Thermal, optical, and dielectric properties of fluoride Rb$_{2}$TaF$_{7}$

The thermal, optical, and dielectric properties of fluoride Rb$_{2}$TaF$_{7}$ were investigated. It was observed that the variation in chemical pressure in fluorides $A^{+}_{2}$TaF$_{7}$ 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 $P$4/$nmm\leftrightarrow 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 Rb$_{2}$TaF$_{7}$ as compared with the values for its ammonium analog.