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

Rutile-type iron oxyfluoride, FeOF, has been synthesized by the reaction of FeF3 with Fe2O3 in a sealed platinum tube at 950 degreesC. The compound wa s previously believed to have a random distribution of oxygen and fluorine anions surrounding each of the Fe3+ cations, In this work, electron diffrac tion experiments have revealed the presence of a characteristic diffuse int ensity distribution in the form of continuous rods of diffuse intensity run ning along both the [110]* and [1 (1) over bar0]* directions of reciprocal space through the (h+k+1) = odd parent rutile reflections. Fe3+ shifts indu ced by local O/F ordering are shown to be responsible for the characteristi c reciprocal space intensity distribution of this observed diffuse scatteri ng. The continuous < 110 >* rods of diffuse intensity require the existence of orthogonal {110} planes within the parent rutile structure which exhibi t long-range, two-dimensional, oxygen/fluorine ordering, but with no correl ation from one {110} plane to the next. A crystal chemical explanation is p roposed to support this argument. (C) 2000 Academic Press.