Low-temperature anomalies of thermodynamic properties of lanthanum trifluoride LaF3 and (SrF2)0.5 (LaF3)0.5 multivalent solid solution

Abstract

Temperature dependences of the heat capacity Cp(T) (2–300 K) and lattice parameter a(T) (5–300 K) of LaF3 and (SrF2)0.5(LaF3)0.5, in addition to the earlier studied SrF2, are experimentally investigated for the first time. The experimental dependences of Cp(T) and the unit cell volume V(T) of the studied fluorides are analysed using the Debye–Einstein model considering the influence of the disorder of their crystal structures. The parameters of the model are determined. An anomalous arrangement of the Cp(T) and V(T) curves of the solution with respect to the corresponding curves of the components is revealed. At low temperatures (T < 150 K), the values of Cp(T) and V(T) of the solution exceed the values calculated by the additivity rule. At higher temperatures (T > 150 K), deviations towards smaller values are observed. We have established that the disorder in solid-solution crystal structure is the common reason of anomalies revealed. It is found that at low temperatures, the cause of these anomalies is a decrease in the vibrational frequencies in the solid-solution metal sublattice upon the replacement of Sr with La, which has a large size and mass. Such a replacement leads to distortions of the original structure and an increase in its disorder. At elevated temperatures, the deviations are caused by an increase in the vibrational frequencies in the F- sublattice due to distortion of the crystal structure, as well as the increased influence of the anharmonicity of the disordered crystal lattice vibration.