Photoinduced self-trapped hole center in TiO2 crystals

Electron paramagnetic resonance is used to identify the intrinsic self-trapped hole center in bulk TiO2 crystals having the rutile structure. Two additional extrinsic trapped hole centers associated with a defect on a neighboring Ti4+ site are also observed. For each center, the unpaired spin (i.e., the hole) is localized in a nonbonding p orbital on an oxygen ion. The three hole centers are photoinduced with 442 nm laser light at 4 K. After the laser light is removed, the majority of the hole centers disappear immediately at 4 K with the remainder disappearing when the temperature is raised above approximately 10 K. Angular-dependence data, taken in the three high-symmetry planes, provide spin-Hamiltonian parameters for the self-trapped hole center and one of the two defect-related trapped hole centers. The principal values of the g matrix for the self-trapped hole center are 2.0040, 2.0129, and 2.0277 and the corresponding principal axes are along the [110], [001], and [1 (1) over bar0] directions in the crystal. Although the principal values of the g matrix for the extrinsic trapped hole center are similar (2.0036, 2.0182, and 2.0307), the directions of two of its principal axes are not along high-symmetry directions in the crystal because of the neighboring perturbation.