Intrinsic small polarons in rutile TiO2

Electron paramagnetic resonance (EPR) is used to identify the intrinsic electron small polaron in TiO2 crystals having the rutile structure. These self-trapped electrons are produced at very low temperature with 442 nm laser light. The defects form when a Ti4+ ion at a regular lattice site traps an electron and converts to a Ti3+ (3d(1)) ion. They become thermally unstable above similar to 15 K. An activation energy of 24 meV describes this release of the electrons (either by a hopping motion or directly to the conduction band). The g matrix is obtained from the angular dependence of the EPR spectrum. Principal values are 1.9807, 1.9786, and 1.9563 and principal axes are along high-symmetry directions in the crystal. The unpaired electron occupies an vertical bar x(2) - y(2)> orbital where x and y are in the equatorial plane of the TiO6 unit and y is the [001] direction. These intrinsic small polarons serve as a prototype for many of the defect-associated Ti3+ ions often observed in this material. They also can be used as a computational test case to evaluate the validity of different approximations presently being employed in density-functional-theory modeling of point defects in TiO2 and other transition-metal oxides. DOI: 10.1103/PhysRevB.87.125201