STRUCTURAL AND CHEMICAL-TRANSFORMATIONS INDUCED BY LASER IMPACT ON TIO2 AND NB2O5

TiO2 (rutile) and Nb2O5 have been irradiated by a 335 nm laser light. This changes the colour of these oxides from white to dark blue. The s tructural and chemical transformations induced by the irradiation have been investigated. Although no severe structural perturbation of the oxides is detected by X-ray diffraction, their electrical resistivity drops from approximate to 10(9) Omega cm to approximate to 10 Omega cm in the case of TiO2 and to approximate to 10(-2) Omega cm in the case of Nb2O5. The temperature dependence of the electrical transport prop erties can be explained by the formation under light irradiation of io ns with unusual valence (Ti3+ and Ti2+, or Nb4+ and Nb2+), leading to energy levels in the electronic gap and to a hopping-driven conduction at room temperature. The presence of such ions is supported by diffus e reflectance, electronic paramagnetic resonance, and X-ray photoelect ron spectroscopy measurements. Comparing with experimental data for th ese and related materials indicates that the defects associated with t he presence of these ions could have the form of shear planes similar to those met in the Magneli phases TinO2n-1 with n > 15. (C) 1997 Else vier Science Ltd.