Thus, the most thermally stable (up to 1100 K) and the widest in concentration (up to x = 0.8) are solid solutions of ZnO with nickel and magnesium oxides . For other 3d transition elements such as iron, cobalt, and manganese, only the limiting concentrations of x = 0.6, 0.5, and 0.4, respectively, can be achieved . Various reasons have been proposed to explain the kinetic stability of such metastable solutions.