Pressure-induced structural changes in NH4Br

摘要

We report angle dispersive X-ray diffraction (XRD) measurements and Raman spectroscopy on NH4Br up to 70.0 GPa at room temperature. Three thermodynamically stable phases (phases II, IV, and V) are confirmed and a new possible phase (phase VI) of P2(1)/m symmetry is proposed whose structure was established from Rietveld refinement of synchrotron XRD data for the first time. The phase sequence observed in NH4Br is in accordance with phase II -> IV -> V -> VI -> Phase V transforms into phase VI at about 57.8 GPa with a huge volume reduction of 30%. Still, the intramolecular distances are analyzed to better understand the nature of structures. The H-H interactions become markedly more important as the N-Br distances are compacted, which is probably the reason of the kink of symmetric stretching band (nu(1)) at the transition pressure. (C) 2015 AIP Publishing LLC.