Ab-initio Hartree-Fock study of Mg+2 as a substitutional impurity in CaF2

The energetics and electronic structure of Mg+2 as a substitutional impurit y in CaF2 have been computed using ab-initio Hartree-Fock theory and the su percell approach. After correcting for electron correlation, a substitution energy (referenced to free Mg and Ca atoms) of +3.30 eV is found. At a Mg2 site, the F- nearest-neighbor shell contracts by about 0.093 Angstrom, re lative to the ideal CaF2 shell radius; however, the Mg+2-F- distance remain s about 0.32 Angstrom greater than in pure MgF2. No new valence states occu r outside the CaF2 valence band, but Mg+2 s-like states are found at the ve ry bottom of the conduction band, lying just below the lowest empty Ca+2 st ates. The position of these states is sensitive to the extent of lattice re laxation around the impurity cation. It is also found that displacement of the Mg+2 from the ideal Ca+2 lattice site can occur easily. (C) 2001 Publis hed by Elsevier Science Ltd.