Electronic absorption by Ti3+ ions and electron delocalization in synthetic blue rutile

Polarized absorption spectra, sigma and pi, in the spectral range 30000-400 cm(-1) (3.71-0.05 eV) were obtained on crystal slabs // [001] of deep blue rutile at various temperatures from 88 to 773 K. The rutile crystals were grown in Pt-capsules from carefully dried 99.999% TiO2 rutile powder at 50 kbar/500 degrees C using graphite heating cells in a belt-type apparatus. Impurities were below the detection limits of the electron microprobe (about 0.005 wt% for elements with Z greater than or equal to 13). The spectra are characterized by an unpolarized absorption edge at 24300 cm(-1), two weak and relatively narrow (Delta v(1/2)approximate to 3500-4000 cm(-1)), slightly sigma-polarized bands v(1) at 23500 cm(-1) and v(2) at 18500 cm(-1), and a complex, strong band system in the NIR (near infra red) with sharp weak peaks in the region of the OH stretching fundamentals superimposed on the NIR system in the sigma-spectra. The NIR band system and the UV edge produce an absorption minimum in both spectra, sigma and pi, at 21000 cm(-1) i.e. in the blue, which explains the colour of the crystals. Bands v(1) and v(2) are assigned to dd transitions to the Jahn-Teller split upper E-g state of octahedral Ti3+. The NIR band system can be fitted as a sum of three components. Two of them are partly pi-polarized, nearly Gaussian bands, both with large half widths 6000-7000 cm(-1), v(3) at 12000 cm(-1) and the most intense v(4) at 6500 cm(-1). The third NIR band v(5) of a mixed Lorentz-Gaussian shape with a maximum at 3000 cm(-1) forms a shoulder on the low-energy wing of v(4). Energy positions, half band widths and temperature behaviour of these bands are consistent with a small polaron type of Ti3+Ti4+ charge transfer (CT). Polarization dependence of CT bands can be explained on the basis of the structural model of defect rutile by Bursill and Blanchin (1983) involving interstitial titanium. Two OH bands at 3322 and 3279 cm(-1) in sigma-spectra show different stability during annealing, indicating two different positions of proton in the rutile structure, one of them probably connected with Ti3+ impurity. Total water concentration in blue rutile determined by IR spectroscopy is 0.10 wt-% OH. The EPR spectra measured in the temperature interval 20-295 K show the presence of an electron centre at temperatures above 100 K and Ti3+ ions in more than one structural position, but predominantly in compressed interstitial octahedral sites, at lower temperatures. These results are ill good agreement with the conclusions based on the electronic absorption data.