Original research articleAnalysis 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 κT, thermal expansion αP and the specific heat CP − CV are calculated through the mode Grüneisen 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 °C) in the phase II of NH4I and the thermodynamic quantities (κT, αP and CP − CV) are calculated as a function of pressure (T = −25 °C) through the mode Grüneisen parameters of the TO and L modes in phase II of NH4I.Calculated thermodynamic quantities (κT, αP and CP − CV) can be compared with the experimental data under the pressure conditions studied here for the phases I, II, III, V and VI in NH4F (T = 293 K) and for the phase II in NH4I (T = −25 °C) when available in the literature. This shows that the thermodynamic properties of molecular crystals such as NH4F and NH4I, can be investigated from the measurements of the vibrational spectra at high pressures.