Thermal‐prehistory dependence of the phase stability in the structural phase transitions of Na 2 SO 4 crystal

After as‐grown Na 2 SO 4 (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 II occurs within a very narrow temperature range of 2–3 °C and the transition temperature of II → III has been known to be much reduced during subsequent cooling scans. Here, we 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 °C. We also found that if the Na 2 SO 4 (I), which is just transformed from the phase V, is heated above 600 °C, then all the peculiarities on transitions surprisingly disappear. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)