Thermal-prehistory dependence of the phase stability in the structural phase transitions of Na2SO4 crystal

After as-grown Na2SO4 (V) crystal is transformed to high-temperature phase (I), the transition sequence is I -> II -> III on cooling and III -> I on reheating scan. The intermediate phase 11 occurs within a very narrow temperature range of 2-3 degrees C and the transition temperature of II -> III has been known to be much reduced during subsequent cooling scans. Here, tare found that all the transitions I -> II -> III as well as III -> I depend greatly on the thermal prehistory. This is possibly related to an incompleteness of the transition from phase V to I near 240 degrees C. We also found that if the Na2SO4 (I), which is just transformed from the phase V, is heated above 600 degrees C, then all the peculiarities on transitions surprisingly disappear. (c) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.