Theory of Phase Transition in Squaric Acid

Abstract

Phase transition in squaric acid is discussed on the basis of a pscudospin model for the hydrogen bond system. A strong coupling between the lattice distortion and proton system is introduced in order to account for the twin structure and improper ferroelastic behaviour in the low temperature phase. Anomalous elastic constant and electric susceptibility are calculated and compared with experiments. Isotope effect in the transition temperature is discussed by taking account of the tunneling motion of proton.