Correlation between the Enthalpy of Melting and the Enthalpy of Anti-Frenkel Defect Formation in Homologous MF2 Fluorite Crystals

The homologous series of MF 2 (M = Ca 2+ , Sr 2+ , Ba 2+ , Cd 2+ , Pb 2+ , Sm 2+ , Eu 2+ , Yb 2+ , Ra 2+ , and Hg 2+ ) crystals having the fluorite structure (CaF 2 , space group Z = 4) holds an important place in fluoride materials science. A correlation between the enthalpy of anti-Frenkel defect formation ( H f ) and the enthalpy of melting ( H m ) was elucidated for CaF 2 , SrF 2 , BaF 2 , and PbF 2 based on the most reliable experimental data set: H f (kJ/mol) = 8.6 H m (kJ/mol) or H f (eV) = 0.089 H m (kJ/mol). Experimentally determined H m values were used to calculate the enthalpies of defect formation H f = 2.0 and 2.6 eV for CdF 2 and EuF 2 crystals, respectively. The correlation is prognostic for the whole series of homologous MF 2 fluorite crystals and for SrCl 2 .