POLARIZED ELECTRONIC ABSORPTION-SPECTRA OF CR2SIO4 SINGLE-CRYSTALS

Polarized electronic absorption spectra, E parallel to a(parallel to X ), E parallel to b(parallel to Y) and E parallel to c(parallel to Z), in the energy range 3000-5000 cm(-1) were obtained for the orthorhombi c the- nardite-type phase Cr2SiO4, unique in its Cr2+-allocation sugge sting some metal-metal bonding in Cr2+Cr2+ pairs with Cr-Cr distance 2 .75 Angstrom along [001]. The spectra were scanned at 273 and 120 K on single crystal platelets parallel to(100), containing optical Y and Z , and parallel to(010), containing optical X and Z, with thicknesses 1 2.3 and 15.6 mu m, respectively. Microscope-spectrometric techniques w ith a spatial resolution of 20 mu m and 1 nm spectral resolution were used. The orientations were obtained by means of X-ray precession phot ographs. The xenomorphic, strongly pleochroic crystal fragments (X dee ply greenish-blue, Y faint blue almost colourless, Z deeply purple alm ost opaque) were extracted from polycrystalline Cr2SiO4, synthesized a t 35 kbar, above 1440 degrees C from high purify Cr2O3, Cr (10% excess ) and SiO2 in chromium capsules. The Cr2SiO4-phase was identified by X -ray diffraction (XRD). Four strongly polarized bands, at about 13500 (I), 15700 (II), 18700 (III) and 19700 (IV) cm(-1), in the absorption spectra of Cr2SiO4 single crystals show properties (temperature behavi our of linear and integral absorption coefficients, polarization behav iour, molar absorptivities) which are compatible with an assignment to localized spin-allowed transitions of Cr2+ in a distorted square plan ar coordination of point symmetry C-2. The crystal field parameter of Cr2+ is estimated to be 10 Dq similar or equal to 10700 cm(-1). A rela tively intense, sharp band at 18400 cm(-1) and three other minor featu res can, from their small half widths, be assigned to spin-forbidden d d-transitions of Cr2+. The intensity of such bands strongly decreases on decreasing temperature. The large half widths, near 5000 cm(-1) of band III are indicative of some Cr-Cr interactions, i.e. delta-delta transitions of Cr-2(4+), whereas the latter alone would be in conflict with the strong polarization of bands I and II parallel [100]. Theref ore, it is concluded that the spectra obtained can best be interpreted assuming both dd-transitions of localized d-electrons at Cr2+ as well delta-delta transitions of Cr-2(4+) pairs with metal-metal interacti on. To explain this, a dynamic exchange process 2Cr(loc)(2+) double le ft right arrow Cr-2,cpl(4+) is suggested wherein the half life times o f the ground states of both exchanging species are significantly longe r than those of the respective optically excited states, such that the spectra show both dd-and delta-delta- transitions.