An electron diffraction and crystal chemical investigation of oxygen/flourine ordering in rutile-type iron oxyflouride, FeOF

Rutile-type iron oxyfluoride, FeOF, has been synthesized by the reaction of FeF₃ with Fe₂O₃ in a sealed platinum tube at 950°C. The compound was previously believed to have a random distribution of oxygen and fluorine anions surrounding each of the Fe³⁺ cations. In this work, electron diffraction experiments have revealed the presence of a characteristic diffuse intensity distribution in the form of continuous rods of diffuse intensity running along both the [110]* and [110]* directions of reciprocal space through the (h+k+l) = odd parent rutile reflections. Fe³⁺ shifts induced by local O/F ordering are shown to be responsible for the characteristic reciprocal space intensity distribution of this observed diffuse scattering. The continuous 〈110〉* rods of diffuse intensity require the existence of orthogonal {110} planes within the parent rutile structure which exhibit long-range, two-dimensional, oxygen/fluorine ordering, but with no correlation from one {110} plane to the next. A crystal chemical explanation is proposed to support this argument.