Vanadium-doped Zircon Research Articles

New spectroscopic and diffraction data to solve the vanadium-doped zircon pigment conundrum

The literature on V-doped zircon pigments offers a puzzling situation with unanswered questions about the origin of turquoise colour. In this study, new diffraction (XRPD) and spectroscopic (EPR, EAS) data were obtained on four industrial pigments, whose vanadium content is much higher than samples from previous investigations. Both EPR features and XRPD results indicate the occurrence of tetrahedrally-coordinated V4+ at the zircon interstitial site 16g. EPR spectra at 5 K exhibit a structured multiplet attributed to isolated V4+ ions and an exchange-narrowed line resulting by clustering of V4+ ions. The intensity of optical bands is stronger in samples affected by V4+ clusters. However, the turquoise colour stems from the window of transmitted green-bluish light, which depends on both the tail of the main optical band (2B ⟵ 2A) and the onset of charge transfer phenomena. Small variations in these two bands readily turn into a chromatic shift from light blue to green.

Colouring of opaque ceramic glaze with zircon pigments: Formulation with simplified Kubelka–Munk model

In this study a simplified Kubelka–Munk model is proposed for colour matching purposes. Opaque glazes were prepared to determine the absorption optical constants from the reflectance curves measured with a spectrophotometer. After the physical and chemical characterization of the glaze components (frit and pigments), to analyze the spectrophotometric results a simplification of the Kubelka–Munk model was suggested. To experimentally verify the model, two target green colour were reproduced in laboratory by adding in an opaque glaze a yellow praseodymium-doped zircon ((Zr,Pr)SiO4) and blue vanadium-doped zircon ((Zr,V)SiO4) pigments. The results were in good agreement with the experimental reflectance curves and the prediction of colour green glazes was possible with a reduced number of experiments.

Ni-doped hibonite (CaAl 12O 19): A new turquoise blue ceramic pigment

A new structure for ceramic pigments was synthesized by a conventional solid state reaction process. It is based on Ni-doped hibonite, CaAl 12O 19, which assumes a turquoise-like blue colour similar to that of V-doped zircon. Hibonite is associated with anorthite, CaAl 2Si 2O 8, acting like a fluxing agent in order to lower the synthesis temperature, and with cassiterite, SnO 2, acting as a tin buffer to promote coupled Ni 2+ + Sn 4+ → Al 3+ + Al 3+ substitutions, in order to ensure the electric neutrality of the hibonite lattice. Since relatively low chromophore contents are required, this new system constitutes an interesting alternative to the common blue ceramic pigments based on cobalt aluminate spinel or vanadium-doped zircon, implying lower cost and environmental advantages. The pigments characterization was performed by X-ray powder diffraction, diffuse reflectance spectroscopy, CIELAB colorimetric analysis, and testing in ceramic glazes and bodies. The substitution of Al 3+ by bigger ions, like Ni 2+ and Sn 4+, increases the cell volume compared to undoped hibonite and is responsible for the turquoise blue colour, as verified by UV–vis analysis. The chromatic mechanism is due to incorporation of Ni 2+ in tetrahedral coordination, likely occurring at the site M3 of the hibonite lattice, where it partially substitutes the Al 3+ ion. While this product shows a strong hue as a pigment, it is not stable after severe testing in glazes and attempts to improve its colouring performance are now under development.

Electrochemistry of vanadium-doped ZrSiO 4: Site-selective electrocatalytic effect on nitrite oxidation

The electrochemistry of vanadium-doped zircon (V x ZrSiO 4, 0 < x < 0.10) has been studied using abrasive-conditioned paraffin-impregnated graphite electrodes. It is compared with that of ZrSiO 4, ZrO 2, and vanadium-doped tetragonal and monoclinic zirconias. In contact with acetic/acetate and HCl + NaCl electrolytes, zirconium materials are reduced to Zr(III) at potentials near to −0.5 versus AgCl/Ag and to Zr metal at potentials more negative than −1.2 V, via proton-assisted reductive processes, influenced by the complexing action of chloride ions. Vanadium-centred oxidation processes appear at potentials from +0.2 to +0.7 V enabling for a distinction between different coordinative arrangements. ZrSiO 4 exerts a significant electrocatalytic effect on nitrite oxidation in acetic/acetate buffers, slightly enhanced in the presence of increasing vanadium loadings. Electrocatalytic data are indicative that only V centres substituting Zr are catalytically active, whereas V substituting Si are catalytically silent.

Electrochemistry of vanadium-doped ZrSiO4Site-selective electrocatalytic effect on nitrite oxidation

The electrochemistry of vanadium-doped zircon (VxZrSiO4, 0 < x < 0.10) has been studied using abrasive-conditioned paraffin-impregnated graphite electrodes. It is compared with that of ZrSiO4, ZrO2, and vanadium-doped tetragonal and monoclinic zirconias. In contact with acetic/acetate and HCl + NaCl electrolytes, zirconium materials are reduced to Zr(III) at potentials near to −0.5 versus AgCl/Ag and to Zr metal at potentials more negative than −1.2 V, via proton-assisted reductive processes, influenced by the complexing action of chloride ions. Vanadium-centred oxidation processes appear at potentials from +0.2 to +0.7 V enabling for a distinction between different coordinative arrangements. ZrSiO4 exerts a significant electrocatalytic effect on nitrite oxidation in acetic/acetate buffers, slightly enhanced in the presence of increasing vanadium loadings. Electrocatalytic data are indicative that only V centres substituting Zr are catalytically active, whereas V substituting Si are catalytically silent.

Atomic site preferences and structural evolution in vanadium-dopedZrSiO4from multinuclear solid-state NMR

Solid state NMR spectra of ${}^{29}\mathrm{Si}$ are reported from pure and vanadium-doped zircon $({\mathrm{V}\ensuremath{-}\mathrm{Z}\mathrm{r}\mathrm{S}\mathrm{i}\mathrm{O}}_{4})$ samples. The vanadium concentration is varied up to $\ensuremath{\sim}1\ensuremath{-}\mathrm{mol}%{\mathrm{V}}^{4+}$ by using both conventional-firing and sol-gel routes, and ${}^{51}\mathrm{V}$ NMR data are also recorded. ${}^{17}\mathrm{O}$ NMR of ${}^{17}\mathrm{O}$ isotopically enriched samples shows that the initial gel is completely amorphous with the whole range of possible $M\ensuremath{-}\mathrm{O}{\ensuremath{-}M}^{\ensuremath{'}}$ linkages detected, and that this structure evolves into a fully ordered ${\mathrm{ZrSiO}}_{4}$ structure with calcination. Static ${}^{91}\mathrm{Zr}$ NMR data is reported from a pure zircon sample. The NMR data are used to quantify the amount of vanadium entering the zircon structure, and to elucidate its site preference within the lattice. Two contact shifted peaks with very different ${T}_{1}$ relaxation from the main zircon peak but attributable to the zircon lattice are observed in the ${}^{29}\mathrm{Si}$ NMR spectra for all samples. These spectra are consistent with vanadium substitution on both the tetrahedral and dodecahedral sites, with a slight preference for the silicon site. The data show that the relative occupation of these two sites is almost independent of the preparation method and vanadium concentration. At a higher vanadium concentration a third additional peak is observed which may indicate another substitution site. Variable temperature NMR and susceptibility measurements indicate the hyperfine nature of the interactions influencing silicon from ${\mathrm{V}}^{4+}$ ions in the different sites.

Theoretical study of the polarized electronic absorption spectra of vanadium-doped zircon

The polarized electronic absorption spectra of a blue vanadium-doped zircon single crystal, grown by the flux method, has been studied by quantum chemical (CI) calculations in order to determine the position of the V 4+ dopant in the zircon host structure. Particularly, the excitation energies and polarizations of V 4+ occupying alternate positions, either the zirconium or silicon position or the interstitial site 16 g, have been considered. It is concluded that the observed electronic absorption spectra and the color of zirconblue can only be explained if the V 4+ chromophore is placed on the respective interstitial position.

Crystallization behaviour and microstructural development in ZrSiO4 and V-ZrSiO4 solid solutions from colloidal gels

Zircon and vanadium-doped zircon blue pigments were prepared by heat treatment of gel precursors. Gels with nominal compositions Vx-ZrSiO4 with x=0.0, 0.002, 0.004, 0.02 and 0.2 were prepared by formation of a silica coating on zirconia colloidal particles previously obtained. The crystallization behavior and microstructural evolution were studied using X-ray powder diffraction (XRD), energy dispersive X-rays microanalysis (EDX), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and selected area electron diffraction (SAED). The results indicated that the vanadia loading in the precursor gels speeds up the crystallization of the vanadium-containing tetragonal zirconia solid solutions and their transformation to the monoclinic form. The overall conversion rate of gel precursors to pigmenting powders increased when the vanadia content was higher. Microstructural data revealed that the used procedure for the preparation of vanadium-zircon pigments allowed high-purity and non-agglomerated powders with controlled particle size to be obtained.

Hyperfine Characterization of Pure and Doped Zircons

The aim of this work has been to investigate the influence of two coloring dopant ions on the ZrSiO4 host lattice. Pure, vanadium-doped and praseodymium-doped zircon powders have been synthesized by the ceramic method and analyzed using X-ray diffraction and the perturbed angular correlations (PAC) hyperfine technique, which probes the nearest environments of zirconium ions. XRD results indicated that vanadium and praseodynamium incorporations in the host lattice result in crystalline solid solutions. In turn, PAC information allowed in both cases the determination of two contributions to the hyperfine interaction. In the case of vanadium-doped zircon, V4+ substitutes mainly for Zr4+ and in a minor fraction for Si4+ or else it localizes at a tetrahedral interstitial site. In praseodymium-doped zircon, a low fraction of Pr4+ cations locate at Zr4+ sites. A slight modification of the zircon lattice is probably caused by the incorporation of the remaining cations at tetrahedral interstitial sites of the solid solution.

Synthesis and Characterization of Vanadium-Containing ZrSiO4 Solid Solutions from Gels

A procedure is reported for the preparation of vanadium-doped zircon pigmenting system with different vanadia loadings which enabled their complete formation and further characterization. Vanadium-zircon solid solutions were prepared by gelling mixtures of ZrO2 and V2O5 colloidal sols and tetraethylorthosilicate and studied over the temperature range up to the formation of zircon. The reaction sequence of gels was evaluated by X-ray powder diffraction (XRD) and ultraviolet-visible (UV-Vis) diffuse reflectance. It was found that the first crystalline phase detected was a vanadium-containing tetragonal ZrO2 solid solution where vanadium was stabilized in the reduced V+4 state. The formation of the V-ZrSiO4 solid solution occurred by the reaction between the monoclinic form of V+4-ZrO2 solid solution and the amorphous silica phase. Energy dispersive X-ray microanalysis (SEM/EDX) data, measurements of lattice parameters and UV-Vis diffuse reflectance of V-ZrSiO4 solid solutions revealed that vanadium was dissolved as V+4 replacing Si+4 in tetrahedral sites in the crystal structure of zircon. The solubility limit of vanadium in ZrSiO4 was about 0.01 mole of vanadium per mole of zircon (0.5 wt% as V2O5).

Ab initio cluster-in-the-lattice description of vanadium-doped zircon: analysis of the impurity centers in vanadium(4+)-doped zircon (ZrSiO4)

A theoretical description of crystal effects induced by the presence of impurity centers V 4+ entering the dodecahedral and tetrahedral sites of ZrSiO 4 is presented. The computation of the electronic structure and equilibrium geometry of the impurity center in the ionic crystal is carried out using an extension of the ab initio perturbed ion method. This theoretical model consists of a new type of cluster-in-the-lattice scheme that involves the rigorous quantum mechanical solution of clusters of varying size embedded in a quantum crystal lattice