VANADIUM CLUSTERS IN DOPED ZRO2-SIO2 TOUGHENED CERAMIC COMPOSITES OBTAINED FROM ALKOXIDES

Gels of ZrO2-SiO2 doped with small amounts of vanadium have been prepa red from alkoxides. The stabilization of tetragonal zirconia in the am orphous silica matrix at low firing temperatures and a blue solid solu tion of vanadium in zircon structure above 1200-degrees-C has been ide ntified, Electron paramagnetic resonance (EPR) shows that vanadium (IV ) occupies three kinds of sites in the composite: one with C4v symmetr y located at the solid surface like [VO(H2O)5]2+, another diluted in t he solid and coordinated to oxygens like in V2O5 structure, and the la test, forming clusters of V(IV) in dipolar interaction. The EPR signal of these clusters increases with the zircon crystallization.