New trends in the crystal chemistry of high- Tc superconductors

Abstract

The La 2CuO 4-type superconductors have opened the route to the research of superconductors with higher critical temperatures. The 90 K superconductor YBa 2Cu 3O 7−δ is a complex oxygen-deficient perovskite whose crystal chemistry is presented here: the high-resolution electron microscopy study of different compositions allows order-disorder and non-stoichiometry phenomena to be understood. The superconducting properties of this oxide are interpreted here in terms of disproportionation of Cu(II) into Cu(III) and Cu(I). Besides this latter oxide, two other families, bismuth-copper and thallium-copper oxides, are described whose T c ranges from 85 to 125 K. These two recent series belong with the La 2CuO 4-type oxide to the same structural family which consists of intergrowths of distorted rock salt-type layers and oxygen-deficient perovskite layers. The possible dynamic transfer of holes from thallium layers towards copper layers in those oxides is discussed.