Abstract
(T − x) phase diagram has been investigated by polarizing optic and birefringence methods, differential scanning calorimetry and X-ray study. A small amount of rubidium in CsLiSO4 as well as cesium in RbLiSO4 reduces the temperature of transition into ferroelastic phase P1121/n which disappear for x ≈ 0.15 and x ≈ 0.9. The region of phase P21/c11 (c = 2c 0) expands and becomes the most stable below ambient temperature for 0.9 > x > 0.2.