Poole Ch.P., Jr. Handbook of Superconductivity

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296

Chapter 8: Crystal Structures of High-To Superconducting Cuprates

internal layers have monocapped square antiprismatic coordination (8ap 1 c, 4212,

(Ti,Gd, Ca)4Baz(Gd, Ca)Cu2012). In (Ti,Sm, Ca)sBaz(Ca, Sm)Cu2014 (5212), where

the number of additional layers is five, AO 2 layers alternating with AO layers, the

cations from the AO layers are 12-fold coordinated (cuboctahedron, 12co),

yielding a local perovskite-like atom arrangement.

b. Interatomic Distances

Displacements of the atoms from their ideal positions distort the coordination

polyhedra, and the interatomic distances from the cations to the surrounding

oxygen atoms exhibit a wide range of values.

The atoms in the conducting DO 2 layers are tightly bonded, and the in-

plane Cu-O distances vary little from one superconducting cuprate to another.

Usual distances range from 1.92 to 1.94 A, the shortest distance (1.89 A) being

reported for 0201 (La, Sr)2CuO4) and the longest one (1.98A) for 1201

(C(Ba, Sr)2CuOs.05) and 0011 ((Sr, La)CuO2). This relatively rigid atom layer

determines the values of the cell parameters a and b, a slight puckering being

possible. When there are two or more DO 2 layers, the copper atoms from the

outside DO 2 layers are slightly moved from the center of the square base toward

the center of the pyramid. The distances from the copper atoms to the oxygen

atoms located in the BO layers, that is, the apical atoms of the square pyramid or

octahedron, are considerably longer (2.10-2.82 A), in agreement with the Jahn-

Teller effect. The large variations observed for this Cu-O distance depend on the

size of the B atom and the chemical nature of the A cations. For structures with

bridging SrO layers, the distances from the copper atoms to the apical oxygen

atoms are in general shorter than for structures containing BaO layers. When

additional layers are present, the same apical oxygen atoms are usually tightly

bonded to the A cations, situated on the other side of the BO layer. For example,

in structures of Hg-based compounds, because of the bonding between the

oxygen atoms from the BO layers and the mercury atoms (and because of the

presence of barium atoms in the BO layers), the apical Cu-O distances are very

long (2.70-2.82 A).

For structures with only one separating C layer between two consecutive

DO 2 layers, the tetragonal prisms around the C atoms are almost regular. The

C-O interatomic distances range from 2.39 to 2.61 A. In general, yttrium atoms

have shorter C-O distances than calcium atoms. For structures with two or more

C layers between consecutive DO 2 layers, the distances from the C atom to the

oxygen atoms in the O2 layers are slightly shorter (2.26-2.39 A).

For distorted perovskite-type structures (one BO layer) the interatomic

distances from the B atoms to the vertices of the surrounding cuboctahedra range

from 2.50 to 3.01 A. For structures with two bridging BO layers, where only half

of the cuboctahedron (square antiprism) remains, there are four distances of

similar length to the oxygen atoms in the DO 2 layer (2.58-2.97 A, depending on

Chemical Families

297

the B cation) and four distances of similar length to the oxygen atoms within the

BO layer (2.73-2.88 A). The distances to the capping oxygen atoms in the

neighboring bridging or additional layer vary from 2.28 to 2.37 A and from 2.61

to 3.19 A, respectively.

The A-O interatomic distances, like the coordination polyhedra of the A

atoms, depend on the chemical nature of the cation. As in the previous cases, they

are of the same magnitude as those commonly observed for the corresponding

cations (e.g., Hg 2+, Cu 2+, C 4+, Bi 3+, Pb 2+, T13+) in other oxides. The distances

between the mercury and oxygen atoms along the stacking direction vary from

1.92 to 2.01 A. The corresponding distances for copper atoms in the same 2-fold

linear coordination range from 1.81 to 1.89 A. Copper atoms in 4-fold coordina-

tion have two distances of 1.94 A to oxygen atoms within the AO' layer and two

shorter distances (~ 1.84 A) to oxygen atoms from neighboring AO' and/or BO

layers. Carbon atoms usually show three similar A-O distances of 1.23-1.29 A.

The displacement of the atoms in the AO layers containing Pb are such that the

square pyramids are strongly distorted with three short distances (including one to

an oxygen atom in the neighboring BO layer) of 2.14-2.48 A and two long

distances of 2.95-3.23 A. The resulting coordination polyhedron can be described

as a tetrahedron with one of the vertices occupied by a lone electron pair (~-

tetrahedron). In a similar way, the ideal octahedra around thallium and bismuth

atoms are transformed into tetrahedra and ~p-tetrahedra, respectively. The intera-

tomic distances from thallium atoms to the two oxygen atoms in the neighboring

AO and/or BO layers vary from 1.82 to 2.20 A, whereas the distances to the four

oxygen atoms within AO layer range from 2.30 to 2.56 (two short) and from 2.77

to 3.16 A (two long distances). Bismuth atoms are not bonded to the oxygen

atoms in the neighboring AO (BiO) layer, the distance to the oxygen atoms in BO

layer being 1.96-2.08 A. Within the AO layer there are usually two short (2.02-

2.61 A) and two long interatomic distances, or three short (2.10-2.70 A) and one

long distance.

Chemical Families

Chemical families of high-T c superconducting cuprates are usually denoted by

the cation in the additional layers. This allows one to emphasize structural

features (including distortions) common to a particular group of compounds.

Cuprates containing halogens and so-called ladder compounds will also be briefly

discussed. References are specified for first reports, whereas other references for

compounds mentioned here can be found with the data sets in Section H,b.

Compounds with structures containing four and more additional layers and

mainly metal atoms, such as Ti, on the A sites are known, but so far no

superconductivity has been reported.

298

Chapter 8: Crystal Structures of High-Tc Superconducting Cuprates

a. Rare-Earth-Alkaline-Earth Cuprates

The structures of the compounds considered in this section contain no

additional

layers, whereas rate-earth-alkaline-earth superconducting cuprates with A or A O'

layers are treated in the next section.

The first 0101 cuprate, LaCuO3, was prepared by Demazeau

et al.

(1972) at

high pressure. It crystallizes with a trigonal structure based on an R Bravais

lattice, a distorted variant of the cubic perovskite-type structure. Copper is in the

oxidation state Cu 3+ and the compound is not superconducting. Decreasing the

oxygen content produces vacancies in the CuO2 layers. Other members of the

structure series with the general formula

BCmDnO2m+3

(pervoskite is the first

member with n = 1 and m = 0), reported so far, were prepared with a mixture of

Cu and Fe or Co on the D site, none of them being superconducting.

The structure series with the general formula

B2Cn_lCUnO2n+2

includes

structures with two

bridging

layers in the stacking unit.

La2CuO4

(Longo and

Raccah, 1973; Grande

et al.,

1977), LaSrCuO4 (Goodenough

et al.,

1973), and

La2_xBaxCuO4 (x--0-0.2) (Michel and Raveau, 1984) crystallize with a body-

centered tetragonal K2NiF4-type structure (Balz, 1953) or with an orthorhombi-

cally distorted variant (n = 1, 0201). The discovery of superconductivity in the

La-Ba-Cu-O system (T c = 30 K) by Bednorz and Mfiller (1986) started the era

of high-temperature superconductivity. Superconductivity for 0201 compounds

can be achieved not only by partial substitution of trivalent La by a divalent

alkaline-earth element, but also by insertion of additional oxygen. At low

temperature and/or for a low content of alkaline-earth element, the crystal

structures of these compounds are distorted (primitive tetragonal or orthorhom-

bic), while at high temperature and/or for a higher Ba(Sr) content the structures

remain of the K2NiF4 type.

The second member of the structure series (n = 2, 0212) was first prepared

by Nguyen

et al.

(1980) for

La2_xMl+xCu206_6 (M ~

Sr or Ca, x = 0-0.14).

Superconductivity is known only for Ca-containing compounds and was discov-

ered by Cava

et al. (1990a)

for Lal.6Sr0.4CaCu206 (T c -- 60 K).

Members with n = 3 (0223) and n = 4 (0234), with superconducting

transition temperatures of about 90 and 70 K for the nominal compositions

Sro.6Ca0.33CuO2.1o

and

Sro.65Cao.3CuO2.10

(synthesized at high pressure), respec-

tively, were first reported by Adachi

et al.

(1993). Later on it was shown that 0223

compounds may also be prepared without calcium.

A superconducting (Tr = 28 K) 0222 compound was reported by Akimitsu

et al.

(1988) in the Nd-Sr-Ce-Cu-O system (nominal composition

Nd2Sro.sCeo.sCUl.2Oy).

In contrast to the structures belonging to the series

mentioned above, where the CuO2 layers are separated by a single C layer

with mainly alkaline-earth metal atoms, here the CuO2 layers are separated by

double C layers with La or rare-earth metal atoms. A 0232 compound with three

separating

layers between consecutive DO 2 layers has also been reported.

G. Chemical Families 299

An infinite-layer compound (0011) was first prepared by Siegrist

et al.

(1988b) by stabilizing calcium cuprate by Sr

(Ca0.86Sro.14CuO2) ,

whereas the first

superconductor, Sro.s6Ndo.14CuO 2 (T c --40K),

was reported by Smith

et al.

(1991). A nonsuperconducting 0021 compound, Nd2CuO4, was prepared by

Miiller-Buschbaum (and Wollschl~iger, 1975), superconductivity (T c = 24K)

being reported by Tokura

et al.

(1989b) for the partly substituted derivative

Nd1.84Ce0.16Cu03.93

9

b. Ba-Y Cuprates

Superconductivity in the Ba-Y-Cu-O system was discovered by Wu

et al.

(1987)

for Ba0.8Yl.zCuO4_~, the first compound that showed superconductivity above

the temperature of liquid nitrogen and later identified as BazYCu307_ ~ (1212)

(T c = 93 K for 6 = 0.07). For 6 = 0-0.6, the compound crystallizes with an

orthorhombic structure. At 6 = 0.6, the structure becomes tetragonal and super-

conductivity is suppressed. On decreasing the oxygen content, the arrangement of

the oxygen atoms in the

additional

CuO layer becomes disordered. The square

planar coordination of the copper atoms (chains of corner-linked CuO4 squares)

is transformed to defect octahedral (random vacancies in the basal plane) and then

reduced to 2-fold linear.

Another compound in the Ba-Y-Cu-O system, BazYCu408 (2212), was

identified as planar defects in BazYCu307_ ~ by Zandbergen

et al.

(1988b) and

was prepared as bulk material (T c = 81 K) by Karpinski

et al.

(1988b). The 1212

and 2212 compounds can be described by the general formula CUkBazYCUzOk+ 6

(k = 1 and 2, respectively). In the structure of the latter, edge-linked CuO4

squares form zigzag chains and, in contrast to the 1212 compounds, the oxygen

content is fixed. A third superconducting compound in the same system,

Ba4YzCu7015_ ~ (T c = 92K), was observed by Zandbergen

et al.

(1988b) and

further studied by Karpinski

et al.

(1988a). The structure consists of an inter-

growth of 1212 and 2212 stacking units. Cu-1222 compounds with a mixture of

rare-earth elements (one of which is Ce) in the

separating

layers have also been

reported.

c. Bi-Based Cuprates

A superconducting Bi-based material was found for the first time in the Bi-Sr-

Cu-O system by Michel

et al.

(1987). A sample of nominal composition

BizSr2Cu207+ 6 was

reported to have a transition temperature of up to 22 K,

the superconducting compound being later identified as 2201. Maeda

et al.

(1988) obtained a critical temperature above 105K in the Bi-Sr-Ca-Cu-O

system for the nominal composition BiSrCaCuzOy. The structures of Bi-

based superconducting cuprates form a series with the general formula

300

Chapter 8: Crystal Structures of High-T~ Superconducting Cuprates

Bi2B2Cn_lCUnO2n+4

with mainly Sr and Ca on the B and C sites, respectively.

The 2201 (n = 1) and 2212 (n = 2) compounds have considerable homogeneity

ranges, and the 2223 (n = 3) compound is generally prepared with a partial

substitution of Bi by Pb. The superconducting transition temperature increases

with an increasing number of conducting CuO2 layers up to 110 K (2223). Bi-

2222, which is not a member of this structure series, has also been reported.

In all structures of Bi-based superconductors, the bismuth and oxygen

atoms in the additional BiO layers are displaced from the ideal positions on the 4-

fold rotation axes, along [1 1 0] of the tetragonal cell. The displacements occur

in a progressive manner, resulting in the formation of voids large enough to

accommodate extra oxygen atoms and incommensurate modulations. The trans-

lation period of the modulation depends on partial substitutions and on the

oxygen content, but for particular compositions the structures can be conveni-

ently described in large supercells. The Bi sites have ff-tetrahedral coordination,

typical for Bi 3+, the lone electron pairs being located between consecutive BiO

layers that are hence only weakly bonded. The asymmetry of the BiO layer is

probably the reason that phases with a single "pure" BiO layer are not known. A

1232 compound, containing a mixture of Bi and Cu in the additional AO layer

and three separating C layers, has also been reported.

d. TI-and Ga-Based Cuprates

The first Tl-based superconducting material was reported in the T1-Ba-Cu-O

system by Kondoh et al. (1988). A critical temperature of 19 K was measured for

a sample of nominal composition Tll.zBao.sCuOy. Independently, Sheng and

Hermann (1988a) reported critical temperatures up to 90 K for the nominal

compositions TlzBazCu308+~, T1BaCu305.5+6, and T11.5BazCu307.3+6, the super-

conducting compound being later identified as 2201. A short time later, the same

authors (Sheng and Hermann, 1988b) succeeded in preparing superconductors

with T c = 120 K in the T1-Ba-Ca-Cu-O system for the nominal compositions

T12BaCal.5Cu308.5+ ~ and Tll.86BaCaCu307.8+ ~. At present, Tl-based cuprates

constitute one of the largest chemical families of high-T c superconductors,

forming two distinct structure series with the general formulas

T1B2Cn_lCUnO2n+3

and

T12B2Cn_lCUnO2n+4 ,

respectively. The B atoms in

compounds with single additional T10 layers are usually Ba and/or Sr (T1

being partly substituted by Pb or Bi in Sr-containing cuprates), whereas Sr-

containing compounds with two T10 layers have not been reported so far. For

both structure series, the C sites are mainly occupied by Ca, with small amounts

of T1. With an increasing number of conducting CuO2 layers, the superconducting

transition temperature progressively increases, reaching a maximum value for

1223, 1234 (~ 120K) and 2223 (125K), and then decreases. Apart from the

compounds that are members of these two structure series, other compounds,

such as 1222 and 2222, have also been reported.

G. Chemical Families 301

A common feature of the structures of Tl-based superconductors is the

displacement of the thallium and oxygen atoms in the

additional

T10 layers from

the ideal positions on the 4-fold rotation axes. The resulting tetrahedral coordina-

tion of the thallium atoms is typical for T13+. Vacancies may occur on the O sites

in the

additional

layers.

Ga-based compounds were first reported by Vaughey

et al.

(1991)

(GaLaSrCuOs, 1201) and Roth

et al.

(1991) (Gao.97Sr2YCu207, 1212), whereas

the first superconductor (T c = 35 K), a 1212 compound (GaSrzEr0.6Cao.4Cu207),

was prepared by Cava

et al.

(1991). The structures of the Ga-based cuprates

known so far contain a single

additional

GaO layer and are described by the

general formula

GaB2Cn_lCUnOzn+3.

The B sites are occupied by Sr (or a mixture

of Sr and La), superconductivity occurring when Ca is present on the C sites. As

in the corresponding structures of Tl-based cuprates, the A cations are surrounded

by oxygen atoms forming tetrahedra; however, the GaO4 tetrahedra are arranged

in chains along [1 1 0] of the tetragonal cell, lowering the symmetry to

orthorhombic.

e. Pb-Based Cuprates

Bi- and Tl-based superconductors are often prepared with a partial substitution of

Bi and T1 by Pb. In the latter case, the substitution can reach 50 at.%. Structures

containing only Pb on the A sites in the

additional

layers are not known so far;

however, a large number of compounds where Pb is the majority element on the A

site are known and form a distinct family of Pb-based superconducting cuprates.

Superconductivity was discovered in the Pb-Sr-Y-Ca-Cu-O system by Cava

al.

(1988), a critical temperature of 68 K being measured for a sample of

nominal composition Pb2Sr2Y0.sCa0.sCu308 (3212). For structures with a

single

separating C

layer between consecutive DO 2 layers, the series with one,

two, or three

additional

layers are described by the general formulas

(Pb,M)kB2Cn_lCunOk+2n+2

(k = 1, 2 or 3). The majority of the compounds

contain tfivalent cations (e.g., y3+) in the

separating C

layers and become

superconducting when part of them are substituted by divalent cations (Ca2+).

The structures with two

separating C

layers between the DO 2 layers can be

grouped into a series with the general formula

(Pb,M)kB2C2Cu2Ok+ 8.

Note that

the general formulas given here consider AO layers but the total number of

oxygen atoms in the

additional

layers may differ from k.

In most Pb-based cuprates, copper atoms are also present in the

additional

layers, whereas for compounds with one

additional AO

layer, mixtures of lead

(Pb 4+) and other elements (alkaline-earth, Sc, Cd) on the A site have also been

reported. In compounds with three

additional

layers, the lead and copper atoms

are always ordered, giving the stacking sequence PbO-Cu-PbO (oxidation states

Pb 2+ and Cu+). Ordering can also occur in compounds with two

additional

layers.

302

Chapter 8: Crystal Structures of High-T~ Superconducting Cuprates

f. Hg-Based Cuprates

Superconductivity for Hg-based materials was discovered in the Hg-Ba-Cu-O

system (HgBazCuO4.10, 1201) by Putilin

et al.

(1993a), the critical temperature

reaching 94 K. Later, Schilling

et al.

(1993) reported two high-T c superconduct-

ing compounds in the Hg-Ba-Ca-Cu-O system, HgBazCaCu206+ 6 (1212) and

HgBazCazCu308+ 6 (1223). The critical temperature of 133 K, attributed to Hg-

1223, is the highest value reported so far at ambient pressure and can be increased

to 157K by applying high pressure (23.5 GPa). Hg-based superconducting

cuprates form a series with the general formula

HgBzCn_lCUnO2n+2+6

(Fig.

8.12) with mainly Ba and Ca on the B and C sites, respectively. On increasing the

number of CuO2 layers, T c progressively increases, reaching a maximum value

for 1223, and then decreases, as can be seen from Fig. 8.13.

For the members of this series, an extra O site in the

additional

layer is

partly occupied and vacancies have often been reported also for the Hg site. The

superconducting transition temperature depends on the oxygen content, each

compound having an optimal value for 5, which increases with the number of

conducting

CuO2 layers. Apart from the compounds that are members of the

series mentioned earlier, a 2212 compound has also been reported.

g. C-Based Cuprates

The first cuprate containing carbonate groups (oxycarbonate), CSr2CuO5 (1201),

was reported by von Schnering

et al.

(see M/iller-Buschbaum, 1989), whereas

superconductivity (T c - 26 K) was discovered by Kinoshita and Yamada (1992a)

Fig. 8.12.

34 l

,~ 22

~) 18

..,

r 14

r,.)

I I I I .... I

,X)"

CO.,,,"

.,. s ~

,,0"

.2)"

3.84t-- " " "0 .... -0-----0-----0---- -0 .... --0 ....

3.80 ~ I I J I I I

1 2 3 4 5 6 7 8

n in HgBa2Ca .]Cu O2,+2+~

Cell parameters vs number of

CuO 2

layers for Hg-based cuprates (Scott

et al.,

1994).

G. Chemical Families

303

Fig. 8.13.

140

130

120

110

100

I I I I I I

/ %

0 /

I %

, 'o

i I ~

I ~,

' b.

'%.

80 I I I I I _. !

2 3 4 5 6 7 8

n in HgBa2Can.lCUnO2.+2+~

Superconducting transition temperature vs number of CuO2 layers for Hg-based cuprates

(T c < 90 K for n = 8" Scott

et al.,

1994).

in the C-Ba-Sr-Cu-O system

(C0.9Bal.lSr0.9CUl.lO4.9+6).

Carbon-based super-

conducting cuprates with a single

additional A

layer form a series with the

general formula

(C,M)B2Cn_lCUnO2n+3.

In most compounds carbon is mixed

with copper; however, partial substitutions of C by B, or even N, have also been

reported. The oxygen atoms in the

additional AO"

layer are displaced from the

ideal positions on the cell edges (tetragonal cell) toward the carbon atoms to

achieve triangular coordination of the carbon site. Depending on the C/Cu cation

ratio in the

additional

layer and the orientation of the CO3 triangles, different

superstructures have been observed. Apart from the compounds with a single

additional

layer, superconducting oxycarbonates (T c =91-113K) with two

carbon-containing layers in the stacking unit have been reported (Kawashima

al.,

1994b,c). In the proposed structural models the (C,Cu)-containing layers are

separated by a single BaO layer which is here also considered as an

additional

layer, these compounds thus having the four-digit codes 3223, 3234, and 3245.

Boron-, phosphorus-, and sulfur-based superconducting cuprates also form

structure series with a single

additional

layer. The phosphorus and sulfur atoms

are 4-fold coordinated, located at the center of tetrahedra formed by oxygen

atoms.

h. Cuprates with Halogens

During the past few years a number of compounds with partial substitution

of oxygen by halogen (fluorine or chlorine), as well as compounds with

"interstitial" halogen atoms (bromine or iodine), have been reported. The

304

Chapter 8: Crystal Structures of High-Tc Superconducting Cuprates

majority of these compounds were prepared by high-pressure synthesis (4-6 GPa)

giving only small amount of the final product, generally insufficient for a

complete structure determination. In the present work, crystallographic para-

meters for such compounds are given only when the classification has been

deduced.

Substitution of oxygen has been intensively studied for cuprates without

additional layers, such as 0201 and 0212. Substitution of 02- by F- (Cl-) takes

place in the bridging BO layers, making it possible to decrease the average

oxidation state of the B cation and in some cases significantly increasing the

critical temperature [e.g., Cal.9Nao.lCuO2C12, T c = 26 K (Argyriou et aL, 1995)

and

Sr2.3Ca0.TCU204.76Cll.24 , r c =

80K (Jin et aL, 1995)]. In the case where

complete substitution has taken place in the bridging layers, an extra layer of

halogen atoms can be inserted between consecutive BO layers, without changing

the relative shifts of these, leading to a local fluorite-type atom arrangement

[Sr2CuO2F2.57 , T c =

46K (A1-Mamouri et aL, 1994) and

Sr2gaCu204.6F2.0,

T c = 99 K (Kawashima et aL, 1994a)].

According to some reports, bromine and iodine atoms can be incorporated

into basic structures, forming an extra additional layer [(Bi2I)Sr2Ca2Cu3Oy

(3223), T c = 100 K (Xiang et al., 1991)].

i. Ladder Compounds

Compounds with the general formula

Srn_lCUn+lO2n

(n = 3, 5, 7 ..... oo) were

first reported by Hiroi et al. (1991). They are generally referred to as ladder

compounds and can only be prepared at high pressure. Their structures contain

CUn+lO2n

layers separated by charge-compensating layers of strontium atoms

(Fig. 8.14). In the former, zigzag chains of edge-linked CuO4 squares share

comers with m chains of corner-linked CuO4 squares, leading to an alternative

general formula:

(m + 1)Sr +

Cu203

-~-

CuO 2

Srm+lCUm+202m+3

(m = 0, 1,2 ..... oo).

In the limiting case where m is infinitely large, square-mesh DO 2 layers are

formed and the infinite-layer compound (0011) is obtained.

Superconductivity at high pressure (T c = 12 K at 3 GPa) has been observed

by Uehara et al. (1996) for

Ca13.6Sro.4Cuz4041.s4,

which crystallizes with an

orthorhombic, (SrgCa6)Cu24041 (McCarron et aL, 1988) or (Casga6)Cu24041

(Siegrist et aL, 1988a) type structure. In this structure Cu203 layers, similar to

those found in the ladder compounds (m = 0), alternate with a new kind of CuO2

layer, consisting of single straight chains of edge-linked CuO4 squares. The

squares of the Cu203 layer are rotated by 45 ~ with respect to those of the CuO2

layer and the translation periods approximately coincide for a ratio of multiples

H. Crystallographic Data Sets 305

Fig. 8.14.

Ladder compounds

Srm+lCUm+202m+3

with layers built up of zigzag chains of edge-linked

CuO4 squares (shaded) and m chains of corner-linked CuO4 squares running along [1 0 0].

Large shaded circles represent Sr atoms located between the layers. The Pearson code is given.

close to ~/2/2 (e.g., 7/10). As for the ladder compounds, the Cu-containing

layers are separated by a layer of cations (C):

7 C + 7

Cu203

-~- 7 C + 10

CuO 2 ~ C14Cu24041.

Crystallographic Data Sets

Next are given complete crystallographic data sets for 69 structures, followed by

information on some 351 related compounds. Crystallographic data from studies

on single crystals and powders by X-ray, neutron diffraction, and transmission

electron microscopy, as well as hypothetical models, have been considered.

a. Conventions Used

Because of the difficulties in growing single crystals suitable for diffraction

studies, the majority of the structural investigations reported in the literature

on high-Tr superconductors were performed on powdered samples. Neutron