Analysis of the Raman intensities near the phase transitions in ammonium halides

This study concentrates on the temperature dependence of the Raman intensities for the lattice modes in ammonium halides (NH4Cl and NH4Br) close to phase transitions. We predict their intensities using the results of a shell model for the Raman polarizability within the framework of an Ising pseudospin–phonon coupled model.From our observed Raman intensities of those phonon modes studied here, we extract the values of the critical exponent for the order parameter in these crystalline systems. The exponent values indicate that the Raman intensities show a logarithmic divergence at higher pressures in NH4Cl, whereas they predict a λ-type phase transition at zero pressure in NH4Br.