherein barium is used as a partial substitute for 
the rare earth, with x < 0.3 and 0.1 5 y 3 0.5. 

6) Compound in accordance with claim t, 
wherein calcium is used as a partial substitute for 
the rare earth, with x < 0.3 and 0.1 £ y £ 0.5. 

7) Compound in accordance with claim 1, 
wherein strontium is used as a partial substitute for 
the rare earth, with x < 0.3 and 0.1 £ y £ 0.5. 

8) Compound in accordance with claim 1, 
wherein the rare earth is lanthanum and the transi- 
tion metal is chromium. 

9) Compound is accordance with claim 1, 
wherein the rare earth is neodymium and the tran- 
sition metal is copper. 

10) Method for making superconductive com- 
pounds of the RE2TM.O4 type, with RE being a rare 
earth, TM being a transition metal, the compounds 
having a transition temperature above 26 K t com- 
prising the steps of: 

- preparing aqueous solutions of the nitrates of the 
rare earth and transition metal constituents and of 
one or more of the alkaline earth metals and 
coprecipitation thereof in their appropriate ratios; 

- adding the coprecipitate to oxalic acid and for- 
ming an intimate mixture of the respective oxalates: 

- decomposing the precipitate and causing a solid- 
state reaction by heating the precipitate to a tem- 
perature between 500 and 1200°C for a period of 



time between one and eight hours: 

• allowing the resultant powder product to cool: 

- pressing the powder at a pressure of between 2 
and 10 kbar to form pellets; 

5 - re-adjusting the temperature of the pellets to a 
value between 500 and 1000°C for a period of time 
between one half and three hours for sintering; 

- subjecting the pellets to an additional annealing 
treatment at a temperature between 500 and 

10 1200°C for a period of time between one half and 
5 hours in a protected atmosphere permitting the 
adjustment of the oxygen content of the final prod- 
uct which has a final composition of the form RE 2 . 
xTM.04.y, wherein x < 0.3 and 0.1 < y < 0.5. 

is 11) Method in accordance with claim 10. 
wherein the protected atmosphere is pure oxygen. 

12) Method in accordance with claim 10. 
wherein the protected atmosphere is a reducing 
atmosphere with an oxygen partial pressure be- 

20 tween 10 1 and 10 s bar. 

13) Method in accordance with claim 10, 
wherein the decomposition step is performed at a 
temperature of 900 9 C for 5 hours, and wherein the 
annealing step is performed at a temperature of 

25 900 °C for one hour in a reducing atmosphere with 
an oxygen partial pressure between 10 1 and 10 s 
bar. 

14) Method in accordance with claim 10, 
wherein lanthanum is used as the rare earth and 

30 copper is used as the transition metal, and wherein 
barium is used to partially substitute for the lan- 
thanum, with x < 0.2. wherein the decomposition 
step is performed at a temperature of 900°C for 5 
hours, and wherein the annealing step is performed 

35 in a reducing atmosphere with an oxygen partial 
pressure on the order of 10 3 bar and at a tem- 
perature of 900 °C for one hour. 

15) Method in accordance with claim 10, 
wherein lanthanum is used as the rare earth and 

40 nickel is used as the transition metal, and wherein 
barium is used to partially substitute for the lan- 
thanum, with x < 0.2, wherein the decomposition 
step is performed at a temperature of 900°C for 5 
hours, and wherein the annealing step is performed 

45 in a reducing atmosphere with an oxygen partial 
pressure on the order of 10 3 bar and at a tem- 
perature of 900 °C for one hour. 

16) Method in accordance with claim 10. 
wherein lanthanum is used as the rare earth and 

50 copper is used as the transition metal, and wherein 
calcium is used to partially substitute for the lan- 
thanum, with x < 0.2, wherein the decomposition 
step is performed at a temperature of 900 °C for 5 
hours, and wherein the annealing step is performed 

55 in a reducing atmosphere with an oxygen partial 
pressure on the order of 10 3 bar and at a tem- 
perature of 900 °C for one hour. 



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17) Method in accordance with claim 10. 
wherein lanthanum is used as the rare earth and 
copper is used as the transition metal, and wherein 
strontium is used to partially substitute for the 
lanthanum, with x < 0.2, wherein the decomposition s 
step is performed at a temperature of 900°C for 5 
hours, and wherein the annealing step is performed 

in a reducing atmosphere with an oxygen partial 
pressure on the order of 10 3 bar and at a tem- 
perature of 900°C for one hour. io 

18) Method in accordance with claim 10, 
wherein cerium is used as the rare earth and nickel 
is used as the transition metal, and wherein barium 
is used to partially substitute for the cerium, with x 

< 0.2. wherein the decomposition step is per- rs 
formed at a temperature of 900°C for 5 hours, and 
wherein the annealing step is performed in a re- 
ducing atmosphere with an oxygen partial pressure 
on the order of 10 3 bar and at a temperature of 
900°C for one hour. so 



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