Phase transitions in as-grown K0.2Rb1.8Cd2(SO4)3 langbeinite crystals: a birefringence study

Basing on temperature dependences of the light intensity transmitted through polarisation system and the birefringence of K 0.2 Rb 1.8 Cd 2 (SO 4 ) 3 crystals, we have determined the phase transition temperature for this solid solution.A conclusion is drawn that the monoclinic and triclinic phases on the (x, T)-phase diagram for the mixed crystals K 2x Rb 2(1-x) Cd 2 (SO 4 ) 3 should disappear at the triple points occurring at x < 0.1.