Structures and Phase Transitions of Na₂SO₄

Sodium sulfate, Na₂SO₄, has been reported to exist in five polymorphous forms labelled I-V. Structure analysis has been reported on three of the polymorphs and thermal analysis on all of them. The system has been reinvestigated by differential scanning calorimetry (DSC) and by X-ray powder diffraction at several temperatures in order to examine the Na₂SO₄ system with the same batch of material in the same laboratory by two independent methods. The X-ray results confirm the existence of the crystalline phases labelled I, II, III and V but not IV. The DSC results do not entirely rule out the existence of five phases. The structure of phase II has been determined and the structures of all phases have been refined by the Rietveld method.