A molecular dynamics study of orientational disordering in crystalline sodium nitrate

Molecular dynamics computer simulations have been made of the ordered low-temperature and orientationally disordered high-temperature phases of crystalline sodium nitrate. The interionic force model used in the simulations is based on a rigid-ion representation of the electrostatic interactions, supplemented by a set of atom-atom potentials of the Buckingham type. Five simulations have been carried out, covering the temperature range from 293 to 570 K. Where possible, detailed comparison is made with experiment, and generally satisfactory agreement is found. Anion disorder is shown to consist primarily in reorientation about an axis parallel to the crystallographic c direction, but only at the highest temperature studied can the motion be described as that of a free, quasi-two-dimensional rotor. The spectra of translational and librational lattice modes have been compared and the way in which these change with temperature is discussed in terms of translational-rotation coupling. Numerical estimates are presented of the degree of correlation between translations and rotations at different points in the Brillouin zone, and possible ordering processes are identified. The diffuse scattering predicted by the model is analysed in detail and related to the results of X-ray and neutron scattering experiments. It is suggested that in the high-temperature phase there exist two major ordering processes and that the competition between the two results in the experimentally observed critical exponent for the macroscopic order parameter being significantly less than its classical value. The same argument is formulated on the basis of a Landau-type expression for the free energy. Although the simulation have been limited to the specific case of sodium nitrate, the results should also be helpful in understanding the nature of the corresponding phase transition in calcite.