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Calculation of the specific heat close to the first-order and second-order phase transitions in ammonium halides

This study presents our calculation of the specific heat C-v1 under the prediction of an Ising model. For this calculation we used our observed Raman frequencies for the l'5LO (177 cm(-1)) mode of NH4Br, the v(5)TO (144 cm(-1)) and v(1)TA (93 cm(-1)) modes of NH4Cl close to the first-order phase transitions (P = 0). We also calculate the specific heat Cvi using our observed Raman frequencies for the v(1)TA (93 cm(-1)) mode of NH4Cl close to the second-order phase transition (P = 2.8 kbar) in this crystalline system.