An electron diffraction and bond valence sum investigation of oxygen/fluorine ordering in Nb n O 2 n -1 F n +2 , n =3

The n=3 member of the NbnO2n-1Fn+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