INFRARED SPECTRA-STRUCTURE CORRELATION STUDY OF VANADIUM-IRON SPINELS AND OF THEIR OXIDATION-PRODUCTS

A series of solid solutions of spinel structures having the general formula Fe3-xVxO4 (0 < x less than or equal to 2) was prepared and investigated by Fourier transform infrared transmission spectrometry. With increasing substitution of vanadium for Fe3+, the conversion from an inverse spinel to a normal spinel is deduced from Fourier transform infrared spectra. Low-temperature oxidation (< 500 degrees C) of these compounds, whose crystallite sizes are less than 400 nm, results in the formation of cation-deficient spinels with a vacancy and cation ordering that depends on both oxidation state and vanadium content. In addition, the absorption bands at 918 and 834 cm(-1) prove that V5+ ions are located at tetrahedral sites of the spinel lattice for x < 1.50. At higher substitution rates the coordination around the vanadium atom changes from tetrahedral to octahedral, namely when any Fe3+ ions are present in the structure. At high temperature (> 500 degrees C), oxidation of the spinels takes place sequentially. Fe-2(1-x)V2xO3 solid solutions, iron(III) orthovanadate, FeVO4, V4O9 and V2O5 are the main separated phases in the heat-treated samples with different vanadium concentrations.