Effect of Hydrostatic Pressure on the Phase Transitions of Langbeinite-Type Crystals

Abstract

Effect of hydrostatic pressure up to 9 kbar has been studied by dielectric measurements on the six langbeinite-type crystals, Tl 2 Cd 2 (SO 4 ) 3 , Rb 2 Cd 2 (SO 4 ) 3 , K 2 Zn 2 (SO 4 ) 3 , K 2 Mn 2 (SO 4 ) 3 , Tl 2 Mn 2 (SO 4 ) 3 and (NH 4 ) 2 Mn 2 (SO 4 ) 3 . The pressure-temperature phase diagrams were deternained. In the former two crystals, Tl 2 Cd 2 (SO 4 ) 3 and Rb 2 Cd 2 (SO 4 ) 3 , a new phase was found between the P1 and P2 1 2 1 2 1 phases at high pressure. In the crystal K 2 Zn 2 (SO 4 ) 3 another phase appeared between the P2 1 3 and P2 1 phases, and the behavior is similar to that in (NH 4 ) 2 Cd 2 (SO 4 ) 3 . The P2 1 3→P2 1 2 1 2 1 transition temperature in K 2 Mn 2 (SO 4 ) 3 linearly rises with increasing pressure. The last two crystals, Tl 2 Mn 2 (SO 4 ) 3 and (NH 4 ) 2 Mn 2 (SO 4 ) 3 do not show any transition down to -196°C even under high pressure studied.