Thermal, optical, and dielectric properties of fluoride Rb2TaF7

Abstract

The thermal, optical, and dielectric properties of fluoride Rb₂TaF₇ were investigated. It was observed that the variation in chemical pressure in fluorides A₂⁺TaF₇ caused by the cation substitution of rubidium for ammonium does not affect the ferroelastic nature of structural distortions, but leads to stabilization of the high- and low-temperature phases and enhancement of birefringence. The entropy of the phase transition P4/_nmm ↔ Cmma is typical of the shift transformations, which is consistent with a model of the initial and distorted phase structures. The anisotropy of chemical pressure causes the change of signs of the anomalous strain and baric coefficient dT/dp of Rb₂TaF₇ as compared with the values for its ammonium analog.