The formation energy and electronic structure of Na+, Cl− and Sr2+ impurity centers were estimated by using ab initio Hartree–Fock theory and a super-cell approach. This extended previous results on Mg2+ as a substitutional impurity. In the case of Na+ which replaced Ca2+ [S(Na)], charge compensation by an F− vacancy, V(F) or by a second interstitial Na+ [I(Na)], were both considered. Geometrical optimization was attained by relaxing the positions of nearest- and next-nearest neighbors so as to minimize the total energy. After correcting for electron correlations, the formation energies were found to increase in the order: Mg2+ < Sr2+ < Cl− < S(Na) + I(Na) < S(Na) + V(F). The Na+ results were in agreement with previous Mott–Littleton formation energies. The ion charges, charge density maps and Mulliken bond populations indicated the nature of the bonding in the vicinity of the defect. The Na+ led to states just above the CaF2 valence-band maximum. Both Na+ and Mg2+ produced states just below the conduction-band minimum. The results were in qualitative agreement with available optical data for Na+ and Mg2+ impurity effects. A Cl− impurity produced a narrow band of states, above the valence-band maximum, which could greatly affect the deep ultra-violet transmission. The Sr2+ did not appear to produce states in the CaF2 gap.

Hartree-Fock Study of Near-Edge Gap States in CaF2 with Na+, Cl- or Sr2+ Impurities. V.M.Bermudez: Computational Materials Science, 2002, 24[4], 501-12