Molecular crystals and ionic crystals which consist of polyatomic groups exhibit “orientational transitions” accompanied by an orientational disorder of polyatomic groups. Some topics on X-ray studies of the orientational disorder, which have been studied by the present author and his collaborators in a recent decade, are summarized.
The observed X-ray diffuse scatterings due to the orientational disorder of NO₃-ions in the CsCI-type cubic phases of NH₄NO₃ and RbNO₃ are analysed on the basis of “12-orientation” model. The intensity formulae of X-ray diffuse scattering for polyorientation system, which have been given by Matsubara (Zernike's theory), by Naya (random phase approximation) and recently by Nagai (Bethe's method), are discussed.
Anomalies of X-ray temperature diffuse scatterings due to C₄₄ mode are observed in the NaCI-type cubic phases of RbNO₃ and NaCN. By taking account of a linear coupling between the orientations and acoustic phonons, the observed temperature dependences of the diffuse scatterings are explained.