Fluorine self-diffusion in CaF₂ and BaF₂

Abstract

The self-diffusion of F-18 in single crystals of BaF₂ and CaF₂ was measured between 350 and 940°C. The results were in agreement with diffusion coefficients deduced from ionic conductivity studies, and yieldedΔH's of 1,6 eV for BaF₂ and of 2.0 eV for CaF₂ for intrinsic diffusion. Below 700°C, extrinsic diffusion behaviour was observed in CaF₂ withΔH=0.9 eV. The formation energy for anion Frenkel defects was estimated to be in the range 2.2 to 3.1 eV. Both anion and cation diffusion rates were increased in CaF₂ doped with SmF₃ as compared with pure CaF₂, indicating an interrelation between anion and cation defects, as observed previously in the related substances, UO₂ and UO_2+x , The migration energy of cations was estimated to be roughly in the range 1.5±0.6 eV. The implications of the results in terms of diffusion mechanisms are discussed and the remaining uncertainties are indicated.