The metastable room-temperature phase III of Na2SO4 crystal transforms from III-->I on heating and I-->II-->III on cooling. Phase II exists in a narrow temperature interval (less than 7 degrees 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(SO4)(1-x)(SeO4)(x) and Na2(1-x)Ag2xSO4 with x less than or equal to 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 Na1.8Ag0.2SO4 phase II persists as wide as similar to 59 degrees C. On heating, phase II, which is otherwise forbidden on heating in pure Na2SO4 crystal, reappears when x is large.