Electron diffraction evidence for continuously variable, composition-dependent O/F ordering in the ReO3 type, Nbv1-xNbIVxO2-xF1+x,,0,≤x≤0.48, solid solution

A spread of compositions within the (Nb1-xNbxO2-xF1+x)-Nb-V-O-IV solid solution have been successfully synthesized in the range 0less than or equal toxless than or equal to0.48. In each case electron diffraction has revealed the presence of strong planar diffuse scattering perpendicular to each of the three <001> directions at the G+/-<hkq>* regions of reciprocal space, where G represents a Bragg reflection of the ReO3 type average structure, h and k are continuously variable and q is a fixed (at any one particular composition), composition-dependent parameter in the range from 1/3 to 1/2. Careful electron probe microanalysis indicates that the value of q is directly related to composition via the equation q = (x + 1)/3 and hence acts as a "chemical ruler" for the composition. A plausible model for the implied one-dimensional O/F ordering as well as for the observed shift in the position of the diffuse versus composition is provided via a crenel type occupational atomic modulation function. (C) 2004 Published by Elsevier Inc.