The λ-phase transitions in NH4Cl

Abstract

Abstract We show here the correlations between the volume changes and the vibrational frequencies of the disorder-induced Raman modes of NH4Cl near the λ-phase transitions of first order (P=0), tricritical (P⋍1.6 kbar) and second order (P⋍2.8 kbar). We obtain the values of the mode Gruneisen parameter for each phonon across the λ-phase transitions and use them to predict the Raman frequencies for the phonon modes studied as functions of temperature in the λ-phase change regions. Our calculated frequencies for the M-point phonons of v7 (93 cm−1) and v5 (144 cm−1) of NH4Cl are in good agreement with our observed Raman frequencies. This shows that the method employed here is satisfactory for predicting the Raman frequencies from the volume changes near the λ-phase transitions in the NH4Cl system.