Selection rules for defect-activated lattice bands and vibronic transitions in face-centred cubic, diamond and zinc blende lattices

The presence of a single substitutional defect in an otherwise perfect crystal lattice reduces the symmetry group of the system from the crystal space group to the point group of a lattice site. The corresponding reduction of the space-group irreducible representations is tabulated for face-centred cubic, diamond and zinc blende lattices. The tables allow the easy determination of selection rules for defect-activated lattice bands. It is shown that all phonons can be defect activated for face-centred cubic and diamond lattices, but that some selection rules occur for rocksalt, calcium fluoride and zinc blende lattices. In the case of vibronic transitions, the selection rules for rocksalt and calcium fluoride are discussed. In favourable cases, both types of measurement lead to a knowledge of the phonon critical-point frequencies in a more direct way than measurement of lattice combination bands. The role of localized vibrations in vibronic spectra is briefly considered.