ARTICLES Structural and Spectroscopic Investigations of Blue, Vanadium-Doped ZrSiO 4 Pigments Prepared by a Sol-Gel Route Silvia Ardizzone,* ,² Giuseppe Cappelletti, ² Paola Fermo, ‡ Cesare Oliva, ² Marco Scavini, ² and Fabio Scime ` ² Department of Physical Chemistry and Electrochemistry, UniVersity of Milan, Via Golgi 19, I-20133 Milan, Italy, and Department of Inorganic, Metallorganic and Analytical Chemistry, UniVersity of Milan, Via Venezian 21, I-20133 Milan, Italy ReceiVed: August 1, 2005; In Final Form: September 30, 2005 A sol-gel reaction starting from silicon and zirconium alkoxides, in water-ethanol mixtures, was employed to obtain vanadium-doped zirconium silicate powders (zircon). The reactions were performed by modulating both (a) the amount of the vanadium salt in the starting mixture and also (b) the amount of mineralizer (NaF). The products of the sol-gel reaction were calcined at 600, 800, 1000, and 1200 °C. The samples were characterized by X-ray powder diffraction (XRPD), electron paramagnetic resonance spectroscopy (EPR), scanning electron microscopy (SEM), X-ray absorption near-edge spectroscopy (XANES), and diffuse UV- vis-near-IR reflectance spectroscopy. Results from the structural, morphological, and optical characterization are examined and cross-compared to produce a consistent picture of the key factors leading to the formation, growth, and optical properties of the reaction products. Introduction Zircon structure-based ceramic pigments have been widely used in the ceramic industry for decades. Zircon offers superior stability at high temperature and in corrosive environments. Furthermore, pigments with zircon as the host crystal yield a wide variety of colors and shades. 1-8 The turquoise blue zircon pigment, which contains vanadium as the dopant, was the first to be introduced commercially and one of the most successful. 9 It is generally accepted that the origin of the blue color, in this pigment, is due to the solid solution of V 4+ in the zircon lattice; 10 however the actual location of vanadium in the zircon structure is still unclear and has been the subject of numerous studies, in some cases leading to controversial interpretations. Furthermore, the role played by the addition of a fluxing agent, or mineralizer (halides), in the promotion of the blue color has not yet been fully clarified. Demiray et al., 10 on the grounds of optical absorption and X-ray diffraction studies, suggested that V 4+ is prevailingly located in the distorted dodecahedral zirconium sites; these conclusions appeared to be in agreement with ab initio cluster calculations of the lattice energy. 11 Exactly the opposite conclu- sion was reached on the basis of energy level calculations, which indicated that V 4+ preferred the tetrahedral silicon site 12 as did Raman spectroscopy. 13 This was further confirmed by electron paramagnetic resonance (EPR) studies, on the basis of a spectrum detectable only at T < 20 K and with A | ) 88 G, A ⊥ ) 31 G. 12 Indeed, the relatively low values of the hyperfine components were more easily attributable to a tetrahedral (low coordination number) than to a dodecahedral (high coordination number, i.e., more ionic bond). 14 Furthermore, the presence of a dynamic Jahn-Teller exchange could introduce a number of low-lying excited vibronic states through which Orbach relax- ation would occur. This could easily explain the fact that the EPR spectrum was not detectable at high temperature. 12 A further possibility some workers have proposed is that both sites can be occupied to a significant extent. This hypothesis was supported by EPR and optical measurements 15 and by lattice energetic calculations 16 which found very little difference in the energy of the V 4+ occupying either of the two principal lattice sites. More recently, Ocana et al. 17 reached similar conclusions, by using a variety of spectroscopic techniques. These authors suggested that the dopant substitutes both for silicon in the tetrahedral site and to a lesser extent for zirconium in the dodecahedral site. Extended X-ray-absorption fine structure measurements (EXAFS) of the vanadium K edge gave the ratio of the occupancy of the two sites as Nt/Nd ) 1.6. This ratio could also be used to rationalize their IR measurements. Furthermore, not only did they observe the above-mentioned low-temperature EPR spectrum attributable to V 4+ in a tetra- hedral site, but, in the presence of large amounts of V 4+ , they recorded also a second EPR feature at temperatures even above that of liquid nitrogen, formed by a single structureless feature attributable to many V 4+ ions dipolarly interacting with each other. However, this broad EPR band remained unresolved and could not bring further information. A further possibility for the site occupancy of the V 4+ is its insertion into a interstitial, strongly distorted tetrahedrally * Corresponding author. Telephone: +390250314225. Fax: +390250314300. E-mail: silvia.ardizzone@unimi.it. ² Department of Physical Chemistry and Electrochemistry. E-mail: (G.C.) giuseppe.cappelletti@unimi.it; (C.O.) cesare.oliva@unimi.it; (M.S.) marco.scavini@unimi.it; (F.S.) fabio.scime@libero.it. ‡ Department of Inorganic, Metallorganic and Analytical Chemistry. Telephone: +390250314425. E-mail: paola.fermo@unimi.it. 22112 J. Phys. Chem. B 2005, 109, 22112-22119 10.1021/jp054254o CCC: $30.25 © 2005 American Chemical Society Published on Web 11/04/2005