The lattice dynamics of the deuterium peroxide crystal

The phonons propagating along the unique fourfold axis in the D2O2 crystal have been measured by coherent inelastic neutron scattering. The measurements covered all branches below 250 cm−1 (31 meV), which include all the motions of the rigid molecule, except rotation about the minor axis (close to the O–O axis of the molecule). The sample consisted of a single crystal of 99% isotopically pure D2O2 in the form of a cylinder 1.5 cm diam and 4 cm long. Group-theoretical selection rules for the observation of the phonons were calculated and used extensively as an aid in the classification and measurement of the phonons. Agreement with previously observed Γ point modes in infrared absorption and Raman scattering was fairly good, but necessitated reassignment of one E mode. A covalent force model, developed in an attempt to reproduce the observations, was only partly successful. The model served to describe the character of the phonon as librational or translational. It is reasonably consistent with the observed intensities of the phonons. The limit success of the model indicates the need for longer range forces than those described as stretching and bending of hydrogen bonds.