Critical behaviour of the specific heat calculated using the Raman frequencies of the lattice and internal modes near the λ-phase transition in NH 4Br

Abstract

We calculate here the specific heat of NH 4Br using our Raman frequency shifts for the lattice mode of ν 7 TA (56 cm −1) and the internal mode of ν 2 (1684 cm −1) near its λ-phase transition ( T λ = 234 K, P = 0). By analyzing our Raman frequency shifts, values of α = 0.19 ( T < T λ and T > T λ ) for the lattice mode, and α = 0.45 ( T < T λ ) and α = 0.57 ( T > T λ ) 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 λ-phase transition in NH 4Br. The specific heat which we calculate here using our frequencies of both Raman modes, agrees well with the observed specific heat C P near the λ-phase transition in NH 4Br. Some discrepancy occurring in the specific heat above T λ due to the lattice mode is explained within the framework of an Ising pseudospin–phonon coupled model for NH 4Br.