Anomalous Thermoelastic Behavior and Molecular Reorientations in Crystals

Abstract

We have derived a microscopic Hamiltonian that takes into account rotational-translational coupling in molecular crystals. This coupling leads to an effective orientational interaction among nonspherical molecules. It is responsible for an anomalous behavior of the elastic constants and of the transverse-acoustic phonons. The temperature behavior of $c_{44}$ and $c_{11}$ in KCN above the 168-K phase transition is explained.