CONTINUITY OF PHONON-SPECTRA FOR DIAMOND AND HEXAGONAL CLOSE-PACKED STRUCTURES

Band representations of space groups are used for showing that crystals with diamond or hexagonal close packed structures have phonon spectra that extend from zero to some maximal frequency without a gap in between. This gapless feature follows from the vectorial nature of lattice vibrations, from the continuity that is introduced by band representations, and from the symmetry of Bloch functions for nonsymmorphic space groups. Some other space groups having two identical atoms in a unit cell and leading to the same continuity feature for their phonon spectrum are listed. © 1995 The American Physical Society.