An Electron Diffraction and Crystal Chemical Investigation of Oxygen/Fluorine Ordering in Niobium Oxyfluoride, NbO2F

A careful electron diffraction study of the ReO3-type oxyfluoride NbO2F has revealed the presence of characteristic transverse polarized planes of diffuse intensity running through the G±* regions of reciprocal space as well as continuous rods of diffuse intensity running through the G±<(1/2)(1/2)xi>* regions of reciprocal space. The continuous planes of diffuse intensity in reciprocal space (perpendicular to each of the major crystal directions a, b and c) imply the existence of one-dimensional, oxygen/fluorine-ordered columns of atoms along <001> in real space but with no lateral correlation in the ordering from such <001> column to the next. The continuous lines of diffuse intensity along the <(1/2)(1/2)xi>* directions of reciprocal space are ascribed to Rigid Unit Mode (RUM) rotations of the constituent MX6 octahedral framework. A combination of bond-valence arguments and simple structure factor calculations are used to support this argument.