Quasi-elastic light scattering at the order-disorder phase transitions of NH4Cl and NH4Br

Abstract

The authors report measurements of quasi-elastic light scattering associated with ammonium reorientation at the order-disorder phase transitions in NH₄Cl and NH₄Br as a function of temperature and hydrostatic pressure. NH₄Cl and NH₄Br show respectively 'ferro' and 'antiferro' ammonium order at low temperatures and zero pressure. The scattering shows critical divergence in NH₄Br where the soft mode is associated with the M points of the Brillouin zone but not in NH₄Cl where the soft mode is at the Gamma point. A theory of the quasi-elastic scattering cross section is developed in which the main scattering mechanism is a second-order process involving two ammonium reorientational excitations of equal and opposite wavevectors and the dominant contributions are from M points of the zone. This is an extension of theories that successfully predict the disorder-induced Raman spectra and it gives a good description of the main features of the quasi-elastic scattering.