An electron diffraction and bond valence sum investigation of oxygen/fluorine ordering in Nb nO 2n−1F n+2, [formula omitted

Abstract

The n = 3 member of the Nb n O 2 n −1F n +2 family of oxyfluoride compounds has been carefully investigated by electron diffraction. This compound was previously believed to have a random distribution of oxygen and fluorine ions at each of the anion sites. Electron diffraction experiments have revealed the presence of strong planar diffuse scattering perpendicular to the a and b directions at the G±[ qkl]* and G±[ hql]* regions of reciprocal space, where G represents a Bragg reflection of the I4/ mmm average structure, h and k are continuous variables and q = 0.39 ± 0.08 . The continuous planes of diffuse intensity imply the existence of strings of ordered oxygen and fluorine atoms for equatorial anion sites along [100] and [010] albeit without correlation from one such string to the next. A bond valence sum argument has been used to derive a plausible site occupation model for the remaining apical and median anion sites.