Crystal Chemistry of Bi-based High-Tc Superconductors. Roman E. Gladyshevskii, Department of Physics of Condensed Matter, University of Geneva, 24 quai E. Ansermet, CH-1211 Geneva 4, Switzerland
Among the different chemical classes of high-Tc superconductors, representing more than 40 structure types, Bi-based cuprates attract high interest because of their physical properties, but also due to peculiarities in their crystal structures. Crystal chemical studies of the first three members of the structural series Bi2Sr2Can-1CunO4+2n+d (n=1,2,3) were undertaken, based on the results of single crystal X-ray diffraction investigations.
The ideal structures of the Bi-based superconductors may be considered as built up of two types of alternating slabs: a rocksalt-derived slab formed by two BiO and two SrO layers and a perovskite-derived slab containing nCuO2 and (n-1)Ca layers. The unit cells of the average structures are pseudo-tetragonal with a(b(5.4 and c(24.6, 30.9, 37.0Å for n=1,2,3 (Tc=20,90,120K), respectively. However, the real phases are nonstoichio-metric, their symmetry is lower and the structures are more complicated than expected, combining displacive and substitutional modulation.
An incommensurate modulation in the direction of one of the short cell vectors has been defined for the first two members of the structural series. The modulation vector strongly depends on partial substitutions and oxygen content. Both Bi2.1Sr1.9CuO6.2 and Bi2.1Sr1.9CaCu2O8.2 can, however, be conveniently described in monoclinic supercells with 5- and 9-fold volume, respectively. Additional O atoms were found to be inserted into the BiO layers, so that Bi-rich regions with a distorted rocksalt-type atom arrangement and Bi-poor regions with chains of corner-linked BiO3 [psi]-tetrahedra are formed. The monoclinic symmetry corresponds to a systematic phase shift of the modulation wave, the phase difference between neighboring BiO layers being 7[pi]/5 and 10[pi]/9. All metal atoms show a transverse displacement wave with largest amplitude for the Cu and Ca sites, the Bi and Sr sites being also displaced longitudinally. As expected, a less pronounced modulation occurs in the third member of the series.