Concentration effects on defect stability in calcium fluoride

Abstract

The results of preliminary investigations of defect stability in CaF₂ are reported. A static-lattice simulation program based on a simulation box of ca. 100 atoms with a periodic boundary condition and a modified Newton–Raphson minimization technique was employed. As the defect concentration approaches zero the energy calculated under conditions of constant volume asymptotically approaches that calculated using Mott–Littleton techniques. The present calculations were performed in both the constant-volume and zero-strain approximations. Calcium fluoride is well characterized and is an excellent system in which the model may be tested. The calculated defect energies were found to depend markedly on concentration. This observation has obvious implications for models which are based on defect energies calculated at infinite dilution and which are then used to describe phenomena that occur at finite concentrations. However, the general trends predicted for various impurity ions associated with defects remain the same as those calculated using the HADES code.