A new theoretical model for the re-entrant phase transitions in malononitrile

Abstract

Malononitrile undergoes a series of 're-entrant' phase transitions from a low-temperature monoclinic phase to an intermediate triclinic phase and, finally, to a high-temperature monoclinic phase whose structure is nearly identical to that of the low-temperature phase. A model is suggested for this system which is based on the interaction between the order parameter associated with the transitions and strains in the unit cell caused by thermal expansion. The energy is calculated as a function of temperature on the basis of this model, using atom-atom '6-exp' potentials along with long-range dipole-dipole forces. The model produces values of the coupling coefficients that agree satisfactorily with values derived from the experimental data and predicts re-entrant behaviour quite similar to that observed experimentally.