THE STABILIZATION OF PHASE-III AND PHASE-I IN SODIUM-SULFATE BY ALIOVALENT CATION SUBSTITUTION

Both phase I and III in Na2SO4 have been stabilized at room temperature by substitution with the sulfate salts of the aliovalent cations Y3+, La3+, Dy3+, Ce4+, and Ca2+. Phase stabilization has been studied using laser Raman spectroscopy to measure the internal optic modes of the sulfate anion. Phase III is stabilized by a few tenths of a mole percent of the substituent, while phase I is stabilized by roughly ten times that concentration. Stabilization of both phase I and phase III does not appear to depend significantly on the charge of the guest ion. The enthalpy and entropy of the phase III to phase I transition was found to decrease markedly with increasing substituent cation concentration.