Short-range order in ND4Br studied by diffuse neutron scattering

Abstract

Diffuse elastic scattering of neutrons has been observed from a single crystal of deuterated ammonium bromide and is interpreted as being due to short-range order among ammonium ion orientations. Both the temperature dependence of the diffuse intensity and its distribution in reciprocal space can be described in terms of a simple Ising model of the order-disorder transition. An expression for the diffuse intensity obtained from the mean-field approximation to the Ising model is least-squares fitted to the data. Interactions between first, second and third nearest neighbour ammonium ions have to be included in the model to give an adequate fit. The interaction energies so obtained are compared with calculations based on a simple electrostatic theory, For agreement, a charge of 0.358e on hydrogen atoms and a polarizability of 1.40 ų for Br⁻ ions have to be assumed in the calculations and the significance of these unlikely values is discussed.