Electron paramagnetic resonance (EPR) measurements revealed the inclusion of nitrogen in yellow anatase single crystals grown by chemical vapor transport using NH 4 Cl as the transport agent. The angular-dependence of EPR triplets suggests that nitrogen occupies a substitutional site of oxygen in the bulk. The paramagnetic nitrogen observed at room temperature becomes diamagnetic at lower temperatures by trapping another electron. Below 50 K, only one symmetric differential EPR signal from titanium with an unpaired electron can be observed. These changes in the electric states of nitrogen and titanium can be explained by the presence of oxygen vacancies and by the contribution of trapped electrons to oxygen vacancies. The optical absorption observed at 3.0 eV in an E k a configuration, which is characteristic of yellow anatase is almost independent of temperature.

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Electron paramagnetic resonance (EPR) measurement under uv light irradiation was performed on Al‐doped anatase TiO2 crystals grown by a chemical vapor transport method. In an as‐grown crystal, characteristic signals of EPR sextuplets are observed at 100–40 K. The intensity of the sextuplets decreases gradually with decreasing temperature. The sextuplets arise from a photogenerated hole of S =1/2 interacting with an Al nuclear spin of I =5/2. The angular dependence of the sextuplets is successfully explained by a model where four symmetrically equivalent paramagnetic species giving the sextuplets are present in the crystal and the principal z‐axis of a diagonal g‐matrix for the paramagnetic species has a small deviation from the crystalline c‐axis. The analysis of the angular dependence of the sextuplets gives anisotropic g ‐values of gxx=2.0254, gyy=2.0016, and gcomplexes=2.0129, and hyperfine coupling constants of Axx=4.93, Ayy=6.04, and Acomplexes=5.98 G. An EPR signal from bulk Ti3+ is also observed at temperatures below 40 K regardless of uv irradiation. In an annealed crystal, the sextuplets are observed at temperatures below 70 K under uv irradiation. The EPR signal corresponding to bulk Ti3+ is absent in the entire temperature range. These experimental results differ greatly from those for the as‐grown crystal (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)