Translational-rotational modes in noncentrosymmetric lattices: A lattice-dynamical interpretation of the phenanthrene phase transition

A lattice-dynamical study has been used to elucidate the mechanism of the phase transition in phenanthrene. The calculation is consistent with the known lattice energy and crystal structure. Combination of experimental results with the calculated phonon dispersion curves and eigenmode symmetries yields a dynamic mechanism for the transition based on increasing density of higher-frequency modes upon approach to the transition temperature to the high-temperature phase. This motion, which may be associated with motion of hydrogen atoms in the molecule, confers a more three-dimensional character to the interactions and correlates well with the observed change in heat capacity.