Optical absorption investigation of Cr3+ ion-bearing minerals in the temperature range 77?797 K

Abstract

Abstract. The effect of raising temperature on spin-allowed dd-transitions of octahedral Cr a+ was studied for various point sym_metries of the Cr 3 +-bearing structural sites, i.e. 3 m and 3 with inversion center in spinel and garnets, respectively, or 32, 3, 2 and 1, lacking the inversion centre, in beryl, corundum, diopside and topaz, respectively. For this purpose, crystals of Cr3+-bearing spinel, pyrope, andradite, grossular, uvarovite, emerald, ruby, diopside and topaz were analyzed by microprobe, oriented, and measured in polarized radiation (except for the cubic minerals) in the spectral range 30000 to 11 100 cm-1 and at temperatures between 77 and 797 K. The evaluation of the intensities, half widths, and energy positions of bands due to Cr 3 +-transitions derived from 4Aa~ ~ 4r2g(F ) and ~ 4rlg(F ) as well as of Dq-and B-values derived, had the following results:In all cases, red shift of the above bands and, hence, independent on the site symmetry of Cr 3+, decreases in the Dq-values were obtained. The dependcies of Dq on T are nearly linear above room temperature and amount between -1.6% in topaz and -5.1% in pyrope in the temperature range studied. From this, values for the local thermal expansion of the Cr 3+-centered octahedra, ~1oo, were derived on the basis of the R~5_o-proportionality of lODq. Such values are consistently higher than those obtained from X-ray refinements, a method averaging RM_ o for all the respective octahedral positions.On increasing temperature, the half band widths increase. The evaluation for spinel, pyrope and uvarovite showed A ~/a to increase by about 60%, for both types of Cr 3+ transitions, in the temperature range studied.The temperature dependence of band intensities is complex, because it depends on the site symmetry and related symmetry selection rules: symmetry forbidden transitions increase strongly on heating. This was observed for Cr 3+ in centosymmetric sites 3 m of spinel and of garnets as well as for the symmetry-forbidden Cr 3 +-transitions in the acentric sites 32 in beryl and 3 in ruby.Where no symmetry-related selection rules exist, as inCorrespondence to: K. Langer point symmetry 1 in topaz, almost no temperature dependence of band intensities was observed. This holds also for diopside M 1-octahedra with point symmetry 2, although here the U-band should be forbidden at least in one direction. As for the intensity of the U-bands in spinel, pyrope and uvarovite, the observed intensity increase with T, agreed with that calculated on the basis of vibronic coupling with an odd vibration near co= 450 cm-1. In order to differentiate between the influences of static and dynamic relaxation of the Laporte rule on the intensities, the relative band intensities, subject to static effects only, were calculated for Cr 3+ in ruby and emerald. This proved the temperature dependence of the U~ and Y~-band, respectively, to be caused by vibronic coupling only.

Optical spectroscopic study of synthetic NaScSi2O6–CaNiSi2O6 pyroxenes at normal and high pressures

“…With the diffuse and uncertain conoscopic figures which we observed, we were unable to orient the crystals precisely enough for preparing oriented samples and measuring polarized spectra in the three polarizations E||X, E||Y and E||Z, as it was done for natural Cr-or Fe-bearing clinopyroxenes by e.g. Taran et al (1994) or Taran and Langer (2001). In the present case the actual orientations were accomplished by extinction between crossed Nicols on sections randomly cut out along the elongation of the crystals.…”

Direct Observation of Cr3+ 3d States in Ruby: Toward Experimental Mechanistic Evidence of Metal Chemistry

“…39 The RIXS spectra (Figure 3b) show seven sharp features in the 0–6 eV range, which are more than the typical three optical absorption bands (Figure 3c). 15,16,42−44 The final state of RIXS is identical to the final state of optical absorption, hence the same electronic states are present a priori. The additional observed states in RIXS, result from the technique’s ability to strongly probe both states with doublet and quartet spin multiplicity that for the ruby XAS and RIXS data this is the first description in a symmetry lower than the ideal octahedral case plus that these are the first soft RIXS data on ruby reported.…”

Reduced radiative conductivity of low spin FeO6-octahedra in FeCO3 at high pressure and temperature

“…The red-shift is likely due to the thermal expansion as the energy separation between the t 2g and e g orbitals is inversely proportional to the fifth power of the metal-ligand distance (Burns, 1993). Intensity variations of CF bands with temperature reported in the literature show a less regular behavior (Shankland et al, 1979;Fukao et al, 1968;Goncharov et al, 2008;Ullrich et al, 2002Ullrich et al, , 2004Thomas et al, 2012;Taran et al, 1994), but often may be understood in terms of temperature-induced changes in the CF symmetry (Taran and Langer, 2001) lifting the Laporte selection rule, which impedes electron transfer among the same parity states (e.g. d-d transitions).…”