Specific features of the lattice dynamics of CaxSr1-x F2 solid solutions

Abstract

The temperature changes in the heat capacity Cp(T) and lattice parameters a(T) of solid solutions of CaxSr1-x F2 are experimentally studied at temperatures of 2–300 K for x = 0, 0.25, 0.5, 0.75 and 1. Deviations from the Kopp-Neumann rule (Vegard's rule) in the values of Cp(T) and a(T) for these solutions are identified.The experimental dependencies of Cp(T) and a(T) for end-member crystals and their solid solutions are analysed using the Debye-Einstein model, taking into account the effects of anharmonicity of lattice vibrations at elevated temperatures, and the parameters of this approximation are determined. An anomalous arrangement of the curves for Cp(T) and a(T) is revealed for these solutions, relative to the corresponding dependence for end-member crystals. Despite cationic substitution taking place during the formation of solid solutions of the system under study, restructuring of the crystal structure mainly affects the nature of the oscillations of the anion sublattice. Most affected by the anharmonicity are the Einstein modes, corresponding to the vibrations of the F− ions, and the coupled vibrations of two different fluorine sublattices.