Neutron scattering studies of crystal dynamics and order-disorder phase transitions in alkali cyanides

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Abstract
The structure, phase transitions and crystal dynamics of KCN and NaCN have been studied by a series of neutron diffraction and inelastic scattering measurements. High resolution powder diffraction results on the three phases of both salts show that the ordered low temperature phase has an “antiferroelectric” orthorhombic structure, based on a primitive orthorhombic unit cell with essentially the same dimensions as the body-centered orthorhombic cell of the intermediate phase. The characteristic superlattice diffraction peak vanishes as the phase transition temperature is approached from below in the manner expected for a continuous order-disorder phase transition. In accord with other workers, we find no evidence of second-order behavior below the orthorhombic-cubic phase transition. Neutron inelastic scattering results are presented for the cubic phase of KCN, including a careful study of the behavior of a soft shear mode down to the cubicorthorhombic phase transition.