A Statistical Theory of the Phase Transition of Squaric Acid (H₂C₄O₄) Crystal

Abstract

An order-disorder theory of the tetragonal-monoclinic phase transition of squaric acid (H₂C₄O₄) crystal is developed where the ice rule of proton configurations on hydrogen-bonds is respected. It is shown that the model adopted previously for the antiferroelectric transition of NH₄H₂PO₄ can be applied, only with a slight modification, and it gives rise to a first order transition. The coupling between the transition parameters and the strain component affects the order of the transition.