The Phase Transition of Potassium Sulfate

The discontinuous phase transition of K 2 S 0 4 is described by a semi-empirical model that uses the elastic stiffnesses cn and the changes in volume and entropy associated with the order-disorder transition. Consideration of the elastic anomaly (existence of soft, damped longitudinal acoustic phonons) in K 2 S 0 4 removes the disagreement between transition enthalpies AHlr measured calorimetrically and AHtr derived from the p-T phase diagram via the Clausius-Clapeyron equation. The model used requires the knowledge of the cu (i = 1, 2, 3) which in many cases is not available in the literature. It is demonstrated that these values can be calculated with sufficient accuracy from the anisotropic displacement (Debye-Waller) factors which are routinely determined in crystal structure analyses and which, therefore, are available for many substances. There is fair agreement between the calculated strain energy and AHlr also for K 2 S e 0 4 . In the case of K 2 C r 0 4 , use of published expansion data resulted in disagreement between calculated strain energy values and published, measured AH,r data.