The reversible phase transformation of potassium sulphate is studied. Differential thermal analysis, dilatometric and dielectric measurements show the occurrence of a reversible phase transition at 571 °C for K2SO4 with heat of transformation H = 2.3 kcal/mol and activation energy E = −215 kcal/mol. The thermal hysteresis in this reversible transformation is examined, the magnitude of the temperature hysteresis does not exceed 5 K. An acceptable agreement is observed between the measured values of the transition temperature obtained by the three different techniques. The dilatometric analysis shows that this transition is accompanied by thermal expansion with a relative thermal expansion coefficient of 6 × 10−3. The mechanism and physical basis for the phenomenon of thermal hysteresis observed in the temperature of the phase transition are proposed and discussed thermodynamically on the assumption that there is a potential barrier between the atomic arrangements of any two phases.

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Some electrical parameters are investigated as a function of temperature (40 °C < T < 120 °C) for single crystals and compressed powder samples of ammonium persulphate (APS), (NH4)2S2O8. The results indicate that APS undergoes a structural phase transition in the vicinity of 90 °C. The DTA thermogram is carried out in the same temperature range as a final confirmation of the observed phase transition. The data are correlated to the reorientational motion of the NH+4 and/or SO−−4 ions as well as the change in the hydrogen bonding system (N‐H … O).