Coupled orientational and displacive degrees of freedom in the high-temperature plastic phase of the carbon tetrabromide α-CBr4

Two-dimensional single-crystal synchrotron x-ray diffraction of the high-temperature plastic phase of CBr(4) reveals relatively sharp G +/- 1/2 {110}* sheets of diffuse intensity (where G represents the set of average structure Fm-3m allowed Bragg reflections). This intense and highly structured diffuse scattering arises from the large amplitude excitation of transverse polarized displacive modes of distortion and shows that a set of six possible orientations of the individual CBr(4) molecules in the plastic phase are strongly coupled with displacive relaxational degrees of freedom. Using Monte Carlo simulations to model the diffuse intensity, the displacive relaxations are shown to be along the molecular nearest-neighbor 1/2 < 110 > real space directions associated with the relative orientations of individual and nearest-neighbor CBr(4) tetrahedra.