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MATURE VOL 336 17 NOVEMBER 1988 



ARTICLES 



211 



Superconductivity near 70 K in a new family of 
layered copper oxides 

R. J. Cava, B. Batlogg, J. J. Krajewski, L. W. Rupp, L. F. Schneemeyer, T. Siegrist, 
R. B. vanDover, P. Marsh, W. F. Peck, Jr, P. K. Gallagher, S. H. Glarum, J. H. Marshall, 
R. C Farrow, J. V. Waszczak, R. Hull & P. Trevor 

AT&T Bell Laboratories, Murray Hill, New Jersey 07974, USA 



A new family of high-temperature superconductors is described, with the general formula Pb 2 Sr 2 ACu l 0^s : Although they 
have the planes ofCuO s square pyramids characteristic of the other copper-oxide superconductors, the new compounds 
belong to a distinct structural series, with wide scope for elemental substitution Their unusual electronic configuration also 
gives new insight into the role of charge distribution among the structural building blocks in controlling superconductivity. 



Since the first observation 1 of high-transition-temperature 
(high-7" c ) superconductivity in La-Ba-Cu-O, progress in the 
understanding of this remarkable phenomenon has been cou- 
pled to the discovery of new materials. Until now, three families 
of copper-oxide-based high-T c superconductors have been 
identified, based on (La,M) 2 Cu0 4l LnBa 2 Cu 3 0 7> and 
(Tl ( Bi) in (Ba,Sr) 2 Ca n+I Cu M 0 (M+2ll+2 (ref. 2). (Here M represents 
a metal cation that may substitute on some La sites, and Ln 
represents a lanthanide.) Here we report the discovery of a new 
family of planar copper-oxide superconductors with general 
formula Pb 2 Sr 2 ACu 3 O fl+a (where A is a lanthanide or a mixture 
of Lfl+Sr or Ca), and describe the synthesis, crystal structure 
and properties of prototype compounds. We find, for example, 
that one preliminary optimal composition Fb 2 Sr 2 Yo.sCao.5Cu 3 08 
has a superconducting T c of 68 K. The new family displays the 
same kind of rich substitutional chemistry as is observed for 
LnBa 2 Cu 3 0 7 , with the phase forming for Y and at least La, Pr, 
Nd. Sm, Eu, Gd, Dy, Ho, Tm, Yb and Lu, spanning the entire 
rare-earth series. Wide ranges of large-metal-atom solid solution 
and oxygen stoichiometry are observed, suggesting many poss- 
ible avenues to be explored for the optimization of supercon- 
ducting properties. 

Superconductivity is induced in the host compounds 
PbjSrjLnCusOg+a (5«0) either by partial substitution of a 
divalent ion (such as Sr or Ca) on the lanthanide site, or possibly 
by the accommodation of excess oxygen (8 > 0), or a combina- 
tion of both. The compounds can be synthesized only under 
mildly reducing conditions, which are necessary to maintain Pb 
in a 2 + oxidation state. Oxidation of 5 = 0 compounds is poss- 
ible, but only at low temperatures, where decomposition to a 
PWtv)-containing perovskite is sluggish. Remarkably, the for- 
mal average oxidation state of copper in the superconductors 
it less than 2 + , but a clear structural distinction between 
different types of copper layers leads us to hypothesize that 
holes are nonetheless present on electronically active CuO 
pyramidal planes. 

Synthesis 

The preparative conditions for the new materials are consider* 
ably more stringent than for the previously known copper-based 
superconductors. Direct synthesis of members of this family by 
reaction of the component metal oxides or carbonates in air or 
oxygen at temperatures below 900 °C is not possible because of 
the stability of the oxidized SrPbOj-based perovskite. Successful 
lynthesis is accomplished by the reaction of PbO with pre- 
reacted (Sr/Ca, Ln) oxide precursors. The precursors are pre- 
pared from oxides and carbonates in the appropriate metal 
ratios, calcined for 16 hours (in dense A1 2 0 3 crucibles) at 920- 
930 *C in air with one intermediate grinding. Some of the 



Pb 2 Sr 2 LnCu 3 0 8+a compounds can be prepared in air from 
PbO+LnS^CusOje precursor mixtures, which are not reacted 
at temperatures below ~850°C. For example, single-phase 
Pb 2 Sr 2 YCuj0 8 -j-« (6 = 0) can be prepared by reacting PbO with 
YSr 2 Cu 3 O x at 920 °C for 1 h, followed by quenching. Slower 
cooling results in partial decomposition through oxidation. 
Short reaction times are generally sufficient to obtain single- 
phase products. The same air-heating/quenching process does 
not appear to work, however, for Pb2Sr 2 LaCu 3 08 +fl or 
Pb 2 Sr 2 LuCu 3 O g+ a. 

The best synthetic conditions found so far involve the reaction 
of PbO with the cuprate precursors in thoroughly mixed pressed 
pellets. Reaction temperatures are between 860 and 925 °C, for 
times between 1-16 h, in a flowing gas stream of 1% 0 2 in N 2 , 
a mildly reducing atmosphere. For Fb 2 Sr 2 Y,-. je Ca JC Cu308 + 3> for 
example, single-phase materials are obtained for 0^x<0.5 in 
1% 0 2 after heating overnight at 865 °C and cooling in the gas 
stream to room temperature in 15 minutes. Using higher tem- 
peratues, higher p Ql in the gas stream er higher Ca contents of 
the starting mixture results in the intergrowth of 123-type 
YSr 2 (Pb,Cu)jO x with the new compound, or the formation of 
an SrPb0 3 -based second phase. Similar procedures are success- 
ful for other Sr/rare-earth/Ca combinations. The oxygen con- 
tents of Pb2Sr 2 Y 1 - x Ca x Cu 3 0 8+s for 0 £ x =s 0.50, prepared under 
these conditions, are measured by reduction in H 2 and are 
uniformly 5 = 0±0.1. Ca is employed as a dopant on the Ln 
site because it has an ionic size similar to the intermediate 
rare-earths. We have not yet found synthetic conditions under 
which Pb^Srj+xLnt-xCusOg+a solid solutions can be prepared 
as single-phase polycrystalline samples that are good bulk super- 
conductors, although superconducting single crystals of that 
stoichiometry have been prepared. 

Single crystals of the superconducting compounds were grown 
from PbO- and CuO-rich melts using a similar precursor tech- 
nique. Melt compositions were generally Pb^SrjYC^Ox. Fol- 
lowing a 30-min soak at 1,025 °C, samples were cooled at 2°C 
min" 1 in the 1% 0 2 atmosphere to temperatures between 800 
and 400 *C f and were then rapidly cooled to room temperature 
in the same gas stream. Crystals are plate-like in habit, but are 
generally more equiaxed than those of LnBa 2 Cuj0 7 . 

Stoichiometry and crystal structure 

Compounds of stoichiometry Pb 2 Sr 2 LnCu s 08 (5 = 0) are not 
bulk superconductors, although we often observe small amounts 
of superconductivity (1% or less) in materials of that 
stoichiometry prepared either by the quench or by the l%-0 2 
synthetic techniques. The non-bulk superconductivity may be 
due to inhomogeneities in either oxygen content or Sr/Ln distri- 
bution* _ • 

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fig.l Two representations of the crystal struc- 
ture for the new superconducting compounds, 
for the case of Fb J Sr 2 . M Nd 0 . w Cu,O, + j. Rep- 
resentation o emphasizes the Cu-O and Po-O 
bonding scheme, and representation b empha- 
sizes the manner in which Cu-0 and Pb-O 
coordination polyhedra are arranged. 





The range of oxygen contents possible for these compounds 
is remarkaW Pb,Ir 2 YCu 3 0 8+ ., 6=0, £ «^ f «* 
oxidized by heating in 0 2 to temperatures below 500 C forshort 
times (2-4 h) to 8 values of -1.6, retaining the same baste crystal 
structure. We have observed values as large as 
PhSrY .Ca««Cu»0«+«. Oxidation at temperatures higher 
5^£^E£££ reaction periods, generally results in 
imposition to the SrPbO r based perovskite. Powder samp es 
of Pb 2 Sr,YCu,0 8+ 9 with large values of S are not superconduct- 
ive cry'stal/of the Pb^r^Ln.Cu^, type are , «pj 
conductors with transition temperatures ^»0«dJK. 
These crystals may have non-zero values of 6 but have not yet 
ieeTfSly characterized. The range of TcS observed suggest a 
complex and interesting relationship between T c , S and the 

Sr po n w?e t r X-ray diffraction indicates that the new phases have 
an o^horhombic unit cell which is based on a ^many- ayer 
perovskite structure. The characteristic X-ray patten, _for the 
prototype compound Pb^YCujO, is presented in Table 1. 
The compound deviates only slightly from tetragonal symmetry. 
The 3«t cell consistent with the X-ray pattern is c-centred, 
with lattice parameters a - 5.40, b - 5.43, and c « 15.74 A. Sys- 
Smaic absence, are consistent with a c-centred cell dowr .to 
ftedete<toMlityUmitof 1% maxim^^ 
bic celt gives an excellent fit to the powder difiracUon pattern 
£tt flint of a shoulder on the high 20 side of the 314 reflection 
indicates that the true symmetry may be weakly monoclimc. 
Although the lattice parameters for this family of compounds 
are very similar to those reported for TlBajCajCujOg (ref. 3), 
the crystal structures are quite different Electron microscope 
investigations indicate that for some crystals, weak (but sharp) 
reflections are present which violate the c-centnng. Furthermore, 
these studies show the presence of long-period, long-range- 
otdered superlattices in the a~b plane, suggesting that avanety 
Strurtural distortions andstoichiometry-dnven atom-ordering 
schemes can occur. 



The crystal structure of compounds in this family, detennined 
for a superconducting Nd-based single crystal °f approx,maU 
toiSetry Pb^Nd^O,*, (determined by structure 
refinement) is shown in Kg. 1. The crystal employed in *e 
structural determination was twinned, as expected from the 
pSSetragonal symmetry. The atomic coordinates «e_repor- 
ted in the centred orthorhombic cell to be consistent With the 
Swder oata, but a primitive cell with a and b rotated by 45 
Jnd reduced by -fl gives an equally good description of the 
sinale-crystal data. The very small scattering cross-sect.on of 
omen Sudes determination of 8 by refinement. The data 
are well fitted by the structural model (refinement parameter 
R = 3 7%) but a microscopic explanation of the orthorhomb.c 
svmmetry is not apparent; if the origin is primarily in the oxygen 
SS we wouldnotbe able to detect it in theX-ray structure 

de ThTS°of the structure comprises infinite planes of corner- 
shared Cu0 5 pyramids separated by eight-coordinate ni^eatth 
££. as arecommon to all the presently known coppertased 
superconductors with T C >50K. The four m-plane copper- 
oxygen distances are -1.9 A, and the distance to the apica 
oxygen is -2.3 A, both of which are very similar to tho e 
observed in YBa,Cu,0,. The structural components unique to 
£ Z Tclaks of materials are the PbO-Cu0 4 -PbO planesshowj 
n the centre of the Fig. 1. For 6 -0, Pb has a distorted flattened 
square pyramid coordination (sharing edges with adjacent 
S with the lone pair pointing toward the vacant six* 
ikeTf the coordination octahedron, The PbO s py^" 0 ' 8 * 
separated by a single copper layer, which, for « -0. »; oxygen- 
freV, wd dbplays an O-Cu-0 coordination characteristic of 
C? 1 (Cu-0 distance -1.8 A), as is observed in non-super*,* 

ducting YBa^O,. During ^^^^^ a \!Z 
cess oxygen is apparently accommodated in this copper layer. 
r^ulSralargeexpanion of the c axis.Tne PbOs and CuO, 
pmmidal planes are joined by the common oxygens at th«r 
apices. The Sr atoms are coordinated.to nine oxygens, as in (La, 

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t 1 — — i 1 r 




Temperature (K> 

Fig. 2 Magnetization data (d.c. field-cooled at 25 Oe) for 
Pb 2 Sr 2 Y 0 , 3 Cao.5Cu3O g . 

Sr)Cu0 4t and the Ln site is eight-coordinate, as in the 
LnBa 2 Cu 3 0 7 family, sandwiched between the Cu0 5 pyra- 
midal planes. In the superconducting compound 
Pb 2 Sr 2 y 1 _ A Ca^Cu 3 08 + fl, Ca partially substitutes for Y in the 
eight-coordinate site. 

The crystal, structures of all the known copper-oxide-based 
superconductors are generally described as many-layered perov- 
skites. The similarities and differences among them are most 
easily illustrated in terms of the sucking sequences of rocksalt- 
like (AO) and perovskite-like (B0 2 ) layers 2 . Taking, for example, 
representatives from- the superconductor families that have 
double CuO-i pyramidal layers, the stacking sequences are: 

Pb 2 Sr 2 (Y, Ca)Cu 3 0 8+a 

-(Y, Ca)-Cu0 2 -SrO-Pb0-Cu0 a -Pb0-SrO-CuO 2 -(Y, Ca)- 
Tl 2 Ba 2 CaCu 2 0 8 

-Ca-Cu0 2 -BaO-T10-TIO-.BaO-Cu0 2 -Ca- 
YBa 2 Cu 3 0 6+6 

-Y-Cu0 2 -BaO-CuO s -BaO-CuQ 2 -Y- 



Table V Characteristic X-ray powder diffraction pattern for 
Pb 2 Sr 2 YCujO a 



hki 


d 


///o 


hki. 


d 




001 


15.74 


7 


116 


2.164 


n 


002 


7.87 


3 


025 


2.057 


12 


003 


5.25 


2 


205 


2.050 


10 


004 


3.94 


10 


008 


1.967 


7 


no 


3.831 


11 


220 


1.915 


25 


in 


3.722 


24 


118,009 


1.750 


2 


112 


3.444 


1 


027,207 


1.730 


1 


005 


3.148 


U 


224 


1.722 


2 


113 


3.094 


11 


130 


1.717 


2 


U4 


2.745 


100 


310, 131 


1.708 


3 


020 


2.717 


43 


311 


1.699 


2 


200 


2.701 


43 


225 


1.636 


3 


021 


2.677 


7 


133 


1.632 


3 


201 


2.662 


7 


313 


1.625 


1 


006 


2.623 


6 


028 


1.593 


11 


023 


2.412 


1 


208,119 


1.591 


n 


203 


2.401 


1 


134 


1.574 


18 


024 


2.236 


2 


314 


1.568 


14 


204 


2.227 


I 









Cu Ktr radiation, 0-60° 20 c-centred orthorhombic cell, preliminary 
indexing, true symmetry may be weakly monoclinic. Lattice parameters 
0 = 5.4019(15), 6 = 5.4333(15), c = 15.7388(33). . 



The new superconductors, then, can be seen to be intimately 
related in structure to those previously described. They can be 
considered as related to Tl 2 Ba 2 CaCu 3 0 8 by insertion of a single 
CuO a layer between adjacent polarizable AO layers, or related 
to YBa 2 Cu 3 0 6+s by sandwiching of the Cu0 6 *chain' layer by 
two PbO layers. We believe that it is the electronic screening of 
the Cu0 2 planes from the CuO s layers by the PbO layers that 
makes the new superconductors of considerable interest. Fur- 
thermore, we expect these materials to be even more anisotropic 
in their physical properties than those previously known, as 
the double pyramidal Cu0 2 -A-Cu0 2 layers are widely 
separated. 

Superconducting properties 

We have studied the composition dependence of the supercon- 
ducting properties of compounds in the series 
PbjS^Yj-^Ca^CujOg for 0^x^0.75, by estimating the flux 
expulsion measured on cooling in a field of 25 Oe in a d.c. 
SQUID magnetometer (S.H.E. model 905). The greatest flux 
expulsion occurs for x = 0.5, and is —20% of the ideal value 
(see Fig. 2). Because flux becomes trapped in the pores of these 
low-density ceramics, this is an underestimate of the true volume 
fraction of superconductivity; For x & 0.5, the materials were 
not entirely single-phase, with one or more impurity peaks 
having a maximum intensity of 5% of the strongest peak in the 
powder X-ray pattern. This, coupled with the estimate of the 
volume fraction of superconductivity, suggests that the optimal 
superconducting composition may have x somewhat greater 
than 0.5. This could be achieved if different synthetic methods 
can be found that allow a larger range of solid solution to be 
attained. We have measured the normal-state susceptibility (in 
a 20-kOe field) for temperatures below 400 K of apparently 
single-phase samples (no unindexed X-ray lines to 0.5% 
maximum intensity) of the non-superconducting endmember 
Pb 2 Sr 2 YCu 3 0 8 and superconducting Pb^SrVWsCaojTsCuaOs. 
The susceptibility of the superconductor ix) is essentially tem- 
perature independent (*»lxl0~ 4 e.m.u. per mole formula 
unit), with only a slight decrease at low temperatures. This 
temperature dependence is similar to that of high-quality 
YBa 2 Cu 5 0 7 , and is characterized by the absence of a Curie- 
Weiss cpntribution. Furthermore, this supports our conclusion 
that the copper atoms between the PbO layers are Cu ,+ . Post- 
oxidation at 500 °C results in oxidation of this copper to mag- 
netic Cu 2 \ Pb 2 Sr 2 YCu 3 08 appears to be magnetic (-0.5 u,B per 
Cu atom), but further studies are necessary to clarify whether 
this is intrinsic or is due to the presence of highly magnetic 
impurity phases that are undetectable by X-ray diffraction. 

Figure 3 shows the temperature dependence of the resistivity 
for a single crystal of Pb 2 Sr 2 Dy l -. < Ca x CuA+»- The midpoint 
of the superconducting transition is at 51.5 K (indicated by an 
arrow in Fig. 3), although there is a small foot which gives 
a zero-resistance T c of 46 K. Above T c the temperature 



Table 2 Crystallographic data for Pb 2 Sr I ^ 4 Nd 0 . 76 Cu30 8+s 



Atom 


Position 


X 


y 


t 




Pb 


41 


1/2 


0 


0.38858 (4) 


1.09 (2) 


Sr 


4k 


0 


0 


0.22184(9) 


0.74 (4) 


Nd, Sr* 


2a 


0 


0 


0 


0.69 (3) 


Cul 


2d 


0 


0 


1/2 


0.86 (9) 


Cu2 


41 


1/2 


0 


0,11074(13) 


0.46 (5) 


01 


41 


1/2 


0 


.0.2546(8) 


liS (5) 


02 


4k 


0 


0 


0.384(3) 


13(5) 


03 


8m 


1/4 


1/4 


0.0995(5) 


0.9(3) 



Orthorhombic cell (pseudotetragonal substructure); a = 5.435(l)A, 
6»5,463(1)A, c = 15.817(3)A; space group Cmmm, *=2; observed 
reflections 707, 0.037. 

* Mixed occupancy site: 0.24(1) Sr, 0.76(1) Nd. 



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100 200 
Temperature [K] 



300 



dependence is fairly linear, but near T » "g B ^ 

positive curvature which, along with the resistivity foot, we 
attribute to small inhomogeneities in the metal and/ or oxygen 
distribution. The scale of the resistivity is a factor of ten greater 
than for previous oxide superconductors. . It is not yet clear 
whether this is an intrinsic property. 

A typical resistivity curve for a ceramics sample is shown in 
the inset to Fig. 3, illustrating the typically broad transitions 
observed. The transition in this sample begins at 79 K (arrow) 
but zero resistance is achieved (within instrumental accuracy) 
as 32 K. Note that the resistivity scale is again quite high. We 
attribute the breadth of the transition and the negative normal- 
state temperature coefficient to inhomogeneity in the metal 
and/or oxygen distribution, rather than to exogenous phases at 
the grain boundaries. The behaviour of this system seems to be 
very similar to that of (La,Sr) 2 Cu0 4 (ref. 4). 

Electronic aspects 

Given that the average formal copper valence of previously 
known superconductors has always been greater than +2, the 
new superconductors are unique and, at first sight, anomalous. 
For the series Pb 2 Sr 2 Y l - x Ca JC Cu 3 0 8t the average formal copper 
valence increases from 1.67 in the non-superconducting x-0 
member to -1.92 at the maximum Ca concentration studied. 
At our current estimate of the optimal superconducting composi- 
tion (x = 0.5), the average formal valence is 1.83. The linear 
coordination of the copper atom sandwiched between the PbO 
sheets, characteristic of Cu l \ and the probable electronic isola- 
tion of this layer from the conducting CuO pyramidal planes, 
imply that the formal charge formulation becomes 
Fb 2 Sr 2 YCu l+ Cur0 8 in the non-superconducting compound. 
When Ca is substituted for Y, we propose that holes are 
accommodated only in the Cu0 5 planes, and at the x = 0.5 
stoichiometry the formal charge formulation becomes 
Pb 2 Sr 2 Y 0 5 Cao jCu^Cu^Og, which is consistent with the cur- 



Ffe 3 Resistivity in the a-b plane as a function of temperature 
for a single crystal of Pb 2 Sr 2 (Dy,Ca)Cu 3 0 8+a . Inset, typical tern- 
oerature^ependent resistivity for a polycrystalHne sample of 
V Pb 2 Sr 2 (Y,Ca)Cu 3 0 B . 



rent assumption for previously known high-T c materials that 
holes are present in the Cu0 5 pyramidal planes. 

For Pb 2 Sr 2 ACu 3 0 8+a compounds with 5>0. excess oxygen 
must be accommodated near the Cu l+ planes, and a more 
complex hole-doping scheme may be operating. We expect thai 
in that case the compound does not respond in a simple fashion 
to the change in charge through doping of a ngid band; tne 
oxygen inserted in the bonding neighbourhood of the reduced 
Cu and Pb ions may create the electronic states in which the 
charge is partly or fully accommodated. 

This new family of compounds has a unique crystal fracture, 
vet it also reflects a concept common to all copper-oxide-based 
superconductors. By now it is well established that superconduc- 
tivity is associated with layers of Cu-0 octahedra, pyramids 
and squares. The remaining structural building blocks are seen 
as the electron acceptors which induce the holes necessary Tor 
superconductivity in the Cu-0 layers. For YBa 2 Cu 3 0 6+ a. for 
example, we have shown in detail how the CuO a chains act as 
charge reservoirs, and how superconductivity depends on charge 
transfer between chains and planes . 

To illustrate the concept of local charge distribution, one may 
rewrite the formulae of the high-T c copper-oxide superconduc- 
tors as follows: YBa 2 Cu 3 0 6 [CuO a ]; Sr 2 CaCu 2 0 6 [Bt 3 Oj]; 
Ba a aCu 2 OJTlA]; Sr 2 (Y,Ca)Cu 2 0 6 [Pb 2 Cu0 2+a ]; where the 
structural components in square brackets act as reservoirs which 
control the charge on the superconducting Cu-0 planes. The 
PbO-Cu0 5 -FbO reservoir layer is likely to be exceptionally 
flexible in accommodation of charge, and we therefore expect 
that a relationship between T c and oxygen stoichiometry as 
unusual as that for YBa 2 Cu 3 0 6+a will eventually be observed. 
The wide ranges of metal-atom and oxygen-atom stoichiometrics 
in this new family of superconductors are of considerable inter- 
est, and warrant further study with the aim of understanding 
and optimizing the superconducting properties. ^ 
We thank D. W. Murphy and K. Rabe for helpful discussions. 



Received 21 October, accepted 28 October 1988. 

1, Pcdnora, -i.'-G. Sc. Muller, K. A. Z Phys. B64, 189-193 (1986), 

2. Santoro, A., Beech, F.. Marexio, M. & Cava, R. I Pbysica C (in the prea). 
31 Parkin, S, S, P. ef o£ Phys. 1 Rn, Uffc 61, 750-753 (1988). . 



4. Van Dover, R. B,, Cm, R. J., fratlou. & & Rkunan, E. A. Phys. JUu B35, 51*7- SM» 

(1987). 

5. Cava, R. J. tt at Physica C (in the preu). 



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