Temperature dependence of the damping constant and the order parameter close to the λ phase transitions in ammonium halides

This study gives our calculation for the damping constant, using the expressions derived for an Ising pseudospin–phonon coupled system in the ammonium halides (NH4Cl and NH4Br). For this calculation of the damping constant, we use the temperature dependence of the order parameter calculated from the molecular field theory. We predict here the damping constants for the v5 (174 cm−1) and v5 (177 cm−1) Raman modes of NH4Cl and NH4Br, respectively, below Tλ and compare them with our observed bandwidths measured as a function of temperature for those phonon modes at zero pressure.Our predictions agree well with the observed bandwidths below Tλ for those modes in both ammonium halide structures, in particular for NH4Br. Some discrepancy that occurs below Tλ for the v5 (174 cm−1) mode of NH4Cl, is explained in terms of the model studied here.