At the same time, the indicated order of decomposition onset of metastable solid solutions with 3d transition metal cations correlates with the number of electrons on their outer electron shell. The most stable are solid solutions of the NiO-ZnO system, in which the Ni 2+ cation has the most stable electronic configuration [Ar]3d 8 and, according to crystal field theory, the greatest energy preference for the octahedral environment of the O 2-anions.