Neutron scattering study of the orientational disorder in potassium cyanide

We report the results of a combined neutron diffraction and total scattering study of the orientational order-disorder phase transitions in potassium cyanide, KCN. The diffraction data are analysed in terms of the spontaneous strains that accompany the phase transitions. The total scattering data are analysed using the Reverse Monte Carlo method, which gives direct access to the distribution of atomic positions and hence molecular orientations in each phase. Incorporating information from diffuse scattering in this way provides a means to measure the coefficients of the orientation distribution function up to almost arbitrarily high orders, and furthermore has the advantage that this function is naturally positive everywhere. The results for the cubic phase show that the distribution of orientations never exceeds 25% difference from an isotropic distribution.