High‐pressure phonon behavior in ammonium halides

This note comments on the paper ‘High‐pressure Raman Study of Ammonium Halides’ [S.‐J. Jeon, R. F. Porter, and A. L. Ruoff, J. Raman Spectrosc. 19, 179 (1988)]. It is shown that their proposed degeneracy of ν 1 and ν 3 modes at high pressure is symmetry‐forbidden, but that a near degeneracy follows logically from the Urey–Bradley force model and plausible assumptions about the pressure dependence of force constants.