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Critical behaviour of the specific heat calculated using the Raman frequencies of the lattice and internal modes near the lambda-phase transition in NH4Br

We calculate here the specific heat of NH4Br using our Raman frequency shifts for the lattice mode Of nu(7) TA (56 cm(-1)) and the internal mode Of nu(2) (1684 cm(-1)) near its lambda-phase transition (T-lambda = 234 K, P = 0). By analyzing our Raman frequency shifts, values of alpha = 0. 19 (T < T-lambda, and T > T-lambda) for the lattice mode, and alpha = 0.45 (T < T-lambda) and alpha = 0.57 (T > T-lambda) for the internal mode, are used as the values of the critical exponent for the specific heat to predict its critical behaviour based on an Ising model near the lambda-phase transition in NH4Br. The specific heat which we calculate here using our frequencies of both Raman modes, agrees well with the observed specific heat Cp near the lambda-phase transition in NH4Br. Some discrepancy occurring in the specific heat above T-lambda, due to the lattice mode is explained within the framework of an Ising pseudospin-phonon coupled model for NH4Br.