Absorption spectra of zircon crystals doped with Cr(IV): ZrSiO4:Cr4+

Crystal structural and optical properties of Cr-doped Y2Ti2O7 and Y2Sn2O7 pyrochlores

“…Crystal field splitting ∆ o of the octahedrally coordinated Cr4+ varies widely in titanates, from 18270 to 19730 cm-1, being roughly increasing with the amount of chromium in pyrochlore lattice, but it is quite steady in stannates, ranging from 19710 to 19840 cm-1 (Table 4). These values confirm those obtained by Pavlov et al [24] and are coherent with a large set of ∆ t obtained for Cr4+ in several tetrahedral sites [40][41][42][43][44][45] once the conversion ∆ o=9/4 ∆ t is made. However, these data are not consistent with the crystal field theory, as a larger ∆ o is expected in titanate pyrochlores that have a shorter metal-oxygen bonding (B-O ~195 pm) with respect to stannates (B-O ~203 pm).…”

“…These values are higher than those determined for Cr4+ in silicate and germanate tetrahedra, i.e. Racah B in the 400-560 cm-1 range [42][43][44][45][46].…”

“…The occurrence of Cr 4+ in tetrahedral coordination though widely documented in oxides and silicates [41][42] and claimed for pyrochlores [27], is improbable for both structural and spectroscopic reasons. The former because it implies localization of chromium ions in interstitial sites of a compact, face-centred array of cations as that of pyrochlore lattice; the latter because d-d transitions of tetrahedrally coordinated Cr 4+ ions are mostly in the 12000-18000 cm -1 range [41][42][43][44][45][46] where our samples have their minimum absorbance. Crystal field splitting ∆ o of the octahedrally coordinated Cr4+ varies widely in titanates, from 18270 to 19730 cm-1, being roughly increasing with the amount of chromium in pyrochlore lattice, but it is quite steady in stannates, ranging from 19710 to 19840 cm-1 (Table 4).…”

Electrochemical Detection of High Oxidation States of Chromium(IV and V) in Chromium‐Doped Cassiterite and Tin‐Sphene Ceramic Pigmenting Systems

“…[9,38] The band in the near-infrared part of the spectrum, centred at 1200 nm, is assigned to 3 A 2 -3 T 1 transitions of Cr IV in tetrahedral surroundings. [9,39] The sphene structure can be described as composed of chains of corner-sharing SnO 6 octahedra running parallel to the cell edges that are cross-linked by silicate tetrahedra to form a SnOSiO 4 framework that accommodates Ca 2+ in irregular hepta-coordinated polyhedra. Here, López-Navarrete et al [9] recently suggested that Cr IV replaced the partially octahedral Sn IV and tetrahedral Si IV sites.…”

Enhancing the Initial Rate of Polymerisation of the Reduced Phillips Catalyst by One Order of Magnitude

“…Similar spectra are shown by several metal oxides doped with isolated Cr IV species in a tetrahedral coordination, which show two strong d-d bands in the 10 000-13 000 and 15 000-25 000 cm À1 regions. [29,30] Other transition metals with a d 2 electronic configuration (such as some V III -halogeno complexes [31] ) show strong d-d bands in approximately the same wavenumber range. The Cr IV oxidation state and the pseudo-tetrahedral coordination of the modified chromium sites is further supported by XANES spectroscopy ( Figure S3 and Table 1).…”