ELECTROCHEMICAL OXYGEN INSERTION INTO LA2CUO4-RELATED COMPOUNDS

We proposed in 1990 a new preparative method ill Solid State Chemistry, the electrochemical oxidation in alkaline solution, for intercalating oxygen atoms at room temperature in ceramic materials, thin films or crystals of La2CuO4 (or substituted compounds), ''214'' structure; this process was used by several groups and a review of the main results is given. The physical properties of these materials show that at least two and likely three superconducting phases are formed with different compositions in oxygen overstoichiometry (delta) and T-c. re surprising insertion of these additional oxygen atoms may result from the formation of holes in both CuO2 planes and La2O2 layers. In addition, electrochemical studies have shown that the electroactive species formed during the oxidation process would be O- with a rather high diffusion coefficient. We assume the existence of a mixed valence of oxygen in these materials and propose that the O- species would be tile mobile species thanks to a coupling with the conduction electrons via tile electron transfer Oxygen --> Copper.