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

Abstract Three types of nanostructured systems: x NbO·(1− x )α-Fe 2 O 3 , x NbO 2 ·(1− x )α-Fe 2 O 3 , and x Nb 2 O 5 ·(1− x )α-Fe 2 O 3 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 Mossbauer spectroscopy measurements. In general, Nb-substituted hematite was obtained at lower molar concentrations for all Nb oxides. For the NbO–Fe 2 O 3 system the favorable substitution of Fe 2+ for Nb 2+ in the octahedral sites in the NbO lattice was observed after 12 h milling for x =0.7. In the case of the NbO 2 –Fe 2 O 3 and Nb 2 O 5 –Fe 2 O 3 systems a formation of orthorhombic FeNbO 4 compound was observed, in which Fe 3+ cations were detected. For the highest concentration of NbO 2 ( x =0.7) iron was completely incorporated into the FeNbO 4 phase after 12 h milling. The molar concentrations of x =0.3 and 0.5 were the most favorable for the formation of ternary FeNbO 4 compound in the Nb 2 O 5 –Fe 2 O 3 system. Influence of ball milling on thermal behavior of the powders was investigated by simultaneous DSC–TG measurements up to 800 °C.