  F+ center, an electron trapped at oxygen vacancy (VO), was investigated in
the oxygen deficient Y3Al5O12 (YAG) crystals by EPR. The measurements were
performed at temperatures 5-450 K and frequencies 9.4-350 GHz with using both
the continue wave and pulse EPR technique. The pulse electron-nuclear double
resonance was applied to resolve the hyperfine interaction of the trapped
electron with surrounding nuclei. The measurements show that at low
temperatures, T < 50 K, EPR spectrum of the F+ center is anisotropic with g
factors in the range 1.999-1.988 and originates from three magnetically
inequivalent positions of the center in garnet lattice according to different
directions of the Al(IV)-VO-Al(VI) chains, where Al(IV) and Al(VI) are the
tetrahedral and octahedral Al sites, respectively. As the temperature
increases, the EPR spectrum becomes isotropic suggesting a motional averaging
of the anisotropy due to motion of electron between neighboring oxygen
vacancies. With further increase of the temperature to T > 200 K, we observed
delocalization of the electron into the conduction band with the activation
energy about 0.4-0.5 eV that resulted in substantial narrowing of the EPR
spectral line with simultaneous change of its shape from the Gaussian to
Lorentzian due to diminish up to zero of the Fermi contact hyperfine field at
27Al and 89Y nuclei. Such temperature behavior of the F+-center electron in YAG
is completely similar to behavior of a donor electron in a semiconductor. Our
findings is further supported by measurements of the conductivity and
dielectric properties. In particular, these data show that the conduction
electrons are not homogeneously distributed in the crystal: there are
high-conductive regions separated by poorly-conductive dielectric layers. This
leads to the so-called Maxwell-Wagner dielectric relaxation with huge apparent
dielectric constant at low frequencies.
