DSC Study on the Heat of III-I Transition in the Undoped and Yttrium-Doped Na₂SO₄

Abstract

The phase transition in Na₂SO₄ was studied by differential scanning calorimetry (DSC). In the undoped Na₂SO₄, the enthalpy changes of the III→I, I→II and II→III transitions were 6.50, -4.85 and -2.15kJmol^-1, respectively. When the reheating was started in the course of the II→III transition, the absolute value of the enthalpy change of the II→I transition increased with an increase in the starting temperature of the reheating, whereas that of the III→I transition decreased. The doping of yttrium up to about 0.7 at.% decreased the transition temperature and the enthalpy changes of the III→I transition. The entropy change of the II→I transition in the undoped Na₂SO₄ was found to be 8.30Jmol^-1K^-1 from the calculation with the enthalpy change and the transition temperature. It suggested that the transition was characterized by the configurational change of 3.