The λ-phase transitions in NH₄Cl

Abstract

We show here the correlations between the volume changes and the vibrational frequencies of the disorder-induced Raman modes of NH₄Cl near the λ-phase transitions of first order (P=0), tricritical (P⋍1.6 kbar) and second order (P⋍2.8 kbar). We obtain the values of the mode Grüneisen parameter for each phonon across the λ-phase transitions and use them to predict the Raman frequencies for the phonon modes studied as functions of temperature in the λ-phase change regions.

Our calculated frequencies for the M-point phonons of v₇ (93 cm⁻¹) and v₅ (144 cm⁻¹) of NH₄Cl are in good agreement with our observed Raman frequencies. This shows that the method employed here is satisfactory for predicting the Raman frequencies from the volume changes near the λ-phase transitions in the NH₄Cl system.

Key Words:

• λ-phase transitions
• mode Grüneisen parameter
• Raman frequency
• NH₄Cl