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, ${\text{Bi}}_{12}M{\text{O}}_{20}$ ($M=\text{Ge},\text{Si},\text{Ti}$, 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 ${\text{Bi}}_{12}\mathrm{Ge}{\text{O}}_{20}$ and ${\text{Bi}}_{12}\mathrm{Si}{\text{O}}_{20}$ 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 $\ensuremath{\sim}25%$ of the hole is in the $\text{Bi}\text{ }6{s}^{1}$ orbital, by delocalization mainly to the neighboring oxygen ions, with extremely small spin densities on the surrounding ${\text{Bi}}^{3+}$ 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.