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

This study presents our calculation of the specific heat C-v1 under the pre diction 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 tra nsitions (P = 0). We also calculate the specific heat Cvi using our observe d Raman frequencies for the v(1)TA (93 cm(-1)) mode of NH4Cl close to the s econd-order phase transition (P = 2.8 kbar) in this crystalline system. Our results Show that our C-v1 values agree with the observed Cp data from the literature. The method of calculating the specific heat using tilt: fre quency data is discussed. (C) 2000 Elsevier Science B.V. All rights reserve d.