The structural phase change in s-triazine: the quasiharmonic approximation

Abstract

A molecular crystal undergoing an elastic phase transition may exhibit rotation of the molecules as well as shear of the unit cell. It is shown that the shear strain is the correct order parameter for such a transition and that the molecular rotation angle need not be exactly proportional to the shear angle. Using the quasiharmonic approximation and the temperature dependence of the experimental Raman frequencies, a model for the phase transition in s-triazine is developed that accurately predicts the temperature dependence of the order parameter and molecular rotation angle over a wide temperature range, and provides a reasonably satisfactory account of other physical properties associated with the transition.