Calculation of the specific heat for the II-III and II-IV phase transitions in NH4Br

The specific heat C-VI is calculated in this study using the prediction of an Ising model for the II-IV phase transition at 0.05 GPa and for the IMV phase transition at the pressures of 0.19, 0.99 and 2.1 GPa close to TC for the NH4Br crystal. The values of the critical exponent for the specific heat of NH4Br which we determine from our analysis, are close to the predicted values of 1/16 (T < T-C) and 1/8 (T > T-C) obtained from the 3d Ising model. Our calculated specific heat C-VI agrees well with the experimental Cp below TC in the antiferro-ordered (phase III) and ferro-ordered (phase IV) phases of NH4Br. Above T-C (disordered phase II) there occur discrepancies between our calculated C-VI and the observed C-P for NH4Br.