Phonon Symmetry Selection Rules for Inelastic Neutron Scattering

A theorem is proven demonstrating the existence of phonon symmetry selection rules, independent of particular structural features, for coherent inelastic neutron scattering by crystals. The resulting systematic absences depend only on the mode symmetry and the Brillouin zone where the measurement takes place. Several examples show their power for identifying the symmetries of measured phonon branches. Despite their importance and simplicity, these structure-independent extinction rules, based only on symmetry arguments, have, to our knowledge, never been formulated and are not currently considered in the analysis of phonon scattering data.