Structural and chemical transformations induced by laser impact on TiO 2 and Nb 2O 5

Abstract

TiO 2 (rutile) and Nb 2O 5 have been irradiated by a 335 nm laser light. This changes the colour of these oxides from white to dark blue. The structural 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 ≈ 10 9Ω cm to ≈ 10 Ω cm in the case of TiO 2 and to ≈ 10 −2 Ω cm in the case of Nb 2O 5. The temperature dependence of the electrical transport properties can be explained by the formation under light irradiation of ions with unusual valence (Ti 3+ and Ti 2+, or Nb 4+ and Nb 2+), 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 diffuse reflectance, electronic paramagnetic resonance, and X-ray photoelectron spectroscopy measurements. Comparing with experimental data for these and related materials indicates that the defects associated with the presence of these ions could have the form of shear planes similar to those met in the Magnéli phases Ti n O 2n − 1 with n > 15.