SIMULATION OF F, F-2(+), AND (F-2(+))ASTERISK CENTERS IN NAF-MG

The object of this work is to determine the atomistic and electronic configuration of the (F2+)* centre in NaF. This centre combines an F2+ centre with an impurity dipole consisting of an Mg2+ substitutional ion and an Na vacancy. There are many conceivable configurations for such a combination, and our calculations lead us to propose a particular one. Our result agrees with an earlier experimental conclusion that, in its ground state, this centre has the excess electron localized in a single vacancy, more like an F centre than an F2+ centre. Our method uses effective potentials derived for interaction of the excess electron with host and impurity ions. We also present a detailed analysis of the F- and F2+-centre ground states, including isotropic hyperfine constants. The F2+-centre results support the picture of substantial localization about this defect's centre.