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THE INFLUENCE OF DISORDER ON THE GROUP-THEORETICALLY DERIVED SELECTION-RULES IN AMMONIUM HALIDES

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 (Tc) 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 Tc will be discussed. The self-consistancy of the various possible approaches to this problem will be shown.