Theoretical study of self-trapped hole diffusion in CaF2, SrF2, BaF2 crystals

In this paper we present the results of ab initio molecular dynamics (MD) study of the Vk-center in CaF2, SrF2, BaF2 crystals. The calculations have been carried using density functional theory in DFT + U approximation. The MD simulation showed three possible diffusion channels of Vk-center: (i) 0°-degree jump, (ii) jump with 90° reorientation, and (iii) jump through an intermediate state in BaF2 and SrF2 crystals. The intermediate state appears when one of the fluorine ions composing Vk-center is displaced into the nearest interstitial position and may be considered as the pair of nearest anion vacancy and H-center. This state cannot be considered as standalone Vk-center configuration but is rather metastable and briefly relaxes into ground state of Vk-center. The existence of this state in SrF2 and BaF2 crystals distinguish them from CaF2 crystal where it has not been observed.