A study of order in deuterated ammonium chloride by neutron scattering

Abstract

The diffuse neutron scattering intensity from a single crystal of ND₄Cl in the room temperature beta phase is studied in detail in the (h,h,l plane as a function of temperature. An Ising model is set up which includes the interaction of the Ising variables with the ionic displacements. The temperature and reciprocal space dependences of the diffuse intensity are calculated using a mean field approximation and are fitted to the observed data, yielding Ising interaction energies. Two models of the ND₄Cl structure are set up to calculate these interaction energies: the first assumes electrostatic interactions only and leads to a value for the Cl^- ion polarizability of 2 AA³ which is too low. The second model is based on a 'shell' model for Cl^-, incorporates short-range interactions, and accounts for the interaction energies assuming the known polarizability. Both models give a deuterium charge of greater than 0.3e which indicates shortcomings in the mean field approximation.