Single Crystal Neutron Diffraction Study of Potassium Cyanide

Abstract

Neutron diffraction measurements were made on single crystals of KCN to obtain information about the disorder of the CN− ions in the cubic phase. The crystals were obtained by pulling from the melt by a modified Czochralski method. Data were collected on two four-circle diffractometers at room temperature and at 180°K, just above the cubic-orthorhombic phase transition. Structure factors were obtained for 24 inequivalent reflections. Free rotation of the CN− ions is ruled out by the different structure factors obtained for reflections with equal values of (h2+k2+l2) but different (h, k, l). However, models fitted to the data assuming orientations along a particular symmetry direction did not establish a preferred direction. The fits indicated that both vibrational and librational thermal amplitudes are large and relatively independent of temperature. These amplitudes could not be reproduced by a dynamical model assuming harmonic forces even when rotational-translational coupling was included. A tentative explanation of these results is given in terms of rapid CN− reorientations caused by the low activation energy for rotation.