Group theoretical classification of halogen nuclear quadrupole resonance spectra of distorted tetragonal antifluorite crystals

Abstract

A classification of the patterns of halogen nuclear quadrupole resonance lines resulting from distortions of the tetragonal R 2MX 6 structures C 4 h 5( I4/ m) and D 4 h 6( P4/ mnc) is given. Formal methods of group thepry are used to identify low symmetry space groups with the symmetries of vectors in an irreducible representation space of the high symmetry space group. Classification charts are presented which permit the identification of those space groups and primary order parameters which are compatible with particular halogen nuclear quadrupole resonance spectra. The discussion is limited to structural phase transitions involving the points of symmetry in the Brillouin zones of the high temperature phases. The sequences of structural transitions in K 2ReCl 6 and (NH 4) 2SnBr 6 are discussed as illustrations of the use of these charts.