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Analysis of vibrational spectra in relation to the thermodynamic quantities close to the phase transitions in ammonium halides (NH4F and NH4I)

The pressure dependences of the vibrational frequencies of the TO and L modes are analyzed from the literature data and the thermodynamic quantities such as isothermal compressibility K gamma, thermal expansion alpha p and the specific heat C-p - C-v are calculated through the mode Grilneisen parameters of those modes in ammonium halides (NH4F and NH4I). For this calculation, the observed frequencies of the TO mode (238 cm(-1) at 1 bar) and the L mode (74.7 meV at 1.9 GPa) are used for the phases of I, II and III in NH4F, whereas for the phases of V and VI in NH4F, the observed frequencies of the 238 cm(-1) are used as a function of pressure at T=293 K. We also analyze the observed frequencies for the TO (17.1 meV) and L (34.1 meV) modes (P=0, T=-25 degrees C) in the phase II of NH4I and the thermodynamic quantities (K-T, alpha(p) and C-p - C-v) are calculated as a function of pressure (T=-25 degrees C) through the mode Gruneisen parameters of the TO and L modes in phase II of NH(4)l.