Ab initio calculations of the BaF2 bulk and surface F centres

A hybrid Hartree–Fock and density functional theory, in which Hartree–Fock exchange is mixed with density functional theory exchange functionals, using Beckes’ three-parameter method, combined with the non-local correlation functionals by Perdew and Wang, allows us to achieve the best agreement with experiment (11.00 eV) for the BaF2 band gap (11.30 eV). The characterization of F centres in BaF2 is still a question of debate. In order to understand the behaviour of the material, we performed ab initio calculations to determine their electronic structure, atomic geometry and formation energy. We also calculated the M centre, the simplest aggregation of two F centres, and the results show that the β band absorption in BaF2 is predominantly due to the presence of M centres. (Some figures in this article are in colour only in the electronic version)