Optical Phonons and Phase Transitions in the Ammonium Halides

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Abstract
The external optical phonon modes in the ammonium and deuteroammonium halides have been studied through their intrinsic phase transitions in the temperature range 4–290°K using infrared and Raman techniques. The number of infrared and Raman active modes are in agreement with group theoretical predictions for the phases II, III, and IV, with the exception that no measurable anisotropic splitting of the out‐of‐phase mode in the tetragonal phase was observed in the infrared spectra. Whilst the frequency of the infrared reststrahlen modes showed only small discontinuities with temperature as the phase transitions were traversed, the phonon lifetimes of these modes showed distinct increases as the temperature was lowered through each phase transition. Estimates of the intrinsic ionic distortion present in NH4Cl and NH4Br are given. The compressibilities of NH4Cl and NH4Br at 4°K are calculated to be 3.86 × 10−12 and 6.79 × 10−12 cm2 dyn−1.