Multiple charge-states of transition metal impurities

Transition metal ions in MgO and CaO are observed in several charge states. We investigate the conditions for stability of the observed ions of manganese, iron, cobalt and nickel. These lead to two main conclusions. First, the stability of so many states results from a balance between the variation of ionization potential with charge state and the dependence on excess charge of the polarization and distortion of the host. Covalency is unimportant, contrary to mechanisms discussed for gold in germanium. Secondly, the detailed quantitative calculations indicate that the full set of experimental data require that these oxides have a negative electron affinity, probably more negative than - 1·3 eV in MgO and more than −0·9 eV in CaO.