X-Ray Study of Orientation-Phonon Coupling in Cubic NaCN

Abstract

In a cubic phase of NaCN, anomalies of X-ray temperature diffuse scatterings due to C₄₄ and (C₁₁−C₁₂)⁄2 modes are observed. The intensity due to C₄₄ mode increases anomalously with decreasing temperature. Orientational disorder of CN⁻ ion in the cubic phase and its temperature dependence are analyzed by the measurement of integrated intensities of X-ray Bragg reflections, and ⟨100⟩+⟨111⟩ mixed model is obtained for the orientations of CN⁻ ion. On the basis of this model, an expression of a linear coupling between the orientations and the acoustic phonons is derived. The observed temperature dependence of the diffuse scatterings is well understood as the coupling effect involving the temperature dependence of the orientational disorder.