Knowledge Bank

In orientationally disordered crystals the molecules are on or near regular positions in space but are irregularly oriented. Because of the positional order the translational lattice vibrations of these crystals can be approximated by those of a regular crystal, and the molecular displacements during the vibrations can be regarded as spatially periodic. Because of the orientation disorder the dipole moment changes accompanying the vibrations are not spatially periodic, and consequently all of the translational vibrations can interact with radiation, and can appear in 1-phonon, or fundamental, infrared and Raman spectra. In this sense the vibrations are mechanically regular but electrically irregular. The spectra arising from disorder-allowed vibrations are thus related to the density of vibrational states weighted by an intensity factor. The translational vibrations of ice 1h and ice 1c absorb strongly between 360 and $50cm-1$. The absorption is primarily disorder-allowed and the spectra are interpreted in terms of features in the density of states curve of the appropriate phase. Some of the assignments can be fitted quite well with a short-range valence force field, but long-range forces clearly influence the translation vibrations in both phases.