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F+ center, an electron trapped at oxygen vacancy (VO), was investigated inthe oxygen deficient Y3Al5O12 (YAG) crystals by EPR. The measurements wereperformed at temperatures 5-450 K and frequencies 9.4-350 GHz with using boththe continue wave and pulse EPR technique. The pulse electron-nuclear doubleresonance was applied to resolve the hyperfine interaction of the trappedelectron with surrounding nuclei. The measurements show that at lowtemperatures, T < 50 K, EPR spectrum of the F+ center is anisotropic with gfactors in the range 1.999-1.988 and originates from three magneticallyinequivalent positions of the center in garnet lattice according to differentdirections of the Al(IV)-VO-Al(VI) chains, where Al(IV) and Al(VI) are thetetrahedral and octahedral Al sites, respectively. As the temperatureincreases, the EPR spectrum becomes isotropic suggesting a motional averagingof the anisotropy due to motion of electron between neighboring oxygenvacancies. With further increase of the temperature to T > 200 K, we observeddelocalization of the electron into the conduction band with the activationenergy about 0.4-0.5 eV that resulted in substantial narrowing of the EPRspectral line with simultaneous change of its shape from the Gaussian toLorentzian due to diminish up to zero of the Fermi contact hyperfine field at27Al and 89Y nuclei. Such temperature behavior of the F+-center electron in YAGis completely similar to behavior of a donor electron in a semiconductor. Ourfindings is further supported by measurements of the conductivity anddielectric properties. In particular, these data show that the conductionelectrons are not homogeneously distributed in the crystal: there arehigh-conductive regions separated by poorly-conductive dielectric layers. Thisleads to the so-called Maxwell-Wagner dielectric relaxation with huge apparentdielectric constant at low frequencies.

Oxygen-vacancy centers in Y3Al5O12 garnet crystals: electron paramagnetic resonance and dielectric spectroscopy study