Fluorine donors and Ti3+ ions in TiO2 crystals

Electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) are used to identify and characterize neutral fluorine donors in TiO2 crystals having the rutile structure. The fluorine ions substitute for oxygen and are present as unintentional impurities. Isolated singly ionized fluorine donors in an as-grown (fully oxidized) crystal convert to their neutral charge state during exposure to 442 nm laser light at 6 K. These donors return to the singly ionized charge state within a few seconds when the light is removed. In contrast, the neutral fluorine donors are observed at 6 K without photoexcitation after a crystal is reduced at 600 degrees C in flowing nitrogen gas. The angular dependences of the EPR and ENDOR spectra provide a complete set of spin-Hamiltonian parameters (principal values are 1.9746, 1.9782, and 1.9430 for the g matrix and -0.23, 0.47, and 5.15 MHz for the F-19 hyperfine matrix). These matrices suggest that the unpaired electron is localized primarily on one of the two equivalent neighboring substitutional titanium ions, i.e., the ground state of the neutral fluorine donor in rutile-structured TiO2 is a Ti3+ ion adjacent to a F- ion.