The Phase Transition of Potassium Sulfate

Abstract

AbstractThe discontinuous phase transition of K2SO4 is described by a semi-empirical model that uses the elastic stiffnesses ci,i 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 K2SO4 removes the disagreement between transition enthalpies ΔHtr measured calorimetrically and ΔHtr 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 ΔHtr also for K2SeO4 . In the case of K2CrO4 , use of published expansion data resulted in disagreement between calculated strain energy values and published, measured ΔHtr data.