Variation of the Raman frequency of a soft mode with the pressure (20 oC) for the phase transitions in NH4F

The Raman frequency of a soft mode (238 cm -1 ) is analyzed as a function of pressure at 20 o C for NH 4 F using the experimental data from the literature. This analysis is performed for the pressure dependence of the Raman frequency shifts (1 / n )( ¶ n / ¶ P) T of the soft mode close to the I - III, III - V and V – VI transitions in NH 4 F. The frequency shifts increase as the pressure increases toward the phase transitions at T = 20 o C (293 K) in this ammonium structure. From the frequency shifts of the soft mode studied, the pressure dependence of the isothermal compressibility is predicted through the mode Gruneisen parameter. Our calculated isothermal compressibility can be compared with the experimental measurements.