THE RAMAN-SPECTRUM OF BROOKITE, TIO2 (PBCA, Z=8)

The Raman microprobe spectra of natural brookite crystals from Switzerland and Brazil and a synthetic brookite powder exhibit a characteristic intense band at 153 cm(-1). In contrast, anatase has a band of similar intensity at 144 cm(-1) and rutile lacks a strong band in this region. Polarization experiments with the Brazilian crystal permit 17 out of a predicted 36 vibrational bands to be readily assigned as A(1g) (127, 154, 194, 247, 412, 640 cm(-1)), B-1g (133, 159, 215, 320, 415, 502 cm(-1)), B-2g (366, 395, 463, 584 cm(-1)) and B-3g (452 cm(-1)). Eight weaker bands and component sub-bands resolved at 172, 287, 545, 618 cm(-1), 254, 329, 476 cm(-1) and 497 cm(-1) are tentatively assigned to B-3g, B-2g, and A(1g), modes respectively. Traces of Si and Fe in the Brazilian crystal and Si, Fe, Al and S in the Swiss sample are indicated by spot electron microprobe analysis. Electron spin resonance spectra suggest small amounts of Fe3+ are substituting for Ti4+ in the structure. Concomitant protonation of oxygens to maintain charge balance yields OH groups which give rise to three sharp, low-intensity absorption bands near 3360, 3380 and 3404 cm(-1) in the infrared spectra. X-ray photoelectron spectroscopy shows that surface-bound OH/H2O species exist in both crystals but no detectable Ti3+.