Crystal Structure Transformations of Alkali Sulphates, Nitrates and Related Substances: Thermal Hysteresis in Reversible Transformations

Reversible phase transformations of alkali sulphates, alkali nitrates, and various other inorganic substances have been studied by making use of differential thermal analysis. Thermodynamic and kinetic data on the transformations have been obtained. Thermal hysteresis in reversible transformations has been examined, and the magnitude of hysteresis is shown to be related to the volume changes accompanying the transformations. The origin of hysteresis probably lies in the strain energies associated with the transformations. Approximate strain energies have been estimated from the analysis of the DTA data. On the basis of considerations from the theory of elasticity, it is possible to show that the strain energy is a function of Delta V. Thermodynamic considerations show that the strain energy is related to Delta T x Delta S.