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

The Raman microprobe spectra of natural brookite crystals from Switzer land and Brazil and a synthetic brookite powder exhibit a characterist ic intense band at 153 cm(-1). In contrast, anatase has a band of simi lar intensity at 144 cm(-1) and rutile lacks a strong band in this reg ion. 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, 5 02 cm(-1)), B-2g (366, 395, 463, 584 cm(-1)) and B-3g (452 cm(-1)). Ei ght weaker bands and component sub-bands resolved at 172, 287, 545, 61 8 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 ind icated by spot electron microprobe analysis. Electron spin resonance s pectra suggest small amounts of Fe3+ are substituting for Ti4+ in the structure. Concomitant protonation of oxygens to maintain charge balan ce yields OH groups which give rise to three sharp, low-intensity abso rption bands near 3360, 3380 and 3404 cm(-1) in the infrared spectra. X-ray photoelectron spectroscopy shows that surface-bound OH/H2O speci es exist in both crystals but no detectable Ti3+.