Raman scattering study of the effect of pressure and temperature on the order-disorder phase transition in ammonium chloride

Abstract

The temperature and pressure dependence of the lattice and internal modes in NH4Cl have been measured. By keeping the temperature fixed, it has been possible to induce the phase transition by changing the external pressure. From the frequency and intensity measurements of several lattice modes as a function of temperature at various pressures, it was found that the character of the order-disorder phase transition in NH4Cl varies significantly at different points along the phase transition line in the P-T plane. An analysis of the Grüneisen parameter for the various lattice modes was presented. The isobaric temperature dependent frequencies of the lattice modes was discussed in terms of the pure-volume contributions. From the pure-temperature contribution, it was found that anharmonic interaction potential is the main mechanism responsible for the temperature dependence of the anomalous 93 cm−1 mode in NH4Cl.