We correlate here the thermal expansivity alphap to the frequency shifts 1/nu(partial differential nu/partial differential P)T for the rotatory lattice (librational) mode in ammonia solid I close to the melting point. This is carried out for the pressures of 0, 1.93, and 3.07 kbar at various temperatures for this solid structure. By obtaining linear plots of alphap versus 1/nu(partial differential nu/partial differential P)T for the pressures studied, we extract the values of the slope dPm/dT according to our spectroscopic relation. Our calculated values of dPm/dT can be compared with the experimental ones for ammonia solid I close to the melting point.