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

We study here the temperature dependence of our Raman bandwidths for the upsilon(5) lattice modes of NH(4)Cl (T(lambda)=241 K) and NH(4)Br (T(lambda)=235 K) and for the upsilon(2) internal mode of NH(4)Cl close to the lambda-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 beta for the order parameter of the ferro-ordered (NH(4)Cl) and the antiferro-ordered (NH(4)Br) phases. Our beta values are close to zero which exhibits a logarithmic behaviour in the vicinity of the lambda-phase transitions in NH(4)Cl and NH(4)Br. It is indicated here that the ammonium halides (NH(4)Cl and NH(4)Br) undergo a weakly first order or nearly second order phase transition close to the lambda transition point.