Molecular dynamics calculations based on atomndash;atom potentials and rigid nonpolarizable ions are used to study the cubic rotator phases of ammonium bromide (NH4Br). The nature of the orientational order exhibited by the ammonium ions is characterized using tetrahedral rotor functions (Mmgr;, mgr;=1,7). The quantities lang;M2mgr;rang; are used as static order parameters whose decay is monitored via the correlation functions lang;Mmgr;(t)Mmgr;(0)rang;. In the high temperature sodium chloride structure the NH+4ions exhibit mostlyC3vorder, while in the room temperature cesium halide structureTdorder is dominant. As in the real crystal, cooling of the latter phase is shown to yield an antiferro arrangement of the NH+4ions which in turn gives rise to a central peak in the zone boundary (Mpoint) transverse acoustic phonon of appropriate symmetry. A crystal field analysis is shown to be useful in understanding the nature of the orientational order which results from a competition between the atomndash;atom and electrostatic interactions. The latter dominate in ammonium bromide. A similar analysis is applied to molecular dynamics calculations on the cubic high temperature rotator phases of KClO4and Li2SO4.
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