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

The homologous series of MF2 (M = Ca2+, Sr2+, Ba2+, Cd2+, Pb2+, Sm2+, Eu2+, Yb2+, Ra2+, and Hg2+) crystals having the fluorite structure (CaF2, space group $$Fm\bar {3}m,$$ Z = 4) holds an important place in fluoride materials science. A correlation between the enthalpy of anti-Frenkel defect formation (Hf) and the enthalpy of melting (Hm) was elucidated for CaF2, SrF2, BaF2, and PbF2 based on the most reliable experimental data set: Hf (kJ/mol) = 8.6Hm (kJ/mol) or Hf (eV) = 0.089Hm (kJ/mol). Experimentally determined Hm values were used to calculate the enthalpies of defect formation Hf = 2.0 and 2.6 eV for CdF2 and EuF2 crystals, respectively. The correlation is prognostic for the whole series of homologous MF2 fluorite crystals and for SrCl2.