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

Abstract

The effect of raising temperature on spin-allowed dd-transitions of octahedral Cr³⁺ was studied for various point symmetries of the Cr³⁺-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 30 000 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 Cr³⁺-transitions derived from ⁴ A _2g → ⁴ T _2g (F) and →⁴ T _1g (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 Cr³⁺, 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 Cr³⁺-centered octahedra, α _loc, were derived on the basis of the R ^-5_M-0 -proportionality of 10Dq. Such values are consistently higher than those obtained from X-ray refinements, a method averaging r_m-o for all the respective octahedral positions.

On increasing temperature, the half band widths increase. The evaluation for spinel, pyrope and uvarovite showed $\Delta \tilde v_{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2}}$ to increase by about 60%, for both types of Cr³⁺ 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 Cr³⁺ in centosymmetric sites $\bar 3$ m of spinel and $\bar 3$ of garnets as well as for the symmetry-forbidden Cr³⁺-transitions in the acentric sites 32 in beryl and 3 in ruby. Where no symmetry-related selection rules exist, as in 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 ω= 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 Cr³⁺ in ruby and emerald. This proved the temperature dependence of the U _π and Y _π -band, respectively, to be caused by vibronic coupling only.