Orientational order in ionic crystals containing tetrahedral ions

Abstract

Molecular dynamics calculations based on atom-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 (M μ, μ = 1,7). The quantities 〈Mμ2〉 are used as static order parameters whose decay is monitored via the correlation functions 〈Mμ(t)Mμ(0) 〉. In the high temperature sodium chloride structure the NH 4+ ions exhibit mostly C3, order, while in the room temperature cesium halide structure Td order is dominant. As in the real crystal, cooling of the latter phase is shown to yield an antiferro arrangement of the NH4+ ions which in turn gives rise to a central peak in the zone boundary (M point) 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 atom-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 KClO 4 and Li2SO4. © 1983 American Institute of Physics.