Low-temperature peculiarities of thermal properties of EuF2.16 europium fluoride and (SrF2)0.5(EuF2.16)0.5 solid solution

Abstract

The heat capacity and lattice parameters of europium fluoride EuF2.16 and (SrF2)0.5(EuF2.16)0.5 solid solution are experimentally investigated at temperatures 2–300 K. The revealed anomalies in the heat capacity are due to magnetic phase transition and disorder in fluorides crystal structures. The dependencies studied were analyzed using the Debye–Einstein approximation, taking into account the influence of the magnetic phase transition, two-level systems, and anharmonicity of lattice vibrations on their thermodynamic properties. We determined the parameters of the Debye and Einstein models – the characteristic temperatures as well as the magnitudes their respective contributions to the overall thermodynamic characteristics, the parameters characterizing crystal structure disorder, and the exchange parameters characterizing interaction in magnetic subsystems of fluorides. Dependences studied for solid solution deviate from the additive values. We further established that the mechanisms causing these are based on the disorder in the crystal structures of europium fluoride and its solid solution.