The lambda-phase transitions in NH4Cl

We show here the correlations between the volume changes and the vibrational frequencies of the disorder-induced Raman modes of NH4Cl near the lambda-phase transitions of first order (P=0), tricritical (P similar or equal to 1.6 kbar) and second order (P similar or equal to 2.8 kbar). We obtain the values of the mode Gruneisen parameter for each phonon across the lambda-phase transitions and use them to predict the Raman frequencies for the phonon modes studied as functions of temperature in the lambda-phase change regions. Our calculated frequencies for the M-point phonons of upsilon(7) (93 cm(-1)) and upsilon(5) (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 lambda-phase transitions in the NH4Cl system.