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 p hase 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 hea t 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 mode l. Our calculated specific heat C-VI agrees well with the experimental Cp b elow TC in the antiferro-ordered (phase III) and ferro-ordered (phase IV) p hases of NH4Br. Above T-C (disordered phase II) there occur discrepancies b etween our calculated C-VI and the observed C-P for NH4Br.