Shearing mechanism in the Bi-Sr-Cu oxycarbonates: HREM study of a new collapsed phase Bi,,Sr2&u,, (C03),056 Maryvonne Hervieu," Maria T. Cald6s,b Denis Pelloquin," Claude Michel," Saul Cabrerac and Bernard Raveau" "Laboratoire CRISMAT, ISMRA et Universiti de Caen, Bd du Mardchal Juin, 14050 Caen Cedex, France bInstitut de CienCa de Materials de Barcelona Campus UAB, 08193, Bellaterra, Spain 'Institute de Investigaciones Quirnicas, UMSA, La Paz, Bolivia A new oxgcarbonate, Bil~Sr,9Cu12(C03)7056, has been synthesized. It crystallizes in a monoclinic cell, u =22.139(7) A,b = 5.498(2) A, c= 39.82(1)A and p= 119.45(2)", with the possible space groups A2/m, Am or A2. Its structure, determined by an HREM investigation, is derived from that of the single intergrowth (Bi,Sr2Cu06) [Sr,Cu(CO,)O,] by a shearing mechanism along the (010) plane, so that it can be described as an assembly of [2201], [S,CC], ribbons which are m= 7 CuO, octahedra wide and shifted by 12 A with respect to each other.In fact, local substitutions exist such that the phase cannot be classified as a true shear structure, and for this reason it is referred to as 'collapsed'. The detailed microstructural study allows other collapsed members (m=8) oriented domains with chemical twins to be identified. New extended defects corresponding to the intergrowth of 2201-type ribbons in the oxycarbonate matrix along [OOl] and [?Ol], are also observed. The correlation between the shearing mechanism in this structure and the amplitude of the modulation in the single intergrowth [2201],[S2CCll compounds is discussed.Two key factors are unanimously recognized as essential for high-T, superconductivity in the layered cuprates, (AO),(A'CuO,-,),: a mixed valence for copper, which can be controlled through the synthesis process or the chemical composition, and the two-dimensional character of the struc- ture which is directly related to the existence of infinite [CuO,], layers. These factors are why every event able to modify such a state, i.e. which would occur at right angles to these layers, is of fundamental importance to the understanding of the mechanisms of superconductivity, no matter how the properties (T,and J,) evolve; creation of columnar defects by heavy ion irradiation,' intergrowths of perovskite-related slice in the tubular and shearing phenomena in layer structures are three mechanisms which induce transversal modifications of the structure.Several copper-based materials have been recently disco- vered, whose structures result from (or are likened to) the periodic arrangement of shear planes in a layered mother structure. These new structures have been termed 'collapsed structures' since some of them are not strictly 'shear structures' due to complex double translation^.^,^ In this respect, the 'collapsed' cuprates and copper oxycarbonates can be classified into two large families whose parent structures exhibit single and double rock-salt layers, respectively. The first family deals with the oxycarbonates corresponding to the general formula (A,M),(Ba,Sr),Cu,(C0,)07 with A = T1, Hg and M=V, Cr,10-12 which derive from the single intergrowth of the '1201' and Sr,Cu(CO,O,)( S2CC) represented by Tlo~5Pbo.5Sr4Cu2(C0,)07'3by the application of a shearing mechanism along (100).In such compounds, two successive [1201]l[S,CC]1 blocks are simply shifted by c/2 with respect to each other, so that they can be considered as true 'shear structures'. An important characteristic of these shear-like oxycarbonates deals with the fact that then [CuO,], layers are not interrupted by the shearing mechanism. Consequently, the shear-like (100) or (110) oxycarbonates, like their parent structures [120111 [S2CC],, remain superconducting. The second family is represented by the cuprates of general -x012formula (Bi2A2Cu06),-2( Bi4+xA4C~2 +x,2) with A =Ba, Sr.8,14,15It results from shear operations along (OlO), in the 2201 structure of Bi2Sr2Cu06 + Nevertheless, the mechanism is more complex than for the first series, due to the fact that double [Bi,O,] layers replace single [TlO] oo layers.Consequently, the copper-oxygen layers are interrupted, and a variation of composition is involved, as well as a rearrange- ment of the polyhedra at the boundary between two successive '2201' blocks so that the structures can no longer be described as pure shear structures, and are better referred to as 2201- collapsed structures. A second consequence is that these col- lapsed bismuth cuprates are not superconductors.A similar mechanism applied to the 2212 cuprate Bi,Sr,CaCu,O, allows a new collapsed phase Bi16Sr28CU1706g to be generated, resulting from a double shearing operation.' In order to understand these complex shearing mechanisms and their influence upon superconducting properties, new compounds must be generated. In this respect, nothing is yet known about the possibility of creating new phases by applying shearing mechanisms to the oxycarbonates (Bi2Sr2Cu06), [S~,CU(CO~)O,]~.'~-'~ This paper is devoted to the first 'collapsed' bismuth oxycar- bonate Bi,,Sr,gC~,2(C03)7056, which derives from the (Bi2Sr2Cu06)[ Sr2Cu(C03)02] single intergrowth by a shear- ing mechanism. Results Synthesis The previous results on the 2201-and 2212-collapsed phases8,9,14.'5 show that there exists, at the level of the shearing boundaries, a local variation of the cation distribution with regard to the 2201 and 2212 mother structures. This results from the direct connection of layers of different natures; consequently, the collapsed phases are stabilized for actual compositions which differ slightly from the ideal ones calcu- lated on the basis of ideal models; they are mainly copper deficient.Starting from the composition of the mother struc- ture, Bi2Sr4Cu,(CO3)Og, we have examined different composi- tions corresponding to the cationic ratios Bi, +3r4+,Cu2-=. The samples were synthesized from mixtures of Bi203, SrCO, and CuO, pressed in the form of bars, wrapped in a gold foil and heated at 800°C in air according to a very short thermal treatment process in order to avoid complete carbon- ate decomposition.The bars were introduced at 915"C, then J. Muter. Chem., 1996,6(2), 175-181 175 after 30 min the temperature was decreased to 800 "C at a rate of 200 "C h-I, after which the samples were quenched to room temperature. The best results were obtained for the nominal composition Bi2~,Sr4~1Cu,~7(C03)08. Nevertheless, SrCO, and the oxycarbonates (Bi2Sr2Cu06),[Sr2Cu( C0,)02], were always detected in trace amounts. XRD and ED characterisation Electron diffraction (ED) was carried out with a JEOL 200 CX electron microscope fitted with an eucentric goniometer (f60"). Energy-dispersive X-ray (EDX) analyses were system- atically performed on numerous crystals with a Kevex analyser; these allowed the nominal cationic ratios to be confirmed.The reconstruction of the reciprocal space from the ED patterns indicates a monoclinic symmetry. The [0101 electron diffraction pattern is given in Fig. 1. Note that some crystals exhibit streaks along a* which have been correlated to the existence of local intergrowth defects. The X-ray powder diffraction (XRPD) studies were per- formed with a Philips vertical goniometer, using Cu-Ka radi- ation in the range 3"<28,<70" by step scanning with increments of 28 =0.02". Lattice constants were determined using the profile refinement computer program FULLPROF.I9 On the bases of the ED results, the cell parameters were :efined from theo XRPD patttrn (Fig. 2) to a=22.139(7)A, b= 5.498( 2) A, c =39.82( 1)A and p =119.45(2)",with the possible space groups A2/rn, Am and A2.Fig. 1 [OlO] Electron diffraction pattern ~.'......l'.'"'"'r.~.--.'l.~.~~~~,.I.'..'....I.."'..'.I'..'.....4000-0N0 0-r.0 HREM study The layer stacking mode along c and the structural mechanism which originates the superstructure along a have been studied by high-resolution electron microscopy (HREM). The grains which were selected for such an investigation indeed exhibited [OlO] orientation. The grains were crushed in alcohol and deposited on a holey carbon film; the study was carried out with a Topcon 002B microscope, yorking at 200 kV and with a point-to-point resolution of 1.8 A.The interpretation of the experimental images was carried out by comparing with images recorded for the 2201, 2212 and bismuth oxycarbonate phases;16-18,20-23the simulated images were calculated using the Mac Tempas program. The comparison of the overall [OlO] HREM image of B~,,S~,,CU~~(CO~)~O~~[Fig. 3(u)] with that of the oxycarbon- ate (Bi2Sr2Cu06)[ S~,CU(CO,)O~]~~ [Fig. 3(b)] which is also [2201], [S2CC], and corresponds to a single intergrowth of the 2201 and S2CC structures, shows the close relationship between the two structures. One can observe [2201], [S,CC],-type ribbons parallel to (100) that are seven octahedra wide on average, and are shifted along c with respect to each other [Fig. 3(a)]. Within these ribbons, the contrast characteristic of the [2201], [SzCC] intergrowth is clearly seen.First, a typical contrast similar to that observed in the layered bismuth c~prates~'-~~indicates the existence of double bismuth layers (black arrowheads). Secondly, a typical contrast indicates the presence of carbonate groups (curved arrow~).~~-~~ According to the different HREM studies devoted to the oxycarbonates, 176 J. Muter. Chern., 1996, 6(2), 175-181 it was indeed shown that the presence of carbonante groups (the cation positions are given in Table 1 and the oxygen in a matrix, in the form of isolated groups or complete rows, atoms were located in ideal positions with regard to the cation positions) for different focus values and different thicknesses can be easily evidenced, but only for a few focus val~es.~~-~~ The enlarged images of Fig.4 illustrate these points. In Fig.4(u), the bright spots are correlated to the heavy atom positions; the contrast can be described as two groups of rows of bright dots: the first one is a group of four rows, oval shaped, correlated to the sequence (SrO),-(BiO)T (BiO),-(SrO), along c (the [BiO] layers are indicated by small arrowheads); the second is a group of two rows correlated to two (SrO), rows. The nature of the layers located between these rows were determined by changing the focus value. In Fig. 4(b), it can be observed that the row located between the two (SrO), rows exhibits a very bright contrast (curved arrow), similar to that observed in the oxycarbonates, and correlated to the row of carbonate groups; the layers located between the two groups of strontium and bismuth rows appear as grey dots correlated to the [CuO,] layers.Then, the layer stacking along c would be (SrO)(BiO)(BiO)(SrO)(CuO,)-(SrO)(CO,)(SrO)(CuO,), which is that observed in the [2201]1 [S2CCI1 oxytarbonates. Two successive (100) ribbons are shifted by cu. 12 A (i.e. ca. 1/2 c~zzol))with respect to each other so that one 'bismuth' layer is directly connected to a 'carbonate' layer through the shearing plane [Fig. 4(b)]. In order to confirm this interpretation of the layer stacking mode within a (100) ribbon, simulated images were calculated. To determine the positional parameters, we began from a theoreti- cal model presented in Fig.5(b) which takes into account the nature of the different layers (space group A2/m) and the way they are shifted. Only the positional parameters of the cations were refined from X-ray diffraction data; in order to limit the number of variables, some x values were constrained. The theoretical images were calculated from these refined values Fig. 4 Enlarged [OlO] HREM image of Bil,Sr29Cu12(C03)7056 recorded for focus values of (a)ca. -55 nm and (b)ca. -20 nm. The calculated images are compared in the left parts of the images for the positional parameters of Table 1. (cTTion of the model used for image calculations; the refined cation positions are given in Table 1; the anion positions have been arbitrarily fixed by considering appropriate interatomic distances.of the crystal; the corresponding projected structure is given in Fig. 5(c). The images c$lculated for focus values of -55 and -20 nm (thicknessx 30 A) are shown on the left-hand side of Fig. 4(u) and (b),respectively. They confirm the layer stacking mode of each (100) ribbon which is characteristic of the [(2201)],[(SzCC)], structure, and the way they are shifted [Fig. 5(u)]. Note that such a structure was previously observed in the form of a defectz4 which originated this work. Starting from the [(2201)]1[(SzCC)]l structure [Fig. 5(a)] which is built up from the intergrowth of the BiZSrzCuO6 (2201) and SrzCuC030z( SzCC) structures, the new oxycarbonate Bi15Srz9C~1z(C03)7056[Fig.5(b)] can be described as a '[( 2201)11[( S,CC)] shear structure' characterized by (010) shearing operations arising, on average, every seven octahedra. The c parameter is similar to c(2201)(s2cc~,i.e. it roughly corre- sponds to [c~zz01)+2 The junction of the (010) ribbons, C(~&J. through the crystallographic shear plane (CSP) involves the connection of layers of different natures and the formation of 'segments' instead of infinite layers as observed in the parent structure, [2201][ S,CC]; in this way, if we consider ribbons of seven octahedra wide: (i) a (BiO), segment is connected to a (CO,), segment on one side and to a (SrO)l4 segment on the other side; (ii) every (CuOZ), segment is connected to another (CuO2), segment, leading to the formation of (CUO~),~ segments; the (CUOZ)~~ segments are connected to a (BiO), segment on one side and to a (SrO),, segment on the other side; (iii) in the same way, every (SrO), segment is systematically connected to another one, leading to the formation of (SrO)14 segments; the (SrO),, segments are connected to a (BiO), segment on one side and to a (CUOZ)~~ segment on the other side.Such an ideal structure would correspond to the composition Bil4SrZ8Cul4; however, as previously mentioned for the 2201 Table 1 Cation positional parameters used for the image calculations, space group A2/m atom site X Y Z Bi 0.0 0.0 0.211 Bi 0.146" 0.5 0.246 Bi 0.146" 0.0 0.326 Bi 0.287" 0.0 0.280 Bi 0.287" 0.5 0.361 Bi 0.4 14" 0.5 0.323 Bi 0.414" 0.0 0.375 Sr 0.293" 0.5 0.026 Sr 0.425" 0.0 0.049 Sr 0.0 0.5 0.044 Sr 0.138" 0.0 0.085 Sr 0.293" 0.5 0.126 Sr 0.425" 0.0 0.166 Sr 0.0 0.5 0.139 Sr 0.138" 0.0 0.180 Sr 0.293" 0.5 0.218 Sr 0.425" 0.0 0.258 Sr 0.138" 0.5 0.383 Sr 0.293" 0.0 0.414 Sr 0.425" 0.5 0.436 Sr 0.138" 0.5 0.487 cu 0.425" 0.5 0.496 cu 0.0 0.0 0.096 cu 0.138" 0.5 0.131 cu 0.293" 0.0 0.169 cu 0.425" 0.5 0.211 cu 0.138" 0.0 0.438 cu 0.293" 0.5 0.470 C 0.0 0.0 0.0 C 0.138" 0.5 0.035 C 0.293" 0.0 0.070 C 0.425" 0.5 0.105 " Constrained values. J.Muter. Chem., 1996,6(2), 175-181 177 Fig.5 Idealized drawing of (a) the [2201][S2CC] parent structure and (b) the Bi,5Sr29C~,2(C03),056collapsed structure, which is directly correlated to the former via a periodic shearing mechanism every seven octahedra and 2212 collapsed c~prates,'-~*'~a strong variation in contrast is observed at the level of the CSP [large arrows in Fig. 4(a)]; this suggests the existence of local cationic substitutions, such as the replacement of copper by Bi or Sr atoms at the extremities of the copper segments, and local variations of the cation environment; for this reason, the polyhedra which ensure the junction at the level of the shear plane are not well defined and are drawn in dotted lines in the idealized model of Fig.5(b).This observation is in agreement with the actual composition, which corresponds to the cationic ratio Bi15Sr2,Cu12 instead of Bi14Sr2,Cu14 for the theoretical composition. The oval shape of the bismuth segments, (BiO)7, clearly observed in the images [Fig.4(a)], may be correlated to an effect of the stereoactivity of the 6s2 lone pair of Bi3+,similar The average width of the (100) ribbons is seven octahedra, but the local stabilisation of ribbons with different widths is not a rare event; an example of an isolated rn=8 octahedra wide ribbon is shown in Fig.4(a) (see rn values in the upper part of the image). An example of crystal where the m=8 members are frequently stabilized is shown in Fig. 6(a).The schematized structure of the rn = 8 member is shown in Fig.6(b); the (100) ribbons are eight octahedra wide; the b and c parameters remain unchanged, whereas a and p are modified. For a member m of this new family, the theoretical parameters of the monoclinic cell can be royghly calculated: a ~os(fl-90)~xm x u,,/2/2; b xap,/2 z 5.4 A; c - C(2201HS2CC) = 39.5 A; tan (p-90)FZ 3 + ,/2/m. In general, the defective mem-bers rn' we have observed in the crystals differ by only one octahedron with regard to the nominal composition, i.e. rn'= to that observed in the layered 2201,2212and 2223 c~prates.~'-~~7fl. In fact, the different layers undulate with a rather large ampli-tude, due on one hand to this oval shape of the bismuth segments and on the other hand to the distortion induced by the direct junction of layers of different natures through the shearing plane; as an example, BiO, octahedra and carbpnate layers, whi$h exhibit different apical distances (dBi4 z 2.1 A and dc4 x1.3 A), are connected.This marked undulation of the layers and the complex microstructural features (see below) hinder structural refinement by the use of X-ray data. Microstructural study Three types of nonstoichiometry features have been systemati-cally observed in the crystals of this new collapsed bismuth oxycarbonate. 178 J. Mater. Chem., 1996,6(2), 175-181 The existence of oriented domains is a rather frequent feature; an example is shown in Fig. 7 (area 1). The c axes of the two domains are at ca. 93" to each other and the domain boundary varies within the matrix while remaining along an average direction which is roughly the [Sol] direction of the [( 2201)], [(S2CC)], parent structure.Although the coincidence is not perfect due to intergrowth defects, the shearing planes generally intersect at the level of the domain boundary. In this image, a second type of oriented domain is observed, in area 2. The (100) plane acts as a role mirror plane; this feature is in fact a chemical twin due to the monoclinic symmetry of the structure; an idealized model is proposed in Fig. 7(b) where it can be seen that the number of bismuth rows of each ribbon is not modified but, through the twin plane, a copper segment, 21 octahedra in width, is formed. Fig. 6 Example of an m=8 member of the family of the collapsed bismuth oxycarbonate: (a) [OlO] HREM image and electron diffraction pattern, (b)idealized structure Crystals are often striated by extended defects running along segments.At the level of the defects, indicated by white arrow- [Ool], [1001 and [ZOl]; an example is shown in Fig. 8(a). In heads labelled ‘a’ and ‘c’, one can observe the disappearance of this image, the carbonate groups are imaged as bright dots so the white segments, suggesting that carbonate groups are locally that the contrast in the thicker part of the crystal consists deleted. The [lOo] and [Ool] defects are generally connected mainly of small white segments associated with the ‘carbonate’ to each other and correspond to the disappearance of the J.Mater. Chem., 1996,6(2), 175-181 179 Fig. 7 Examples of oriented domains observed in the B~,,S~,,CU,~(CO~)~O,~crystals: (a) [OlO] image; in the area labelled 1, the two c axes are at 93” to each other and the shearing planes intersect at the level of the boundary. In area 2, the twin boundary is parallel to (100). (b) Idealized drawing of the twinning domains in area 2. carbonate groups, Pividing the crystal into Jwo parts, which are translated by 12A along c and by 5.4A along [loll with respect to each other. Taking into consideration the positions of the carbonate and bismuth layers, and these translations, a structural model of this defect can be proposed [Fig. 8(b)]. The [loo] defect corresponds to the introduction of a 2201-type layer along the (001) plane, { [1001-oriented arrowhead labelled ‘a’ in Fig.8(b));the COO11 defect corresponds to the substitution of carbonate groups by copper octahedra and the formation of a 2223-type slice parallel to (100) (arrowhead ‘c’ whose width may vary. Thus, the defective [1001 regions of dark contrast in the crystals correspond in fact to the replacement of oxycarbon- ate regions by pure bismuth cuprate-type regions as shown schematically in Fig. 8(b). The defects running along [?Ol] can be explained easily on the same basis, since due to the symmetry of the structure, similar staircase-like [(2201)], [(S,CC)], slices are observed parallelly to (102). Thus, the defects running along [ZOl] correspond to the intergrowth of additional [(2201)], slices parallel to (102).Note that the formation of such defects resulting from the intergrowth of additional [( 2201)], slices has been reported in a collapsed ferrite, Bi13Ba2Sr2sFe,306~3 whose structure results from a similar mechanism; the structure of this iron oxide can indeed be described as a double collapsed structure derived from the [(2201)], [(0201)], parent structure. Discussion The first important observation deals with the fact that shear- ing mechanisms applied to the bismuth oxycarbonate lead 180 J. Muter. Chern., 1996, 6(2), 175-181 Fig. 8 Examples of defects striating the B~,,S~,,CU,,(CO~)~O,, crys-tals. (a) Overall [OlO] image; defects running along (100) are labelled ‘a’ and those running along (001) are labelled ‘c’.They result mainly from the replacing of oxycarbonate regions by pure bismuth copper oxide regions. (b)Idealized drawing of the defects. systematically, as previously observed for the ‘2201’ and ‘2212’ collapsed phases, to a rupture of the [CuO,], layers so that no superconductivity should be expected. One indeed observes that this phase does not superconduct, in contrast to the mother structure Bi2+xSr4Cu2(C03)0816 which exhibits a T, of 30K. The rupture of the [CuO,], layers involves the formation of ‘copper tapes’ running along [OlO]. The width of these tapes, which corresponds tq 14 octahedra for the collapsed 2201 oxycarbonate (ca. 35 A) is significantly larger than that observed for the collapsed 2201 yprate Bi17Sr16CU704814 which is eight octahedra (cu.21.6 A) and close to that observed in the 2212 collapsed phase (17 polyhedra). In this respect, these new structures are intermedi- ate between the collapsed 2201 cuprates and the [22011, [S2CC], oxycarbonates16-18 which exhibit infinite copper layers instead of ‘tapes’. A second interesting characteristic of these collapsed phases deals with the oval-shaped segments observed in all the bismuth compounds. This property may be correlated to the stereoactiv- ity of the 6s2 lone pair of Bi3+, which controls the undulation of the bismuth-oxygen layers in the mother structures. It is necessary to determine whether there exists any correlation between the amplitude of the modulation in the mother structures and the periodicity of the shearing mechanism in the collapsed oxycarbonate Bil,Sr2,Cu12(C03)7056. For this purpose, we can compare the four bismuth oxycarbonates which have been prepared so far: Bi,,5Pb0.5Sr3.5C~2(C03)08;27 Bi2Sr3.5C~2(C03)08;27 Bi2+xSr4Cu2(C03)1 and-x08+s16 Bi15Sr29C~12(C03),056(this work), which are shown sche- matically in Fig.9 (where only the bismuth and carbonate layers are represented). Comparing the two oxycarbonates Bil,,Pb0.,Sr,,sCU,(C03)08and Bi2Sr3.,Cu2(C03)08 that both exhibit the same cationic ration Bi +Pb +Sr:Cu =2.75, one can 000000000 0 0 00000oo 00000 000000000 DDDDDDDDD * ooooooo~oo~ooooooo DDDDDDDDDDDDDDDDDD 000000000 0000ooooo~ooooo0000 000000000 0 0 ooooooo00ooooooo0 <-- 9.5 --> (Bi+Sr)/Cu = 3.1 ooooooo ooooooo DDVDDDD (d1 DDDDDDDooooooo ooooooo ooooooo o~ooo~oDDVDDDD <--7 --> 0000000 ooooooo Fig.9 The four bismuth-based oxycarbonates schematically drawn through the double bismuth layers (as dark spots) and the carbonate groups (as triangles) see that the first compound exhibits a non-modulated structure, whereas the second has a modulated structure with a period- icity b=9.5upJ2. When the (Bi +Sr) content increases, the amplitude of the modulation decreases as shown for Bi,+,Sr,Cu,(CO,), -x08fd,whose periodicity is only b= 8.8.uPJ2. An important feature deals with the fact that for lower (Bi +Sr) contents such as Bi2Sr3.,Cu,(C03)08 the bis- muth bilayers undulate in phase opposition [Fig.9(b)], whereas for higher (Bi+Sr) contents such as Bi,+xSr,Cu,(CO,), -x08f6,they undulate in phase [Fig. 9(c)]. This can be understood easily if we consider that when larger cations occupy some sites of the carbonate layers, the minimiz- ation of the strain between the different layers stacked along cwill occur for a centred configuration between the undulating bismuth layers and the mixed C/Bi layers. When the (Bi +Sr) content is increased further, the strains can longer more be minimized and the structure collapses as shown for B~,,S~,,CU~~O~~(CO~)~[Fig. 9(d)]; nevertheless, the bismuth and carbonate segments remain connected as in the Bi,+,Sr,Cu2(C0,), -,08,sstructure [Fig.9(c)]. Note that a similar effect is observed in the iron-based bismuth oxides (2201)(0201) where shearing mechanisms are observed with decreasing iron content.33 From the comparison of these phases which all exhibit double bismuth layers, it appears clear that the composition of every layer through the different cationic ratio, (Bi+Sr):Cu and Bi:Cu, plays an important role in the stabilisation of the different structures. The great ability of the rock-salt and perovskite layers to accommodate various cat- ions and nonstoichiometry allows large homogeneity ranges to be observed for one given structure, through the formation of mixed layers; however, the strains and the variation of the oxygen content which are then involved are accommodated up to a certain point and, beyond this point, lead to the formation of new structures.The authors are grateful to the ISC Programme of the European Commission for financial support. References 1 V. Hardy, A. Ruyter, Ch. Simon, J. Provost, D. Groult, M. Hervieu and B. Raveau, Proc VIIth Int. Workshop on Critical Currents in Superconductors, IWCC, Alpbach, Austria, 1994, pp. 1-6. 2 A. Fuertes, C. Miravitlles, J. Gonzales-Calbet, M. Vallet-Regi, X. Obradors and J. Rodriguez-Carjaval, Physica C, 1989,157,529. 3 M. T. Caldes, M. Hervieu, A. Fuertes and B. Raveau, J. Solid State Chem., 1992,97,48. 4 B. Domenges, M. T. Caldes, M. Hervieu, A. Fuertes and B. Raveau, Microsc. Microanal. Microstruct., 1992,3,415. 5 M. T. Caldes, M.Hervieu, B. Raveau and A. Fuertes, J. Solid State Chem., 1992,98,301. 6 M. T. Caldes, A. Fuertes, P. Gomez, X. Obradors and J. Rodriguez, Physica C, 1992,185-189,681. 7 M. T. Caldes, M. Hervieu, A. Fuertes and B. Raveau, J. Solid State Chem., 1992,98,48. 8 M. Hervieu, C. Michel, A. Q. Pham and B. Raveau, J. Solid State Chem., 1993,104,289. 9 M. Hervieu, M. T. Caldes, S. Cabrera, C. Michel, D. Pelloquin and B. Raveau, J. Solid State Chem., 1995,119, 169. 10 F. Goutenoire, M. Hervieu, A. Maignan, C. Michel, C. Martin and B. Raveau, Physica, 1993,210,359. 11 M. Uehara, S. Sahoda, H. Nakata, J. Akimitsu and Y. Matsui, Physica C, 1994,222,27. 12 A. Maignan, D. Pelloquin, S. Malo, C. Michel, M. Hervieu and B. Raveau, Physica C, 1995,249,220.13 M. Huvt, C. Michel, A. Maignan, M. Hervieu, C. Martin and B. Raveau, Physica C, 1993,205,219. 14 Y. Ikeda, H. Ito, S. Shimomura, Y. Oue, K. Inaba, Z. Hiroi and M. Takano, Physica C, 1989,159,93. 15 M. Hervieu, C. Michel, M. T. Caldes, A. Q. Pham and B. Raveau, J. Solid State Chem., 1993,107, 117. 16 D. Pelloquin, M. T. Caldes, A. Maignan, C. Michel, M. Hervieu and B. Raveau, Physica C, 1993,208,121. 17 D. Pelloquin, A. Maignan, M. T. Caldks, M. Hervieu and B. Raveau, Physica C, 1993,212, 199. 18 D. Pelloquin, M. Hervieu, A. Maignan, C. Michel, M. T. Caldes and B. Raveau, J. Solid State Chem., 1994,116,53. 19 J. Rodriguez-Carjaval, Proc. Satellite Meeting on Powder Diflraction of the XVth Congress of Int. Union of Crystallography, Toulouse, France, 1990, p.127. 20 H. W. Zandbergen, W. A. Groen, F. C. Mulhoff, G. Van Tendeloo and S. Amelinckx, Physica C, 1988,156, 325. 21 0.M. Kwei, J. B. Shi and M. C. Ku, Physica C 1991,174,180. 22 0.Eibl, Physica C, 1990,168,215. 23 Y. Matsui, S. Takekawa, H. Nozaki, A. Umezono, E. Takayama-Muromachi and S. Moruichi, Jpn. J. Appl. Phys., 1988,27, L1241. 24 M. Hervieu, M. T. Caldes, C. Michel, D. Pelloquin and B. Raveau, J. Solid State Chem., 1993, 108, 346. 25 M. Hervieu, C. Michel, M. Huve, C. Martin, A. Maignan and B. Raveau, Microsc. Microanal. Microstruct., 1993,441. 26 B. Domenges, M. Hervieu and B. Raveau, Physica C, 1993,207,65. 27 X. F. Zhang, G. Van Tendeloo, S. Amelinckx, D. Pelloquin, C. Michel, M. Hervieu and B. Raveau, J. Solid State Chem., 1994, 113, 327. 28 H. W. Zandbergen, W. P. Groen, F. C. Mijhoff, G. Van Tendeloo and S. Amelinckx, Physica C, 1988,156, 325. 29 Y.Gao, P. Lee, D. Coppens, M. A. Subramanian and A. W. Sleight, Science, 1988,241,954. 30 A. Yamamoto, M. Onada, E. Takayama-Muromachi, F. Izumi, T. Ishigaki and H. Asano, Phys. Rev. B, 1990,42,4228. 31 C. Michel, C. Martin, M. Hervieu, D. Groult, D. Bourgault, J. Provost and B. Raveau, Proc. Int. Symp. on Superconductivity, Hsinshu, Taiwan, 1989. 32 H. Leligny, S. Durcok, P. Labbe, M. Ledesert and B. Raveau, Acta Crystallogr., Sect. B, 1992,48,407. 33 M. Hervieu, M. T. Caldes, D. Pelloquin, C. Michel and B. Raveau, J. Solid State Chem., 1995,118, 357. Paper 5/04982B; Received 27th July, 1995 J. Muter. Chem., 1996, 6(2), 175-181 181