Electronic States ofF-Type Centers in Oxide Crystals: A New Picture

A strong case is made to show that the $F$-type color centers in oxide crystals are qualitatively different from the color centers of alkali halides. A semiquantitative discussion is followed by the results of an $X\alpha$ self-consistent-field calculation for the $\alpha$-${\mathrm{Al}}_2$ ${\mathrm{O}}_3$ crystal. An oxygen-ion vacancy in $\alpha$-alumina is a deep electron trap, the ground state of which lies between the two valence bands, unlike the $F$ center of alkali halides whose ground state lies between the valence and conduction bands. The two electrons in the trap are stabilized by a hole.