Diffuse neutron scattering in sodium and potassium cyanide

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Abstract
Diffuse neutron scattering has been measured in NaCN at 295 K and KCN at 175 and 295 K. In both samples, strong scattering from the soft shear mode (related to the translation-rotation coupling which leads to the phase transition from the high-temperature cubic NaCl phase to the low-temperature orthorhombic phase) is observed. The asymmetry of this scattering about certain reciprocal-lattice points is direct evidence both of the bilinear nature of the coupling, and of its sign (and hence of the relative importance of overlap and quadrupolar interactions). In NaCN, additional structured diffuse scattering is observed which is absent in KCN at both measured temperatures. This scattering is assumed to arise from short-range order in CN orientations, but the data are not consistent with simple models of steric hindrance that have been proposed for the short-range correlations in NaCN.