THE LAMBDA-PHASE TRANSITIONS IN NH4CL

We show here the correlations between the volume changes and the vibra tional frequencies of the disorder-induced Raman modes of NH4Cl near t he lambda-phase transitions of first order (P=0), tricritical (P simil ar or equal to 1.6 kbar) and second order (P similar or equal to 2.8 k bar). We obtain the values of the mode Gruneisen parameter for each ph onon across the lambda-phase transitions and use them to predict the R aman frequencies for the phonon modes studied as functions of temperat ure 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 frequencie s. This shows that the method employed here is satisfactory for predic ting the Raman frequencies from the volume changes near the lambda-pha se transitions in the NH4Cl system.