perovskite stacking in the structures of the high temperature cuprate superconductors

Pergamon

Applied Superconductivity Vol. 3, No. 1-3, pp. 73-83, 1995 Copyright 0 1995 Elsevier Science Ltd

0964-1807(95)00035-6 Printed in Great Britain. All rights reserved 0964-1807195 $9.50 + 0.00

PEROVSKITE STACKING IN THE STRUCTURES OF THE HIGH TEMPERATURE CUPRATE SUPERCONDUCTORS

CHAN PARK and ROBERT L. SNYDER

Institute for Ceramic Superconductivity, New York State College of Ceramics at Alfred University, Alfred, NY 14802, U.S.A.

Abstract-Most of the known cuprate superconductors belong structurally to one family and are closely related to each other. Ignoring odd structural details and considering the idealized structures only, most of them adopt a Ruddlesden-Popper (R-P) or an oxygen-deficient R-P structure with or without intergrowth layers which are composed of either a rock-salt type atomic arrangement, chains, bare cations, or a mixture of them. The rock-salt layers found in R-P structures are simply the result of the stacking of perovskite unit cells. Sometimes a fluorite type structural unit can be found, but the structures containing fluorite blocks may also be considered to be of the R-P structure type with minor readjustment of anions in the rock-salt layers to stabilize the structure. A general formula can be used to describe the basic structures of all the cuprate superconductors, even those having fluorite units.

1. INTRODUCTION

The crystal structures of the known high temperature copper-oxide superconductors have been classified in terms of well-known structural types such as those of K2NiF4 or Aurivillius phases. They, however, have many things in common in addition to the presence of CuOZ sheets, the presence of which is strongly believed to be indispensable to high temperature superconductivity.

The purpose of the present article is to review the crystal structures of high temperature cuprate superconductors with an emphasis on the inter-relation of the structures of the high T, phases. There are considerable differences in the crystallographic details of these structures such as the buckling of CuOd octahedra, cation site disorder, oxygen vacancies, intercalation of more than two phases, and nonstoichiometry. Here the similarities of the structures are highlighted rather than the differences. For this purpose only the idealized structures are treated. We start with simple perovskite and the infinite layer (ACu02) structure which is nothing but the oxygen- deficient perovskite with oxygens removed in the A cation plane (usually the alkaline earth atom). Stacking these structural units results in a Ruddlesden-Popper structure, which can be regarded as the parent structure of most of the known cuprate superconductors. A general formula which can describe the basic atomic arrangement of these superconducting phases is proposed and the inter- relations of the structures are described.

2. RUDDLESDEN-POPPER STRUCTURE

There are two ways to stack perovskite unit cells (Fig. la). One is by putting a second cell on top of the first, so that two unit cells share a single (A-O) face shown as X in Fig. lb. The (A-O) face shared by two perovskite units has the atomic arrangement of the rock-salt (RS) structure and will be referred to as an RS plane. The other way is to put the second cell on top of the first without sharing a common plane. The second cell undergoes a a/2 + b/2 displacement relative to the first (Y in Fig. 1 b) in order to minimize electrostatic repulsion. This configuration leads to two consecutive A-O faces displaced in a manner identical to the rock salt structure. This structural feature will be called an RS block in this paper. When perovskite units are stacked in the second way, it produces the so called “Ruddlesden-Popper” (R-P) structure series [ 1, 21.

In R-P structures which can be formulated as An+lBnX3n+l or better as (AX)(ABX3)*, the unit cell consists of two identical halves (each having n perovskite units) which are related to each other by a translation vector of $(a + b) (Y in Fig. lb). K2NiF4 [l] (Fig. 2a) and Sr3Ti207 [2] (Fig. 2b) are typical examples of the n = 1 and n = 2 cases. The perovskite itself can be described as

73

74 CHAN PARK and ROBERT L. SNYDER

0 a

J Ruddlesden-Popper

Rock-Salt Plane

Fig. 1. (a) Perovskite structure. (b) Stacking of perovskite unit cells.

the n = 00 structure of R-P series (Fig. 2~). It is well-known that La,CuO, adopts the structure of K2NiF4. The structure of LazCu04 is usually viewed as made of alternating layers having the perovskite (LaCu03) and rock-salt (Lao) structure, which is implied in the formula (Lao) (LaCu03). In case of n = 2 of the series (Sr3TiZ07), two perovskite unit cells together are stacked with the i $ 0 translation between them. The stacking sequence of (RS-P-P)-(RS-P-P)- may be thought of as (SrO)(SrTi03)2. The full R-P series may be viewed as an accordion-like sequence with n perovskite units stacked between RS end caps. The RS layers do not represent additional atoms, they are just the outcome of the stacking of perovskite unit cells with the & f 0 translation between them.

When the total cation valence is lowered by either substitution or replacement, oxygen vacancies are formed to compensate for the charge difference. The oxygen vacancies can be randomly distributed but are usually ordered. In the cuprate superconductors having an oxygen- deficient R-P structure, the coordination of the Cu changes into square planar or square pyramidal from octahedral coordination (Figs 2 and 3). For the n = 1 case, when Sr replaces La in La&u04 to make Sr2Cu03 (Fig. 3a), two oxygens must be removed in each unit cell (there are two formula units in one unit cell), which changes Cu coordination from 6 to 4 (from octahedral to square planar). The same phenomena can be seen in the n = 2 case (Fig. 3b) in which the octahedral configuration becomes pyramidal as oxygen content decreases. In the extreme case of n = 00 (Fig. 3c), the lower oxygen content caused by a smaller positive charge changes the coordination

Perovskite stacking in cuprate HTSCs 75

Ruddlesden-Popper Series A”+lB”XB”+l or wwwrl

K,NiF, La,CuO,

( 1 a

- --

n=2 Sr3Ti,0,

w

888888 n ==

PEROVSKITE

( 1 C

Fig. 2. Ruddlesden-Popper structures. (a) n = 1, (b) n = 2 and (c) n = co.

of copper from 6 to 4 producing square planer sheets. Oxygen atoms are missing along a single plane changing the coordination of A atoms from 12 to 8. The oxygen-deficient n = cc R-P structure is nothing but bare CuOz sheets made of corner-sharing Cu04 squares separated by A atom (usually an alkaline earth) layers without oxygen and these bare Cu02 sheets are what is believed to be indispensable to the presence of high temperature superconductivity. The Cu02 sheet which is sandwiched by two A layers is called an “infinite layer” (IL), and this oxygen-deficient n = 00 R-P structure is called an IL structure. The stacking sequence is -Cu02-(A)-Cu02-.

3. INFINITE LAYER SUPERCONDUCTING COMPOUNDS

Beginning with the Siegrist et al. [3] report of the first structure of an IL compound, various compounds having the IL structure (ACu02) have been reported to superconduct at up to 110 K [4-l 11. These compounds have the simplest possible structure which contains the CuOz sheets and may yield further insights into high temperature superconductivity because their simpleness can make the interpretation of the physical measurements unequivocal. The IL structure is the very one from which structures of all the cuprate superconductors can be derived.

4. GENERAL FORMULA OF CUPRATE SUPERCONDUCTORS

The structures of most of the high temperature superconducting cuprates (except those with T’ or T* structure, which are explained below) can be described as a combination of an R-P (or


Oxygen-deficient Ruddlesden-Popper A,BX,_6 or (AX)(ABX,..&

n=l Sr,CuO,

(a)

“infinite layer”

: . . . . . . i-) oxygen-less A layer

“infinite layer”

n ==J

ACuO, A = (Sr, Ba, Ca)

n=2 La,SrCu,O,

(W

“infinite-layer(lL)” structure

(c) Fig. 3. Oxygen deficient Ruddlesden-Popper structures. (a) n = 1, (b) n = 2 and (c) n = cm.

oxygen-deficient R-P) structure and extra layers intercalated between the building blocks of the R-P structure. The extra layers usually adopt a RS atomic arrangement. The general formula can be written as

(M”X),[(M’X)(MCuX3-a),]

or

(M”X),[(M’X)(MCuX,+,).I which can also be expressed as (M”X),[oxygen deficient R-P], where m = number of extra

layers intergrown between the R-P building blocks and n = number of CuOZ sheets (ILs). All of the HTSC cuprate phases with R-P or R-P type layers intercalated between the

perovskite blocks follow this formula and are characterized by four integers: m2(n - l)n, each of which represents the number of four different types of layers. The value of m ranges from 0 to 3, and that of II from 1 to 6 or 7.

Table 1 provides a list of the four integer short-hand notation for the superconducting compounds used in this paper and the formulas rewritten to follow the formula. In the case of well known compounds like the 123 we have kept the recognized (but incompatible) name and added one extra line beneath the usual compound name with the cations re-ordered according to the above general formula.

The first number represents the number of insulating layers (usually, but not necessarily RS layers) which can be also called counter cation layers intergrown between the R-P building


Table 1. The HTSC phases described by the four integer m2(n - 1)n notation used in this paper. M in the generalized formula includes atoms in both the interface and separating layers.

Compound Notation m2(n- 1)n Generalized formula

La-020 1 0201 [(LaOXLaCu03-6)1)

La-02 12 0212 [(LaO)(LaCu03-6)21

Y-1236 1212 (Cu)[(Bao)(MCuo3-6)21

Y-1237 1212 (Cuo)[(Bao)(Mcuo3-,)21

Y-124 2212 (CuO)2[(BaO)(MCu03-6)21

Y-247 1212-2212 a Tl-12(n - 1)n 12(n - 1)n (Tlo)[(Bao)(MCuo3-~).1 Tl-22(n - I)n m2(n - 1)n (T10)2[(BaO)(MCu03-6),1 B&22@ - 1)n m2(n - 1)n (BiO)2[(s~)(MCUo3-6)nl Hg-12(n - 1)n 12(n - 1)n (Hgo~)[(Bao)(MCuo3-6),1 Pb-3212 3212 (Cu)(PbO)z[(SrO)(MCu03-s)21)

Pb-3201 3201 (Cu)(PbO)z[(SrO)(SrCu03)1

Pb-22 12 2212 (Cu)(PbO)[(Bao)(MCuo3)21

PbCu-1212 Pb-1212

1212 1212

PbCu-1201 1201 Cu-12(n - 1)n 12(n - I)n Sr-OZ(n - 1)n 02(n - 1)n Nd-0201 -T’ 0201-T’ Nd-0201 -T* 0201-T* Pb-1201-T* 1201-T* Cu-1201-T* 1201-T* Bi-2201-T* 2201-T*

(Pbo.~Cuo.~O)[(Ba~)(M~~~~)21 (Pbo.sSro.5O)[(BaO)(MCu03)21

(Pbo,5Cuo.~O)[(SrO)(LaCuO3~6)1 (Cu)[(BaO)(MCu03~6),1 [(MO)(MCu%d$)

b

b

b

b

‘Sum of Y-1236 and Y-124. bCannot be expressed following the general formula due to the presence of fluorite-type structure unit.

blocks. These layers are composed of either an RS type atomic arrangement (Tl-12(n - l)n, Tl-22(n - l)n, Bi-22(n - l)n, Hg-12(n - I)n, Pb-1201-T*, B&2201-T*), chains (Y-1237, Y-124, Y-247), bare cations (Y-1236, Y-247), or a mixture of them (Pb-32 12, Pb-3201, Pb-2212). These insulating layers typically contain the heavy metal atoms such as Bi, Tl, Pb or Hg, and the Cu atoms which are sometimes found in these layers (Cu-12(n - l)n, Pb-3212, Pb-3201, Pb-22 12, PbCu-1212, Cu-1201-T*, Pb-1201-T*) as well as in the conducting CuOz planes.

The second number gives the number of interface layers located at the top and bottom of the perovskite block containing n CuOz layers. There are always two of these interface layers (top & bottom) per n CuOz layers, and these oxide layers typically have an RS atomic arrangement. The metals found in these layers include La, Ba and Sr.

The third number identifies the number of layers separating adjacent Cu02 sheets within the perovskite blocks. The layers correspond to the oxygenless A layers in the IL compound ACu02. The number of these layers are always one less than the number of CuOz sheets because these bare cation layers are always sandwiched by two CuOz layers. The metals commonly found in these separating layers are Ca, Sr, Y, and other rare earth elements.

Finally, the fourth number is the number of CuOz planes (ILs) within the perovskite block. The T1Ba2CaCu20, (Tl-12 12) structure, for example, can be described as m = 1, n = 2

structure containing one T10 insulating layer, two BaO interface layers, one Ca separating layer without oxygen, and two Cu02 planes. The IL structure is a special case of the m = 0 series 02 (n - 1)n where n is 00.


La,CuO, (Nd,Ce),CuO,(T’) (Nd,Ce,Sr),CuO,(T*)

( ) a (b) ( 1 C

Fig. 4. Structures of (a) La2Cu04, (b) (Nd,Ce)&u04 and (c) (Nd,Ce,Sr)2Cu04.

Some well known structures have names that do not follow the above naming scheme. For example, the YBa2Cu307 and YBa,Cu,Os are referred to by 123 and 124, respectively. These names describe the total number of cations regardless of their positions in the structures. Following the four-number naming scheme, 123 and 124 are actually 1212 and 2212. It is easier to recognize the atomic arrangement if they are written as CuBa2YCu207 and Cu2Ba2YCuzOs respectively (extra lines in the first column of Table 1). The M sites in the general formula is usually occupied by more than two cations. Therefore M which can be found in the 4th column of Table 1 indicates a mixture of corresponding cations.

For example, T1Ba2CaCu207 can be expressed as (T10)[(BaO)(BaCu03_~)(CaCuO~-~)] or (TlO)[(BaO)((Ba,Ca)CuOs_&], where M” = Tl, M’ = Ba, M = (Ba,Ca), m = 1, and n = 2. The formulas of all the high temperature cuprate superconductors can be rewritten this way (Table 1) except those with T’ and T* structures.

There are a few cuprate HTSC structures that fail to fit into the above sequence. The most common occurs in the structures with a fluorite-type block. In the case of (Nd,Ce)&u04 [12-141 and (Nd,Ce,Sr)2Cu04 [15-l 71, which have the parent structures of these, the metal atoms have the same positions as those of LazCu04 while the oxygen atom arrangements are different in all three cases (Fig. 4). Therefore if the oxygen positions are ignored, these structures can also be represented by the four-number naming scheme even if they can not be classified as R-P structures. To avoid ambiguity, the notation T’ and T* is used. T’ indicates a structure which contains CuOZ planes with Cu atoms having 4-fold oxygen coordination separated by fluorite-type layers (Fig. 4b). T* indicates a structure consisting of Cu05 pyramids with 5-coordinate Cu atoms separated by fluorite-type layers (Fig. 4~). And so, the structures of (Nd,Ce)$u04 and (Nd,Ce,Sr)&u04 can be represented as 020 1-T’ and 020 1 -T*. Those with T’ and T* structures are also included in Table 1.

5. STRUCTURES OF CUPRATE SUPERCONDUCTORS

Figure 5 shows the structures of cuprate superconductors schematically. One, two and three perovskite (or oxygen-deficient perovskite) unit cells are alternating with a/2 + b/2 displacement


u one perovskite cell

EI two perovskite cells

three perovskite cells

counter cation layers - intergrown between

perovskite cells

m=l El cl a iYi - 0 El n! s

n=2

I m=O

R-P

n n

cl 0

n=l

El El El E

nd?

A tl

El El II

n=3

m=2 R u - -

0 - III - - - u Fi- El u -

g n=2

m=3 .r 1_1 - -

n=l I I

n=2

Fig. 5. Schematic stacking sequences of all high temperature superconducting cuprate phases.

n = 1 compounds

.

La,CuO,

m=O

TIBa,CuO, m=l

TI,Ba,CuO, Pb,(SrLa),Cu,O,

m=2 m=3 Fig. 6. Structures of n = 1 compounds.


n=2 compounds

TIO

: TIO , !

ta,SrCu,O,

m=O

TI,Ba,CaCu,O,

Y Ba,Cu,O, m=2

m=l Pb,Sr,CaCu,O,

m=3

Fig. 7. Structures of n = 2 compounds.

TIO TiO

PbO cu

n=3 compounds n=4 compound

TIBa,Ca,Cu,O, m=l TI,Ba,Ca,Cu,Ol,

m=2

TIBa,Ca,Cu,O,, m=l

Fig. 8. Structures of n = 3 and n = 4 compounds.

81

L&i& n=l

Perovskite stacking in cuprate HTSCs

m=O compounds

;+J_.&&: ..; $i

La,SrCu,O,

(Sr,Ca),Cu,o,

n=2

w,w,cu,o, n=3 w,ca),cu,o,,

n=4 Fig. 9. Structures of m = 0 compounds

relative to each other in the case of m = 0 (R-P structures). As m increases from one, an extra layer is inserted in between those building blocks of R-P structure. Whether they are not displaced at all or translated by a/2 + b/2 (or b/2), is totally determined by their need to minimize electrostatic repulsion. The structures of those with fluoride blocks can also find their position in Fig. 5. When the cations in the RS block which originates from the stacking of perovskite cells (Y in Fig. lc) are replaced by smaller cations, the whole structure becomes less stable. For example, when La ions in the Laz CuO4 structure are replaced by smaller ions like Nd, the difference in ionic size makes the structure less stable. The oxygen which was in the LaZOz plane (apical oxygens of Cu-O6 octahedra) displaces to the middle of the RS block (tetrahedral sites of Nd ions) and make their own planes without cations, which results in a fluorite block (Nd-Os-Nd) in place of rock-salt block (Lao-Lao). Therefore the structures containing fluorite blocks can be said to be of R-P structure with minor readjustment of anions to stabilize the structure. Figure 5 can be a help to understand that most of the cuprate superconductors are one family in terms of their basic structure.

Figures 6-8 show the structures of n = 1, n = 2, n = 3 and n = 4 with different m values respectively. As m increases in the n = 1 case (Fig. 6), an extra insulating layer is added in between the single perovskite unit cells which are the building blocks of the y1 = 1 R-P structure. In the n = 2 compounds (Fig. 7), the extra layer is inserted in between two perovskite unit cells with oxygen deficiency. When no fluorite blocks are included in the structures, the n = 1 compounds have Cu-0 octahedra only. Therefore these structures contain ILs but no full IL blocks. It should be noted that the CuOZ sheets which are symmetrical in the c-direction are flat


m=l compounds

YBa,Cu,O,

Nd-0201 -T’

(a)

TIBqCuO, TIBa,CaCu,O,

(Eu,Ce),(Ba,Eu),Cu,O,,

(Pb,Cu)(Eu,Ce),(Sr,Eu),Cu,O,

Fig. 10. Structures of m = 1 compounds.

T’ & T* compounds

:

F

F

\. :;: Al%BmjW PbO G5is%* cue,,

Nd-0201 -T* Pb-1201 -T* Cu-1201 -T* W-2201-T”

(b) @I (d) Fig. 11. Structures of T’(a) and T*(b-e) compounds.

03


but those nonsymmetrical are puckered resulting in different Cu-0 bond lengths. A feature of n = 1, 2, and 3 groups is that all the CuO;! sheets in n = 1 group are flat, those in n = 2 are buckled, and n = 3 compounds have both flat and buckled Cu-0 bonds in the CuOz planes. In n > 3 compounds, there is essentially nothing new in the kinds of structural units. Figures 9 and 10 show the structures of m = 0 and m = 1 compounds with different n-values respectively, which shows the addition of one extra ACuOz block as n increases. Figure 11 compares the structures containing fluorite blocks. All the structures of T’ and T* can be derived from 0201-T’ or 0201-T* whose structures differ only in the oxygen positions with the LazOz rock-salt layers, and can be classified as R-P if the anion sites are ignored.

6. SUMMARY

Most of the known cuprate superconductors belong structurally to one family and are closely related to each other. They may be considered as resulting from the stacking of perovskite units with extra layers intercalated in between them. All the p-type cuprate superconductors have a Ruddlesden-Popper (R-P) or an oxygen- deficient R-P structure with or without intergrowth layers which are composed of either a rock- salt type atomic arrangement, chains, bare cations, or a mixture of them. Sometimes a fluorite type structural unit can be found in place of rock-salt layers. A general formula can be used to describe the common factors in the structures of cuprate HTSCs, even those having fluorite units.

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perovskite stacking in the structures of the high temperature cuprate superconductors

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