Hexagonal-to-cubic phase transition by Li2O evaporation from ordered LixNi1-xO solid solution

The hexagonal (ordered)-to-cubic (disordered) phase transition in LixNi1-xO with nominal x = 0.46 was investigated by X-ray diffraction (XRD) and ther mogravimetric (TG) measurements. The order-disorder phase transition is due to Li2O loss and, as a consequence to the substitution of Ni at the Li sit e to give a Li-deficient hexagonal phase, in equilibrium with the cubic pha se. The phase transition is completed for total lithium atomic fraction of x = 0.14, widely below the minimum critical value a = 0.31 required for the formation of the layered structure. This behaviour has been attributed to the different Li2O evaporation rates in the hexagonal and cubic phases. The hexagonal phase present in the sample with total lithium atomic fraction x < 0.31 is a metastable phase, disappearing following prolonged thermal tre atment at 750 degrees C, in the absence of lithium loss.