Microscopic theory of phase transitions in hydrogen-bonded phenol-amine adducts

Second-order reversible ferroelastic phase transitions in a recently discovered class of hydrogen-bonded phenol-amine adducts has already been analyzed by Landau theory. The analysis is however phenomenological and does not directly indicate the microscopic origin of this phase transition. In this paper, a microscopic theory is presented. It is proposed that the main mechanism responsible for the phase transition is the interaction of hydrogen bonds with the lattice vibrations or phonons of the crystal. These interactions with the phonons induce long range cooperative interactions between the hydrogen bonds, which causes the phase transition behavior at the critical temperature. Critical exponents for unit cell parameters and beat capacity are derived with a variational meanfield approach, and shown to be consistent with the prediction of Landau's theory.