J. MATER. CHEM., 1994,4(5), 773-774 (BaTiO,),(Gd,Ce),Cu20,: A New Homologous Series of Layered Cuprates containing Various Layers of Perovskite Units Rukang Li Department of Applied Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, People's Republic of China Two compounds in a new homologous series (BaTiO,),(Gd, Ce),Cy207 with m =2 and 3 have been synthesized and characterized. The tetragonal unit-cell parameters, a =3.874 04(4) A, c =36.888(1) A (for m =2) and a =3.881 45(5) A, c =44.775(1) A (for m =3) for the two compounds were determined by X-ray and electron diffraction. From the diffraction patterns, we deduce that the structures of the two compounds are built up by alternative stacking of multiple perovskite layers, copper-oxygen planes and double fluorite layers.The unusual structural characteristics in the series are the wide separations between the CuO, planes. Although both compounds contain CuO, planes, as is common in high-T, cuprates, we have not yet succeeded in making the samples superconducting. As it is now widely accepted that copper-oxygen planes in cuprates are crucial for the appearance of superconductivity, we have aimed to explore cuprates containing new interleaving layers which connect or separate the CuO planes as potential high-T, superconductors.' A single perovskite layer, MTiO, has long been incorporated into the high-T, cuprates for connecting the Cu-0 planes.2 Recently, Gormezano and Weller3 found that a double perovskite layer can also act to joing the CuO, pyramidal layers.Based on these studies, we have succeeded in preparing a new cuprate: Ti,(Ba,Gd) (Gd,Ce)2Cu20y, which is built up by alternative stacking of (Gd,Ce),02 fluorite layers and a double perovskite unit joining CuO, pyramidal layers4 in the sequence (Gd,Ce),O2-CuO5-MTiO3-MTiO3-Cu0,-( Gd,Ce),O,. The structure of the compound has been assigned as Ti-2322, in accordance with the structures of other layered cuprates. However, if one rewrites the chemical formula as: (BaTiO3),(Cd,Ce),Cu2O7, the Ti-2322 compound corre-sponds to the m=2 member in this family. Thus it is plausible to expect other members in the family with different m could also exist. Here, I report the preparation and the characteriz- ation of new cuprate compounds with m=2, 3 in the (BaTi03),(Gd,Ce)3Cu207 family.The (BaTiO,),Gd, -,Ce,Cu,O, samples were prepared by solid-state reaction from appropriate ratios of the starting compounds Ti0,(99.99%), BaC0,(99%), Gd203( 99.95%), Ce0,(99.9%) and CuO(99%). The thoroughly ground mix- tures of the reagents were first heated at 1050 "C for 48 h in covered corundum crucibles with an intermittant grinding. After cooling to room temperature, the samples were reground, pelleted and finally calcined at 1100 "C for another 48 h, followed by furnace cooling. The as-prepared samples were then subjected to X-ray powder diffraction, electron diffrac- tion, resistance measurements and further heat treatment. The X-ray diffraction (XRD) patterns were recorded with a Rigaku D,,, -rA diffractometer equipped with a Cu-Ka rotating anode source.A step width of 0.02" (28) and a counting time of 1 s were applied. A curved graphite mono- chromator was set in the diffraction path and silicon powder added as the internal standard. Selected-area electron diffrac- tion (SAED) was performed on a Hitachi H-800 transmission electron microscope. The sample resistances were measured by the standard four-probe method down to 15 K in a commercial He-circulating refrigerator. Taking account of the proposed structural relationship among members in the (BaTi03),(Gd,Ce)3Cu207 family, we derive the structural models shown in Fig. 1 (up to m= 3). All Fig. 1 Ideal structural models of the members in the (BaTi0,),(Gd,Ce),Cu20, family (FL=fluorite layers, PU .=perov-skite units).The upper parts of the models are shifted by (3, +:I relative to the lower parts the models can be viewed as combinations of two types of structural unit. One unit is the multiple perovskite layer connecting copper-oxygen layers, i.e. Cu05-( Ba?'iO,),-CuO, (m=O, 1, 2,3). In combination with the other type of unit, the double fluorite layers Ln202, the units of the per- ovskite containing CuO, bilayers are shifted alternatively by a translation 1/2, 1/2 in the basal plane. This translation causes the primitive lattice to become body centred. The members of the family differ from each other by the number of perovskite layers between pairs of CuO, planes.Thus the lattice constant c correspondingo to each structure should follow the relation: c, + =c, +8 A. Since (Y,Ce)2SrCuFeOy(c,=20.5 A), and NbSr,(Nd,Ce),Cu,O, (cl =28.8 A), may be taken as model examples for the structures with m=O and 1, respectively, the c paramtters of the mepbers with m=2 and 3 would be c2=36.8 A and c3=44.8 A from the above relation. Fig. 2 shows the XRD patterns of the as-prepared samples with the nominal compositions of Ti2Ba2Gd2.33Ce0.67(:u20, (rn=2) and Ti3Ba3Gd2.4 Ceo&u20y (rn=3). The m =2 samples are nearly single phase with a minor phase of BaTiO,. For the samples with m =3, a large amount of BaTiO, was found to coexist with the main phase. Although the patterns for the samples m=2 and 3 seemed to resemble each other, we have J.MATER. CHEM.. 1994, VOL. 4 Fig. 2 X-Ray and electron diffraction patterns (insets) of the Ti,Ba,Gd2,33Ceo,67C~,0y and Ti,Ba,Gd,,,Ce,,,Cu,O, samples. The asterisked peaks in the patterns are due to BaTiO, indexed the two patterns successfully by least-squares fitting the XRD peaks in the range 20-65" and 9btained the cell parameters: a 3.87404(4) A, c,= 36.888(1) A for m =2, and a =3.88145(5) A, c =44.755(1)A for m =3, respectively. The systematic absences of diffraction indices, h +k +1 =odd, as well as the magnitudes of the c parameters all agree with the ideal structure models [Fig. l(c), (d)] and space group The SAED patterns of the two samples (Fig. 2, inset) generally agree with our indexing of the XRD patterns.For m=2 samples, the strongest 001 spot is 0018 (the 9th spot) whereas that for m=3 is 0022 (the 11th spot). This indicates that the m=2 sample has a nine-layer structure and the rn= 3 has an eleven-layer structure. Such evidence further supports the proposed structural models for the two compounds. However, broad, weak streaks along the c* direction at (3,2, 1) were observed in a pattern of sample with m=3. Those streaks reveal the existence of distortion from the ideal 14,mmm structure. The distortion may originate from rotation of the TiO, octahedra in the perovskite units similar to the SnO, rotations found in L~,B~,S~,CU,O,,.~ It is worth noting that an important structure feature of the compounds in this family is that the CuO planes are well separated in the unit cel!.Normally, the thickness of the double fluorite layer is 6 A, whereas those of !he perovskite layers in the m=2, 3 compounds are 8 and 12 A, respectively. If one assumed that the Cu0,-Ln,O,-CuO, units were the source for superconductivity, the present compounds would provide model examples to study whether well isolated CuO planes are able to support high-T superconductivity. In fact, both samples showed low room-temperature resistances and semiconductive behaviour down to 15 K. Although the resis- tivities and their temperature dependences became smaller and weaker, all the samples remained semiconductive after treatment in flowing oxygen at 1100 "C. A preliminary high oxygen pressure treatment (20 atm, 800 "C) was also unsuc- cessful to make the samples superconducting or metallic.In conclusion, we have succeeded in preparing two new layered cuprates with the general composition (BaTiO,),(Gd,Ce),Cu,O,: rn =2 and 3. The compounds con- tain units with multiple layers of BaTiO, sandwiched between CuO, planes. The structures of the compounds are alternative stacking of this unit and double fluorite layers. Although the structures possess complete CuO, planes, we have not yet succeeded in inducing superconductivity in them. Financial support from the Fok Ying Tung education foun- dation is gratefully acknowledged. References 1 Li Rukang, Appl. Phys. Commun., 1992,11,295. 2 Li Rukang, Zhu Yiangjie, Xu Cheng, Chen Zuyao, Qian Yitai and Fan Chengao, J.Solid State Chem., 1991,94,206. 3 A. Gormezano and M. T. Weller, J. Muter. Chem., 1993,3,771. 4 Li Rukang, J. Solid State Chem., in the press. 5 Li Rukang, Tang Kaibin, Qian Yitai and Chen Zuyao, Muter. Res. Bull., 1992,27, 349. 6 M. T. Anderson K. Poeppelmeier, J. P. Zhang, H. J. Fan and L. D. Marks, Chem. Muter., 1992,4, 1305. Communication 4/01335B; Received 8th February, 1994