Modified Pippard relationship describing the Raman frequency shifts of the rotatory lattice mode of ammonia solid II in the vicinity of its melting point

We relate in this study the thermal expansivity, aP, to the Raman frequency shift (1/n)(n/P)T for the rotatory lattice (librational) mode in ammonia solid II near its melting point. We have used our calculated Raman frequencies of this mode for pressures of 3,65, 5,02 and 6,57 kbars for this crystalline system. The values of the slope, dPm/dT, which we deduced from our spectroscopic relation, are compared with those obtained experimentally. In particular, our computed slope value for the pressure of 502 kbar is in very good agreement with the empirical result.