Layered Cuprates with Double Copper Layers: Relationships Between Structure, Nonstoichiometry and Superconductivity

Abstract

Layered cuprates with double copper layers form a rather large family which appears as very promising for superconductivity at high temperature. The structure of those phases, the nonstoichiometry phenomena, especially extended defects observed by high resolution electron microscopy and oxygen nonstoichiometry are presented here. The relationships between those structural features and the superconducting properties of those materials are discussed.Many superconductive layered cuprates have been synthesized since the beginning of 1987. In spite of the variety of their chemical compositions, all those materials belong to the same structural family. They consist of an intergrowth of multiple oxygen deficient perovskite layers (p) with multiple rock salt-type layers (r.s) according to the general formula (ACuO3-x) m p (AO) n r.s where m and n are the number of Cu-O and AO planes respectively. They can also be represented by the symbol [m,n]. The rock salt slabs are, in fact, built up from n + 1 planes: (n — 1) AO planes sandwiched by 2 AIIO planes (AII = Ba, Sr) which ensure the junction between the two structures. Thus, the structural principles which allow these phases to be synthesized appear as very simple, taking into consideration the fact that besides the two-dimensional character of the structure, the mixed valency of copper Cu(II)-Cu(III), should be introduced in order to create hole carriers on the copper-oxygen framework. In reality, the physics and chemistry of those materials are more complex than expected. We present here some recent issues dealing with oxygen nonstoichiometry, extended defects, in connection with superconductivity and new structures in the m = 2-members, involving single rock salt layers.