Spectroscopic modifications of the Pippard relations have been established for ammonium halides near their X-phase transitions. A linear relationship has been obtained between the specific heat and the frequency shifts using our observed Raman frequencies of the nu(5)TO(134 cm(-1)) and nu(5)LO(177 cm(-1)) lattice modes of NH4Br near its first-order phase transition (P = 0). Similar linear relationships have also been obtained using our observed Raman frequencies of the nu(5)TO(174 cm(-1)) lattice mode and the nu(2)(1708 cm(-1)) internal mode of NH4Cl near the tricritical (P similar or equal to 1.6 kbar) and the second-order (P similar or equal to 2.8 kbar) phase transitions in this crystal system. Our values of the slope (dP/dT)(lambda) deduced from the spectroscopic modifications of the Pippard relations are comparable with the literature value for the NH4Br crystal near its first-order phase transition. We also give in this study our (dP/dT)(lambda) values which are the first reported in the literature for the tricritical and the second-order phase transitions in NH4Cl. (C) 1997 Elsevier Science B.V.