Thermal, optical, and dielectric properties of fluoride [Rb.sub.2]Ta[F.sub.7]

The thermal, optical, and dielectric properties of fluoride [Rb.sub.2]Ta[F.sub.7] were investigated. It was observed that the variation in chemical pressure in fluorides A+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 [left right arrow] 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.sub.2]Ta[F.sub.7] as compared with the values for its ammonium analog.