Electron diffraction evidence for continuously variable, composition-dependent O/F ordering in the ReO 3 type, Nb V1 −xNb IVxO 2− xF 1+ x, 0⩽ x⩽0.48, solid solution

Abstract

A spread of compositions within the Nb V 1− x Nb IV x O 2− x F 1+ x solid solution have been successfully synthesized in the range 0⩽ x⩽0.48. 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 ReO 3 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.