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.