Thermo-Raman spectroscopic studies on polymorphism in Na2SO4

The phase transformations involved in Na 2 SO 4 (Na 2 SO 4 10H 2 O as starting material) and predried Na 2 SO 4 in a dynamic thermal process of heating from 30 to 500 °C and then cooling down to 30 °C, with a heating/cooling rate of 2 °C min -1 in two consecutive thermal cycles, were monitored at 1 °C intervals by thermo-Raman spectroscopy (TRS). The transformations were characterized on the basis of the disappearance of the bands, appearance of new bands, splitting in the internal modes, change in the spectral profile, intensity (area) changes in the internal modes, full width at half maximum (FWHM) of the totally symmetric stretching mode and even the variations observed in the background during the thermal process. This thermo-Raman investigation provided some evidence for the existence of an intermediate phase between phase V and phase I and also for the changes observed in the phase I structure at higher temperatures, in addition to the dynamic transformations between the well known phase V, phase I, phase II and phase III. The intensity study on the 3 mode of SO 2- 4 clearly demonstrated the thermal hysteresis involved among the phases during the heating and cooling process. The differences observed in the polymorphisms of Na 2 SO 4 (Na 2 SO 4 10H 2 O as starting material) and predried Na 2 SO 4 were also discussed.