Lattice Modes of Molecular Crystals

Abstract

The lattice vibrations of molecular crystals arise from the translational and rotational motions of the molecules, which are unhindered motions in the gas phase. They are intermolecular vibrations and are sometimes called external vibrations, while the vibrations that exist in the gas phase are intramolecular vibrations and are sometimes called internal vibrations. The lattice vibrations can be usefully thought of as translational vibrations and rotational vibrations, although the two types of motion often contribute to the same vibration. Translational and rotational vibrations are sometimes called translatory and rotatory vibrations, and rotational vibrations are frequently called librations.