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 ± 〈hk1/3〉* regions of reciprocal space as well as continuous rods of diffuse intensity running through the G ± 〈1/21/2ξ〉* 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 〈1/21/2ξ〉* 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.