THE LAMBDA-PHASE TRANSITION IN NH4BR

This study gives the predicted frequencies for the Raman modes of v(7) TA(56 cm(-1)), v(5)LO(177 cm(-1)) and v(2)(1684 cm(-1)) of NH4Br at va rious temperatures for zero pressure across the lambda-phase transitio n in this crystal system. We calculate the Raman frequencies by means of a constant mode Gruneisen parameter for each mode across the lambda -phase change region of NH4Br. Our calculated frequencies are in good agreement with those observed experimentally. This indicates that this form of analysis for predicting frequencies using a single-mode Grune isen parameter is adequate to describe the observed features of lattic e vibrations near the lambda-phase transition in NH4Br.