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

A correlation between thermal expansivity alpha(p) and frequency shifts (1/nu)(partial derivative nu/partial derivative P)(T) was established for the ammonia solid I near its melting point. For this correlation, calculated frequencies for the translational modes of nu(T) (100 cm(-1)) and nu(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 alpha(p) vs (1/nu)(partial derivative nu/partial derivative P)(T). Slope values at zero pressure are in very good agreement with those obtained experimentally. It is also shown that the spectroscopic modi. cation of the Pippard relations applies satisfactorily to ammonia solid I.