V-doped zircon: new diffraction and optical spectroscopy data on industrial pigments

Vanadium bestows a deep turquoise color to zircon, making it an excellent ceramic pigment. Questions about vanadium valence and location in the zircon structure gave rise to many diffraction and spectroscopy studies. From the literature, an overall convergence emerges about the occurrence of vanadium as V 4+ , while controversial are the results about its accommodation at the Si tetrahedral site or at the Zr cubic site. Convincing diffraction (Siggel & Jansen, 1990) and spectroscopic studies (Niesert et al., 2002) point to V 4+ hosted at a peculiar four-fold coordinated interstitial site. However, current knowledge is not able to explain the variation in color shade usually observed in pigment manufacturing. In order to overcome this limit, four industrial pigments were considered (representing high V concentrations) together with zircon literature data (low to intermediate V contents) and isostructural phases (Hf and Th silicates and germanates). Industrial zircon pigments were characterized by XRF, XRPD and DRS to get chemical composition, unit-cell and structural parameters (e.g. metal-oxygen distances, polyhedral volumes, and distortion parameters), energy of the main optical bands and crystal field strength. The V 4+ incorporation into the zircon structure is testified by a progressive increase of the unit-cell volume with the V concentration. XRF analysis indicates a deficiency in Si suggesting a Si‒V balanced substitution. Such a substitution might occur through a Si by V 4+ replacement at the tetrahedral site 4b (point symmetry -4m2) or with V 4+ hosted at the interstitial site 16g (point symmetry ..2) coupled to a Si vacancy. Very different from those of structures containing VO 2 2- vanadyl complexes, the optical spectra of V-doped zircon exhibit three main bands in the 4000‒22000 cm -1 range which can be attributed to electronic transitions of V 4+ at tetrahedral coordination. Although challenging to calculate because of three-fold splitting of the 2 T 2 band, the crystal field strength values obtained from samples containing different amounts of V and isostructural phases give rise to a linear relationship with the mean V‒O distances at the interstitial tetrahedron from diffraction data (on average 1.894 Å). Such findings support the occupancy of an interstitial site by tetravalent vanadium when incorporated into the zircon crystal structure. The changes in turquoise color appear to depend in complex way on the amount of vanadium actually incorporated, the local environment at the interstitial site, the Zr/Si ratio in zircon, and particle size distribution. Niesert A., Hanrath M., Siggel A., Jansen M. & Langer K. 2002. Theoretical study of the polarized electronic absorption spectra of vanadium-doped zircon. J. Solid State Chem., 169, 6-12. Siggel A. & Jansen M. 1990. Rontgenographische Untersuchungen zur Bestimmung der Einbauposition von Seltenen Erden (Pr, Tb) und Vanadium in Zirkonpigmenten. Z. Anorg. Allg. Chem., 683, 67-77