OXIDE SUPERCONDUCTORS - INFINITE STACKS OF COPPER-OXIDE

It is known that Bi//2Sr//2CaCu//2O//8 belongs to a family of structurally related high-temperature superconductors which contain the elements Bi-Sr-Ca-Cu-O or Tl-Ba-Ca-Cu-O and have the general formula A//2**I**I**IB//2**I**ICa//n// minus //1Cu//nO//4// plus //2//n A equals Bi,Tl; B equals Sr,Ba). Structural studies of phases with n equals 1-3 have established that copper-containing blocks, each with n left bracket CuO//2 right bracket // infinity layers interleaved with n minus 1 calcium layers, are separated by left bracket ABO//2RB// infinity units with NaCl structure. Nevertheless, the preparation of members with larger n appears essential for further progress on a report by Siegrist et al. on the synthesis and structural characterization of Ca//0//. //8//6Sr//0//. //1//4CuO//2 represents an impressive leap to the n equals infinity end member of this series. The structural study by Siegrist et al. clearly demonstrates the very simple n equals infinity structure in which only calcium or strontium ions separate left bracket CuO//2 right bracket // infinity layers. This highly defective structure, a perovskite with a third of the oxygen positions vacant, has copper bonded to only four coplanar oxygen ions. The most pronounced difference between the parent compound and its bismuth- and thallium-containing relatives, n equals 1-3, is the absence of left bracket ABO//2 right bracket // infinity connecting units, which is likely to modify the electrical properties by the consequent changes to the copper environment and, probably, also its valence. 9 Refs.