We correlate here the thermal expansivity alpha(p) to the frequency shifts 1/v(partial derivative v/partial derivative 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 alpha(p) versus 1/v(partial derivative v/partial derivative P)(T) for the pressures studied, we extract the values of the slope dP(m)/dT according to our spectroscopic relation. Our calculated values of dP(m)/dT can be compared with the experimental ones for ammonia solid I close to the melting point.