Anomalous thermoelastic behavior and molecular reorientations in crystals

Starting from a repulsive overlap potential, we derive a microscopic Hamiltonian that takes into account single particle orientational potential effects and rotational–translational coupling in molecular crystals. This coupling leads to an effective orientational interaction between nonspherical molecules. Theoretical expressions for the elastic constants in the presence of this interaction are derived. The anomalous thermoelastic behavior which was discovered by Haussühl for c44 and c11 in KCN and NaCN, and found by Rowe et al. in neutron measurements of TA phonons, is explained. The qualitative and quantitative agreement between theory and experiment is convincing.