A practical direct-space refinement cycle for structure images

A direct-space refinement method is described based on the procedure of placing a digitized electron- microscope unit-cell image in the frame store of an image-simulation computer to permit an iterative calculation. The material under investigation is Rb_22-5xNb_{54 + x}O₁₄₆, previously refined by X-ray single-crystal diffraction, which has tunnels with non-stoichiometric ionic occupation. It is concluded from this study that for problems where the major part of the structure is known the weak-phase-object approximation is adequate for the inversion component of a refinement provided only the x, y coordinates of the missing ions are required. An improved procedure which could allow refinement of the interatomic potential is suggested, incorporating the assumption that the structure consists of spherical atoms.