The influence of disorder on the group-theoretically derived selection rules in ammonium halides

Abstract

Abstract There is no significant disagreement about the basic selection rules operating in the ferro-ordered (δ) and antiferro-ordered (γ) phases of the ammonium halides. The effects of residual disorder just below the transition temperature (T c ) are still treated differently by different research groups and therefore this remains a subject of some interest. In this study it will be shown how our approaches lead to assignments of the modes observed in the disordered β - phase, Raman spectrum at temperatures just below T c will be discussed. The self-consistancy of the various possible approaches to this problem will be shown.