Cluster Theory of Phase Transition in Squaric Acid. I: Formulation and General Discussion

Abstract

In view of the importance of the ice rule imposed on the proton configurations in hydrogen bonded crystals, a cluster approximation is developed and applied to the phase transition in squaric acid. A particular emphasis is laid on the fact that in squaric acid the strong coupling between the lattice distortion and proton system is important and two order parameters are necessary to account for the twin structure in the low temperature phase. It is shown that by including the antiferroelectric inter-layer interaction the quasi two dimensional proton net-work in squaric acid may exhibit a phase transition of the first order or second order depending on the model parameters.