Effect of niobium valence on the mechanochemical activation of niobium oxides–hematite magnetic ceramic nanoparticles

Three types of nanostructured systems: xNbO·(1−x)α-Fe2O3, xNbO2·(1−x)α-Fe2O3, and xNb2O5·(1−x)α-Fe2O3 were synthesized by ball milling at different molar concentrations (x=0.1, 0.3, 0.5, and 0.7). The effect of Nb valence and milling time on mechanochemical activation of these systems were studied by X-ray diffraction and the Mössbauer spectroscopy measurements. In general, Nb-substituted hematite was obtained at lower molar concentrations for all Nb oxides. For the NbO–Fe2O3 system the favorable substitution of Fe2+ for Nb2+ in the octahedral sites in the NbO lattice was observed after 12 h milling for x=0.7. In the case of the NbO2–Fe2O3 and Nb2O5–Fe2O3 systems a formation of orthorhombic FeNbO4 compound was observed, in which Fe3+ cations were detected. For the highest concentration of NbO2 (x=0.7) iron was completely incorporated into the FeNbO4 phase after 12 h milling. The molar concentrations of x=0.3 and 0.5 were the most favorable for the formation of ternary FeNbO4 compound in the Nb2O5–Fe2O3 system. Influence of ball milling on thermal behavior of the powders was investigated by simultaneous DSC–TG measurements up to 800 °C.