DEFECTS IN TIO2 CRYSTALS PRODUCED BY NEUTRON IRRADIATIONS AT 20-K

The single crystals rutile (TiO2), cut parallel and perpendicular to t he c-axis, are irradiated by reactor neutrons at 20 K (8.0 x 10(16) n/ CM2; E > 0.1 MeV). By means of optical measurements an intense absorpt ion band, which has a maximum peak near 1 mum (having FWHM approximate ly 0.87 eV), is observed and is annealed out at about 220 K. Also, som e kinds of defect centers can be distinguished by ESR measurements. Th e broad band has similar characteristics to that in reduced TiO2 Cryst al, in which the band has a maximum peak at 1.5 mum. With heavy reduct ion, the intensity of the broad band enhances with increasing electric al conductivity. It has been proposed that the origin of the band in r educed crystals may be attributable to the absorption of donors due to the polaron effects. The evidence for the assignment to the defect in the irradiated crystals is obtained by optical, ESR, and electrical r esistivity measurements. The results lead to quite a different origin for the irradiation produced defect. centers.