Spectroscopic Studies on the Localization of Vanadium(IV) in Vanadium-Doped Zircon Pigments

The origin of the blue color observed for vanadium-doped zircon powders that have been prepared either in the presence or the absence of fluxes (NaF) has been investigated by analyzing the localization and distribution of the vanadium species in the zircon matrix. In both cases, the unit-cell parameters determined from the X-ray diffraction patterns of the samples and the results obtained from X-ray absorption (X-ray absorption near-edge structure and extended X-ray absorption fine structure), infrared, and electron spin resonance spectroscopies indicated that V⁴⁺ cations form a solid solution with the zircon lattice, substituting for both Si⁴⁺ and Zr⁴⁺ cations, although to a greater extent for the former. These vanadium cations are heterogeneously distributed in the zircon matrix, being mainly located in the outer layers of the particles in two different situations: "aggregated" in nearby lattice positions with V⁴⁺-V⁴⁺ distances ( d _V-V) of <8 ? (0.8 nm), and isolated with respect to other V⁴⁺ cations ( d _V-V > 8 ?). The strongest blue color that is observed for samples prepared in the presence of NaF seems to be due to the higher amount of V⁴⁺ cations incorporated to the zircon lattice, because this flux agent does not have any structural role.