It should be noted that, the main role in the stabilization of cubic ZnO-NiO solid solutions seems to be played by the small size of Ni 2+ cation (0.69 Å versus 0.74 Å for Zn 2+ ) and its strong preference for the octahedral environment in oxide compounds . Another feature of nickel compared to other 3d transition metals is the high resistance to oxidation of the Ni 2+ cation. In fact, oxidation to Ni 3+ is only possible under rather harsher conditions, such as exposure to high-energy plasma , whereas bivalent cobalt, iron and manganese cations can be relatively easily oxidized to the +3 state by atmospheric oxygen upon heating. As will be shown below, oxidation can contribute to the thermal decomposition of cubic solid solutions with other 3d transition metals.