Temperature dependence of the Raman bandwidths for the lattice and internal modes in ammonium halides close to the λ-phase transitions

Our Raman bandwidths of those modes are studied using a power-law formula on the basis of the soft mode-hard mode coupled model. From our analysis, we extract the values of the critical exponent β for the order parameter of the ferro-ordered (NH4Cl) and the antiferro-ordered (NH4Br) phases. Our β values are close to zero which exhibits a logarithmic behaviour in the vicinity of the λ-phase transitions in NH4Cl and NH4Br. It is indicated here that the ammonium halides (NH4Cl and NH4Br) undergo a weakly first order or nearly second order phase transition close to the λ transition point.