Vm. Khomenko et al., ELECTRONIC ABSORPTION BY TI3+ IONS AND ELECTRON DELOCALIZATION IN SYNTHETIC BLUE RUTILE, Physics and chemistry of minerals, 25(5), 1998, pp. 338-346
Polarized absorption spectra, sigma and pi, in the spectral range 3000
0-400 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 t
han or equal to 13). The spectra are characterized by an unpolarized a
bsorption 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 r
egion 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) an
d 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 su
m of three components. Two of them are partly pi-polarized, nearly Gau
ssian bands, both with large half widths 6000-7000 cm(-1), v(3) at 120
00 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, hal
f band widths and temperature behaviour of these bands are consistent
with a small polaron type of Ti3+Ti4+ charge transfer (CT). Polarizati
on dependence of CT bands can be explained on the basis of the structu
ral model of defect rutile by Bursill and Blanchin (1983) involving in
terstitial titanium. Two OH bands at 3322 and 3279 cm(-1) in sigma-spe
ctra show different stability during annealing, indicating two differe
nt positions of proton in the rutile structure, one of them probably c
onnected with Ti3+ impurity. Total water concentration in blue rutile
determined by IR spectroscopy is 0.10 wt-% OH. The EPR spectra measure
d in the temperature interval 20-295 K show the presence of an electro
n centre at temperatures above 100 K and Ti3+ ions in more than one st
ructural position, but predominantly in compressed interstitial octahe
dral sites, at lower temperatures. These results are ill good agreemen
t with the conclusions based on the electronic absorption data.