Charge-state stability of Ni and Cu impurities in MgO

Total energies for substitutional Ni and Cu ions in MgO are studied for their 1+, 2+, and 3+ charge states. These are used to assess charge-state stability. Specifically, electron loss to the conduction band for ${\mathrm{Ni}}^{+}$ and electron capture from the valence band for ${\mathrm{Ni}}^{3+}$ and ${\mathrm{Cu}}^{3+}$ are considered. The impurity and its near-neighboring ions are analyzed in the unrestricted Hartree-Fock self-consistent-field approximation, as a molecular cluster embedded in a consistently relaxed shell-model lattice. The results indicate that both electron loss for n=1 and electron capture for n=3 should occur, so that disproportionation of the various charge states is intrinsically unstable.