Ionic conduction and structural phase transitions of Na 2SO 4 doped with various impurities

Abstract

The metastable room-temperature phase III of Na 2SO 4 crystal transforms from III→I on heating and I→II→III on cooling. Phase II exists in a narrow temperature interval (less than 7°C). To obtain better microscopic understandings about the existence and nature of the intermediate phase II, electrical conductivity and Raman spectrum were measured for mixed crystals of Na 2(SO 4) 1− x (SeO 4) x and Na 2(1− x) Ag 2 x SO 4 with x≤0.1. On cooling, the intermediate phase II is stabilized in a wider temperature interval with the incorporation of guest ions. In the case of Na 1.8Ag 0.2SO 4, phase II persists as wide as ∼59°C. On heating, phase II, which is otherwise forbidden on heating in pure Na 2SO 4 crystal, reappears when x is large.