Electron diffraction study of diffuse scattering due to atomic displacements in disordered vanadium monoxide

The diffuse intensity pattern observed by electron diffraction from VO_1.23 annealed above the ordering temperature, has been interpreted in terms of short- range-order parameters and by model calculations. Emphasis was put on the ordering of displacements, which gives the major contribution at large scattering angles - a range where the intensity variations at 1 MV are well suited for recording. Through detailed dynamical calculations it is shown that a greatly simplified procedure for calculation of the diffuse intensity is justified when the number and strength of dynamical interactions is moderate. A few displacement parameters were derived through a linear, multiple-regression method. Models for the displacements were based on these results and from information about the ordering of defect tetrahedra. These results agree with previous proposals which indicate large static displacements associated with ordering of defects. Apart from the presence of (100) acoustic phones, the indications of an expansion of the nearest-neighbour distance with a large transverse component were most significant. Such rotation may result from movement of atoms towards selected defect clusters. Model calculations demonstrated that rotated tetrahedra or a zig- zag string of atoms is probable - both fitting well into the assumed surroundings of the clusters.