emperature dependence of the Raman frequencies and bandwidths close to phase transitions in ammonium halides

In this study, we give the temperature dependence of our observed frequencies and bandwidths for the Raman optical modes in the ammonium halides close to the phase transitions of the first order (NH4Br), tricritical (NH4Cl) and second order (NH4Cl). Using the predictions of an Ising pseudospin-phonon coupled model, which considers interactions between two spin and two phonons, our observed Raman data have been interpreted qualitatively. Our results show that an Ising model considered here can explain the observed behaviour of the Raman phonons near the phase transitions of the first order, tricritical and second order in the ammonium halides.