Pressure and temperature-dependent Raman scattering study of the phase transitions in ammonium bromide

Abstract

The pressure and temperature dependences of the lattice phonons and internal ν4 modes in NH4Br have been measured, with special emphasis on the Raman bands connected with the recently discovered high pressure phase (OII). It has been shown from this work that the ammonium ions in phase OII are ordered and the crystal structure of the OII phase is cubic. The pretransition effect associated with the OII–OT transition has also been found. The frequencies of the 68 cm−1 phonon mode and the LO component of the ν4 mode in phase OII have been found to decrease with increasing pressure, indicating that the pressure decreases the curvature of the potential function of the Br− ion at equilibrium. This work has shown that the phonon and internal vibrational spectra of NH4Br in phase OII closely resembles the ordered phase of NH4Cl. This contrasts with the suggestion made previously that the structure of the OII NH4Br is similar to the disordered phase of NH4Cl.