ELECTROCHEMICAL OXIDATION OF LA2CUO4 IN FUSED NITRATES

La2CuO4 has been electrochemically oxidized in fused alkali nitrates, a medium that stabilizes peroxo and superoxo groups. The oxidation tak es place either in neutral (NaClO4 electrolyte) or basic (KOH, Na2O el ectrolytes) media, in the presence or absence of protons, and in the p resence or absence of added oxo groups (e.g. NaClO4). The oxidation po tentials are lower as the basicity of the medium increases (NaClO4 > K OH > Na2O). The resulting La2CuO4+delta materials have large supercond uctivity fractions when the oxidation is performed at 150 degrees C, a nd very small fractions (along with phase decomposition into CuO) when it is performed at 235 degrees C. The oxidized materials are totally unreactive versus iodide. That, along with the reported existence of p eroxo and superoxo groups in this media, suggests that intercalation o f oxygen in intermediate -1 and/or -1/2 oxidation states takes place. However, the existence of such reaction in the absence of protons elim inates the possibility of a mechanism by which OH- groups are being in tercalated. Potentiostatic experiments yield a phase-segregated materi al with phases of T-c 45 and 35 K, while galvanostatic experiments all ow to prepare the 35 K phase by itself. (C) 1997 Elsevier Science B.V.