Mean field statistical mechanics of model Hamiltonian for hydrogen bonded phase transitions

Abstract

Recently, a new prototype model has been proposed to explain structural phase transitions in single crystals of organic hydrogen bonded phenol–amine adducts. A variational calculation of the thermodynamic properties was able to explain the main features of the phase transition. We report an alternative statistical mechanical calculation which confirms the results of the earlier calculation even though self‐energy terms appear in the present calculation. The interpretation of the results within the pseudo spin representation is clarified further. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Phase Transition in Hydrogen-Bonded 1-Adamantane-methanol

“…Within this image the decrease of the order parameter on increasing temperature (Figure ) would be accompanied by an increase of the “symmetrization” of the occupational factor of the H atom of the hydroxyl group up to equal values (50%) at T = T c , the temperature at which the equality of the occupational factors (together with the value of β = 90°) makes the appearance of the symmetry plane and thus leading to the lattice the Pnnm symmetry. Second-order phase transitions involving organic hydrogen-bonded crystals have recently been described, − and even a Landau theory using an expansion of the free energy has been proposed. , …”