The Raman spectrum of brookite, TiO 2 ( 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 1 g (127, 154, 194, 247, 412, 640 cm −1 ), B 1 g (133, 159, 215, 320, 415, 502 cm −1 ), B 2 g (366, 395, 463, 584 cm −1 ) and B 3 g (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 3 g , B 2 g and A 1 g 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 Fe 3+ are substituting for Ti 4+ 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/H 2 O species exist in both crystals but no detectable Ti 3+ .