High-frequency electron paramagnetic resonance of the hole-trapped antisite bismuth center in photorefractive bismuth sillenite crystals

The hole-trapped antisite bismuth center has been directly observed by W-band (94 GHz) electron paramagnetic resonance (EPR) in the series of sillenite crystals, Bi12MO20 (M=Ge,Si,Ti, denoted as BMO), either nondoped or doped with transition ions (Cr, Cu, Ru, Ce). Blue light illumination influences the EPR intensity in most crystals, while in nondoped Bi12GeO20 and Bi12SiO20 the signals only appear upon illumination. The spectra can be attributed to a single species and no anisotropy could be detected eliminating any significant deviation from tetrahedral symmetry due to a perturbing defect in the near neighborhood or to static lattice distortion. The large and isotropic hyperfine parameter, in good agreement with previous optically detected magnetic-resonance measurements [Phys. Rev. B 47, 5638 (1993)], reveals that only similar to 25% of the hole is in the Bi 6s(1) orbital, by delocalization mainly to the neighboring oxygen ions, with extremely small spin densities on the surrounding Bi3+ lattice ions as derived from the EPR linewidths. The parameter variations between the three crystalline hosts are very small, showing a near-identical degree of delocalization of the trapped hole.