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Triplet ground state of the neutral oxygen-vacancy donor in rutile TiO 2

2014; American Physical Society; Volume: 89; Issue: 11 Linguagem: Inglês

10.1103/physrevb.89.115206

1550-235X

A. T. Brant, E. M. Golden, N. C. Giles, Shan Yang, Md. Amran Sarker, Satoshi Watauchi, Masanori Nagao, Isao Tanaka, Donald A. Tryk, A. Manivannan, L. E. Halliburton,

Advanced Photocatalysis Techniques

Electron paramagnetic resonance (EPR) is used to investigate the triplet ($S$ = 1) ground state of the neutral oxygen vacancy in bulk rutile ${\mathrm{TiO}}_{2}$ crystals. This shallow donor consists of an oxygen vacancy with two nearest-neighbor, exchange-coupled Ti${}^{3+}$ ions located along the [001] direction and equidistant from the vacancy. The spins of the two trapped electrons, one at each Ti${}^{3+}$ ion, align parallel to give the $S$ = 1 state. These neutral oxygen vacancies are formed near 25 K in as-grown oxidized ${\mathrm{TiO}}_{2}$ crystals by illuminating with sub-band-gap 442 nm laser light. The angular dependence of the EPR spectra provides the principal values and axes for the $g$ and $D$ matrices. Observations of the Ti and Ti hyperfine lines when the magnetic field is along high-symmetry directions show that the two Ti${}^{3+}$ ions are equivalent; i.e., they have equal hyperfine $A$ matrices. The $A$ matrix for each Ti${}^{3+}$ ion in the neutral $S$ = 1 oxygen vacancy is approximately half of the $A$ matrix reported earlier for the one Ti${}^{3+}$ ion in the singly ionized $S$ = 1/2 oxygen vacancy [Brant et al., J. Appl. Phys. 114, 113702 (2013)]. The neutral oxygen vacancies are thermally unstable above 25 K. They release an electron to the conduction band with an activation energy near 63 meV and convert to singly ionized $S$ = 1/2 oxygen vacancies. When undoped ${\mathrm{TiO}}_{2}$ is sufficiently oxygen deficient (i.e., reduced), this combination of conduction band electrons and singly ionized oxygen vacancies may result in carrier-mediated ferromagnetism at room temperature.