Since the discovery of a sharp anomaly in the specific heatof ammonium bromide(NH4Br)at about -30 ℃ by Simon,1which is accompanied by a volume anomaly in the same crystalstructure above and below the region of anomaly,ammoniumhalides have attracted much experimental and theoretical interest.1–3 Hydrostatic compression also decreases volume,so wehave explored the effect of pressure to complement previouswork at low temperatures.In addition,polymorphism underpressure also attractedmuch interest,because of the order-disorder transition typeinvolvingthe relative orientations of the tetrahedral NH4+ionsand their grossly analogous polymorphism to the alkali halides.Ammonium halides have linear or nearly linearN– H…Xconfigurations in the CsCl structure,which might beideal for the formation of a hydrogen bond.