STUDIES ON THE ELECTRICAL-PROPERTIES OF VANADIUM IRON SPINELS AT STOICHIOMETRY AND AT OXIDATIVE NONSTOICHIOMETRY

The de conductivity (sigma) and the thermoelectric power (theta) of single-phase cubic vanadium spinels, Fe3-xVxO4 with 0 < x < 2 are measured under vacuum (at stoichiometry) in the range 300 to 750 K. For x < 1.50, the electrical conduction is n-type and shows an activated mobility similar to that found in Fe(2)MO(4) (M = Mo, Cr, Al) where mixed Fe3+/Fe2+ valences on both octahedral and tetrahedral sites have been demonstrated. For x > 1.50, the conductivity strongly decreases and p-type conduction is found which is consistent with a normal near-spinel structure. The temperature dependence of the electrical conductivity in O-2 atmosphere (at oxidative nonstoichiometry) is compared to that of Fe2MoO4 and Fe2AlO4 and is explained through the disappearance of the Fe2+-Fe3+ and V2+-V3+ ion pairs on octahedral sites for x < 1.50 and the formation of Fe2+-Fe3+ ion pairs on tetrahedral sites for x > 1.50.