Effects of reaction atmosphere on physico-chemical properties of V-doped FeNb11O29

FeNb11O29, pure or doped with metal transition ions, is a very promising material with advanced multifunctionalities
and interesting applicative perspectives. It is isostructural with Nb12O29, one of the rare compounds
in which Nb displays a local magnetic moment and shows both antiferromagnetic ordering and metallic
conductivity at low temperatures. In this work we have synthesized and studied Fe0.8V0.2Nb11O29 monoclinic
powders. In particular we monitored the effects on structural, electronic and magnetic properties in samples
produced in different atmospheres to stabilize cations with different oxidation states.
We have demonstrated that the reaction atmosphere influences the phase homogeneity, the crystallite size
and the amount of paramagnetic centres, with a transformation of Fe3+ in Fe2+ when an inert atmosphere is
used, as proved by the absence, in this case, of any electron paramagnetic resonance signal. Also the Raman
spectra result to be affected due to the change of coordination polyhedra. Subsequent re-oxidation of reduced
powders brings to the monophasic iron niobate again containing Fe3+ demonstrating the reversibility of redox
process. This reversibility is accompanied by a complete restoring of monoclinic structure evidenced by X-ray
diffraction data and by Raman measurements, which allowed also to follow in situ the spectral changes induced
by laser heating.