Optical investigations of the effect of gradual substitution NH4 → Cs on the ferroelastic phase transition in a CsLiSO4 crystal

Abstract

Crystals of Csx(NH4)1−xLiSO4 (0.39≤x≤1.0) solid solutions are grown and investigated using polarized light microscopy and measurements of the birefringence in the temperature range 100–530 K. The (x-T) phase diagram of the Csx(NH4)1−xLiSO4 solid solutions is constructed. It is demonstrated that, upon substitution of ammonium for cesium in the CsLiSO4 crystal, the phase transition temperature gradually increases to such a degree that the ferroelastic phase can exist at room temperature. The triple point of intersection of the Pmcn, P21cn, and P1121/n phase boundaries is determined. It is established that the introduction of ammonium in small amounts has an unusually strong effect on the refractive properties and character of the ferroelastic phase transition in the CsLiSO4 crystal.