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 impurity in CaF2 have been computed using ab-initio Hartree–Fock theory and the supercell approach. After correcting for electron correlation, a substitution energy (referenced to free Mg and Ca atoms) of +3.30 eV is found. At a Mg+2 site, the F− nearest-neighbor shell contracts by about 0.093 Å, relative to the ideal CaF2 shell radius; however, the Mg+2–F− distance remains about 0.32 Å greater than in pure MgF2. No new valence states occur outside the CaF2 valence band, but Mg+2 s-like states are found at the very bottom of the conduction band, lying just below the lowest empty Ca+2 states. The position of these states is sensitive to the extent of lattice relaxation around the impurity cation. It is also found that displacement of the Mg+2 from the ideal Ca+2 lattice site can occur easily.