Spectroscopic modifications of Pippard relations for ammonium halides

Abstract

Abstract Spectroscopic modifications of the Pippard relations have been established for ammonium halides near their λ-phase transitions. A linear relationship has been obtained between the specific heat and the frequency shifts using our observed Raman frequencies of the ν5TO(134 cm−1) and ν5LO(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 ν5TO(174 cm−1) lattice mode and the ν2(1708 cm−1) internal mode of NH4Cl near the tricritical ( P ⋍ 1.6 kbar ) and the second-order ( P ⋍ 2.8 kbar ) phase transitions in this crystal system. Our values of the slope ( d P d T ) λ 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 ( d P d T ) λ values which are the first reported in the literature for the tricritical and the second-order phase transitions in NH4Cl.