Sodium Sulfate Polymorphism

The crystal structure of anhydrous sodium sulfate purified by double recrystallization from an aqueous solution was investigated by thermal analysis and X-ray phase analysis in the temperature range from 25°C to 300°C. When heated at a rate of 2–5 K/min, Na 2 SO 4 undergoes a first-order phase transition (onset at 240°C) from an orthorhombic structure (space group Fddd , phase V) to a hexagonal structure (space group P 6 3 / mmc , phase I). However, lines of orthorhombic phase III (space group Cmcm ) appear on the X-ray pattern at 225°C; phase III coexists with phases V and I at 240°C.