Thermal behaviour of cobaltic and cobaltous oxides as influenced by doping with some alkali metal oxides

The role of Na2O- and Li2O-doping on the thermal decomposition of Co3O4 to CoO and the re-oxidation of cobaltous to cobaltic oxide has been investigated using DTA, with controlled rates of heating and cooling, IR and X-ray diffraction spectrometry techniques. The DTA investigation revealed that both Li2O and Na2O increased the thermal stability of Co3O4. However, the effect was much more pronounced in the case of lithium oxide. Doping Co3O4 with 1.5 mole% Li2O was found to prevent any thermal decomposition of cobaltic oxide even by heating at 1100°C. The maximum thermal stabilization effect induced by doping with sodium oxide (4.5 mole%) was 30%. The sodium oxide- and lithium oxide-doping enhanced the reactivity of the produced CoO towards the re-oxidation by O2 yielding Co3O4. The X-ray diffraction and IR spectrometric investigations showed that part of Li2O and Na2O was effectively incorporated in the Co3O4 lattice, affecting the thermal stabilization of the solid, and another part of the dopant oxide interacted with the produced CoO and also with Co3O4 giving a new sodium cobalt compound, and with Co3O4 producing, also, a new lithium cobalt oxide phase. However, the amount of Li2O dissolved in the Co3O4 lattice was greater than that of Na2O. The sudden cooling of doped solids, from 1000°C to room temperature, favoured the formation of the new sodium cobalt oxide compound, and exerted no effect on the production of the new lithium cobalt oxide phase. The characteristic d spacings and IR absorption bands of these new compounds have been determined. The possible mechanisms of dissolution of Li2O and Na2O in cobaltic oxide lattice are discussed.