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

A series of solid solutions of spinel structures having the general fo rmula Fe3-xVxO4 (0 < x less than or equal to 2) was prepared and inves tigated by Fourier transform infrared transmission spectrometry. With increasing substitution of vanadium for Fe3+, the conversion from an i nverse spinel to a normal spinel is deduced from Fourier transform inf rared spectra. Low-temperature oxidation (< 500 degrees C) of these co mpounds, whose crystallite sizes are less than 400 nm, results in the formation of cation-deficient spinels with a vacancy and cation orderi ng that depends on both oxidation state and vanadium content. In addit ion, the absorption bands at 918 and 834 cm(-1) prove that V5+ ions ar e located at tetrahedral sites of the spinel lattice for x < 1.50. At higher substitution rates the coordination around the vanadium atom ch anges from tetrahedral to octahedral, namely when any Fe3+ ions are pr esent in the structure. At high temperature (> 500 degrees C), oxidati on of the spinels takes place sequentially. Fe-2(1-x)V2xO3 solid solut ions, iron(III) orthovanadate, FeVO4, V4O9 and V2O5 are the main separ ated phases in the heat-treated samples with different vanadium concen trations.