Raman study of the high pressure phase transitions and the volume dependent lattice anharmonicity of NH4Br

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Abstract
Raman spectra of NH4Br are reported for temperatures between 195 and 407 K and pressures between 1 bar and 42 kbar. This domain includes the stability fields of the disordered NH4Br II, the parallel ordered NH4Br IV, and the high pressure phase of NH4Br V and the II–V and V–IV transitions. A detailed phase diagram including the high pressure transtions determined in this work is also presented. The Grüneisen constants for the lattice phonons at 296 K are 3.2 and 1.2 [ν5(TO)], 0.72 and 0.46 (2ν6), and 3.3 and 0.75 (ν7) in NH4Br II and NH4Br V, respectively. NH4Br V is interpreted as a mixed phase and its spectrum as the superposition of the spectra of two other phases, III (antiparallel arrangement of NH4+ ions) and IV (parallel arrangement). The most salient features of the results for NH4Br V, namely the small volume‐dependent anharmonicity and the anomalous and strongly temperature‐dependent intensities of the Raman bands, can be explained in terms of a mixed phase. The intensity of one group of peaks (antiparallel) decreases, while that of the other (parallel) increases as the temperature is lowered in the field of NH4Br V. The five lattice phonons of NH4Br V which become Raman inactive in the phase IV belong to cluster or domains in which NH4+ ions are aligned as in NH4Br III.