Electron paramagnetic resonance (EPR) is used to identify a new and unique photoactive silicon-related point defect in single crystals of rutile TiO2. The importance of this defect lies in its assignment to interstitial silicon ions and the unexpected establishment of silicon impurities as a major hole trap in TiO2. Principal g values of this new S=1/2 center are 1.9159, 1.9377, and 1.9668 with principal axes along the [¯110],[001], and [110] directions, respectively. Hyperfine structure in the EPR spectrum shows the unpaired spin interacting equally with two Ti nuclei and unequally with two Si nuclei. These silicon ions are present in the TiO2 crystals as unintentional impurities. Principal values for the larger of the two Si hyperfine interactions are 91.4, 95.4, and 316.4 MHz with principal axes also along the [¯110],[001], and [110] directions. The model for the defect consists of two adjacent Si ions, one at a tetrahedral interstitial site and the other occupying a Ti site. Together, they form a neutral nonparamagnetic [Siint−SiTi]0 complex. When a crystal is illuminated below 40 K with 442-nm laser light, holes are trapped by these silicon complexes and form paramagnetic [Siint−SiTi]+ defects, while electrons are trapped at oxygen vacancies. Thermal anneal results show that the [Siint−SiTi]+ EPR signal disappears in two steps, coinciding with the release of electrons from neutral oxygen vacancies and singly ionized oxygen vacancies. These released electrons recombine with the holes trapped at the silicon complexes
