Local structural properties of (Mn,Fe)Nb2O6 from Mössbauer and X-ray absorption spectroscopy

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 Mossbauer 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 Mossbauer spectra are consistent with variations in the average environment, as recorded by X-ray data. Line broadening of the Mossbauer 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.