Crystal Chemistry and Properties of Mixed Valence Copper Oxides

Abstract

The role played by the mixed valence of oxides of transition elements in the metal-like properties is well known since the discovery of the sodium tungsten bronzes Na{sub x}WO{sub 3} by Hagg and Magneli. The synthesis of mixed valence copper oxides with metal-like properties has opened the route to the discovery of superconductivity at high temperature in the La{sub 2}CuO{sub 4}-type family. The chemistry of copper oxides offers great possibilities for nonstoichiometry on the oxygen sublattice because of the Jahn-Teller effect of cooper than can induce several coordinations in the same framework: octahedral (CuO{sub 6}), pyramidal (CuO{sub 5}), and square planar (CuO{sub 4}). Thus, one can easily imagine that starting from a perfect octahedral framework it could be possible to create tunnels or cage structures, or lamellar oxides by removing oxygen atoms. Similarly, it can be expected that such oxygen- deficient copper oxides will be able to intercalate oxygen and return to the stoichiometric phase. The perovskite structure, with its great flexibility, is a potential candidate for such a realization. However, the Cu(II)-Cu(III) mixed valence in oxides is not so easily obtained. Before 1980 trivalent copper could only be stabilized in some oxides under oxygen pressures of several kbars. Moremore » recently basic cations like the alkaline earths were introduced as guest elements in the copper oxygen host lattice in order to stabilize the upper oxidation state Cu(III) under normal oxygen pressure. Several series of oxides were produced that can be classified mainly as having either a tunnel structure or a layered structure.« less