This paper deals with the columbite Fe x Mn 1Àx Nb 2 O 6 (x ¼ 0, 0.25, 0.50, 0.75 and 1) solid solution. After synthesis under vacuum, structural and magnetic properties were studied by means of X-ray diffraction, Rietveld refinement, electron paramagnetic resonance and magnetization measurements. This investigation showed that (i) the homogeneity range of the solid solution is complete and the A (Fe and/or Mn) cations are randomly distributed in the A-octahedral sites of the structure; (ii) the oxidation state of Fe and Mn is always þ2; (iii) there is no cation disorder between the A and Nb sites; (iv) an antiferromagnetic transition is present only for the two end-members (FeNb 2 O 6 and MnNb 2 O 6 ).

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The MnNb 2 O 6 -FeNb 2 O 6 solid solution has been investigated by Fe-K-and Mn-K-edge X-ray absorption (XANES and EXAFS), and Mö ssbauer spectroscopy. The first-shell M-O bond lengths deduced from EXAFS show a fairly small compositional dependence. A degree of static disorder, which increases with increasing manganese content, is clearly seen by the loss of correlation for the next-neighbour (NN) interaction. Hyperfine parameters from Mö ssbauer spectra are consistent with variations in the average environment, as recorded by X-ray data. Line broadening of the Mö ssbauer spectra provides evidence for next-neighbour effects and is consistent with there being no significant clustering of Fe or Mn within the samples. There appear to be differences in the way the columbite structure accommodates Fe 2+ and Mn 2+ions. In ferrocolumbite all the Fe octahedra are close to being identical, while there are local structural heterogeneities at a longer length scale, presumably in ordering the precise topology of polyhedra immediately adjacent to the octahedron. By contrast, the manganocolumbite seems to have some diversity in the precise coordination at the MnO 6 octahedra, but a greater uniformity in how the adjacent polyhedra are configured around them.

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0.25, 0.50, 0.75, and 1 are characterized by powder XRD, EPR, and magnetization measurements. The powder XRD patterns of all samples are indexed in space group Pbcn. The Fe and/or Mn cations are randomly distributed over the A-octahedral sites of the structure and there is no cation disorder between the A and Nb sites. The oxidation state of Fe and Mn is always +2. An antiferromagnetic transition is observed only for the two endmembers FeNb2O6 and MnNb2O6. -(TEALDI, C.; MOZZATI, M. C.; MALAVASI*, L.; CIABATTONI, T.; AMANTEA, R.; AZZONI, C. B.; Phys.