The crystal chemistry of birefringent natural uvarovites. Part III. Application of the superposition model of crystal fields with a characterization of synthetic cubic uvarovite

Synthetic, flux-grown uvarovite, Ca3Cr2 [SiO4](3), was investigated by optical methods, electron microprobe analysis, UV-VIS-IR microspectrometry, and luminescence spectroscopy. The crystal structure was refined using single-crystal X-ray CCD diffraction data. Synthetic uvarovite is optically isotropic and crystallizes in the "usual" cubic garnet space group Mid [a = 11.9973 Angstrom, Z = 8; 21524 reflections, R1 = 2.31 % for 454 unique data and 18 variables, Cr-O 1.9942(6), Si-O = 1.6447(6), Ca-O-a = 2.3504(6), Ca-O-b 2.4971(6),]. The structure of Ca3Cr2[SiO4](3) complies with crystal-chemical expectations for ugrandite group garnets in general as well as with predictions drawn from "cubically averaged" data of non-cubic uvarovite-grossular solid solutions (Wildner and Andrut 2001). The electronic absorption spectra of Cr3+ in trigonally distorted octahedra of synthetic uvarovite were analyzed in terms of the superposition model (SM) of crystal fields. The resulting SM and interelectronic repulsion parameters are (B) over bar (4) = 9532 cm(-1), (B) over bar (2) = 4650 cm(-1), power law exponent t(4) = 6.7, Racah B-35 = 703 cm(-1) at 290 K (reference distance R-0 = 1.995 A; fixed power law exponent t(2) = 3 and spin-orbit parameter = 135 cm-1). The interelectronic repulsion parameters Racah B-55 = 714 cm(-1) and C = 3165 cm(-1) were extracted from spin-forbidden transitions. This set of SM parameters was subsequently applied to previously well-characterized natural uvarovite-grossular solid solutions (Andrut and Wildner 2001a; Wildner and Andrut 2001) using their extrapolated Cr-O bond lengths to calculate the energies of the spin-allowed bands. These results are in very good agreement with the experimentally determined band positions and indicate the applicability of the superposition model to natural 3d(N) prevailing systems in geosciences. Single-crystal IR absorption spectra of synthetic uvarovite in the region of the OH-stretching vibration exhibit one isotropic absorption band at 3508 cm(-1) at ambient conditions, which shifts to 3510 cm(-1) at 77 K. This band is caused by structurally incorporated hydroxyl groups via the (O4H4)-hydrogarnet substitution. The water content, calculated using an integral extinction coefficient epsilon = 60417 cm(-2) l mol(-1) is C-H2O = 33 ppm.