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This study gives our calculation for the specific heats Cvi due to an Ising model using the observed Cp data for the Ⅱ-Ⅲ and Ⅱ-Ⅳ phase transitions in NH4Ⅰ. By fitting to the Cp data we determine the values of the critical exponent for the pressure of 0.14GPa (Ⅱ-Ⅲ phase transition) and for the pressures of 0.75, 1.35 and 1.97GPa (Ⅱ-Ⅳ phase transition) in NH4Ⅰ. Our exponent values are close to the predicted values of the specific heat in a three-dimensional Ising model. Our calculated Cyi are in good agreement with the experimental Cp for NH4Ⅰ in most cases.