GAMMA-GRUNEISEN RELATIONS CLOSE TO LAMBDA-PHASE TRANSITIONS IN NH4CL

In this study we have correlated the frequency shifts of the acoustic modes to the volume changes by means of our gamma-Gruneisen relations near the lambda-phase transitions in NH4Cl. Using the acoustic frequen cies and the length-change data for NH4Cl from the literature, we firs t calculated the Gruneisen gamma values for the LA modes along the (10 0) and (110) directions and for the TA mode along the (110) direction. We then predicted the acoustic mode frequencies across the lambda-pha se transitions in NH4Cl. Our calculated frequencies are in good agreem ent with the observed frequencies of those acoustic modes studied for the NH4Cl crystal. This indicates that our gamma-Gruneisen relations a re adequate for predicting the observed behaviour of NH4Cl near its la mbda-phase transitions.