Critical behaviour of Raman frequency shifts for translational modes near the melting point of ammonia solid I based on Pippard relations

Abstract

A correlation between thermal expansivity α _p and frequency shifts (1/ν) (∂ν/∂P) _T was established for the ammonia solid I near its melting point. For this correlation, calculated frequencies for the translational modes of ν _T (100 cm⁻¹) and ν _T (130 cm⁻¹) were used to acquire the Raman frequency shifts at pressures of 0, 1.93 and 3.07 kbar in ammonia solid I. In this crystalline system, values of the slope dP_m /dT were deduced for the fixed pressures from linear plots of α _p vs. (1/ν) (∂ν/∂P) _T . Slope values at zero pressure are in very good agreement with those obtained experimentally. It is also shown that the spectroscopic modification of the Pippard relations applies satisfactorily to ammonia solid I.