Optically detected magnetic resonance of the bismuth-on-metal-site intrinsic defect in photorefractive sillenite crystals

Applying the method of optically detected magnetic resonance (ODMR) via the magnetic circular dichroism (MCD) of absorption, paramagnetic resonance data are provided for the ground state of the intrinsic defect ${\mathrm{Bi}}_{\mathit{M}}$ in sillenites of the type ${\mathrm{Bi}}_{12}$M${\mathrm{O}}_{20}$ (M=Si, Ge, Ti). This is a characterization on a microscopic scale of this defect, generally being present at very high densities in these crystals, which are very interesting for photorefractive applications. The g values are slightly larger than ${\mathit{g}}_{\mathrm{free}}$ and the hyperfine constants A, accounting for the interaction with the central bismuth nucleus, are of the order of 19 GHz [g/A for ${\mathrm{Bi}}_{12}$${\mathrm{SiO}}_{20}$:2.049(5)/19.35(6) GHz; ${\mathrm{Bi}}_{12}$${\mathrm{GeO}}_{20}$:2.041(3)/19.21(5) GHz; ${\mathrm{Bi}}_{12}$${\mathrm{TiO}}_{20}$:2.05(1)/19.2(1) GHz]. Both parameters were found to be isotropic within the experimental error. The theoretical interpretation of g and A leads to the picture of a highly covalent defect, which consists of a hole mainly located on the oxygen neighbors of the tetrahedrally coordinated ${\mathrm{Bi}}_{\mathit{M}}^{3+}$ ion. The MCD spectra of the sillenite crystals are linked to the ODMR results by the method of tagged MCD and are thus identified to arise from this defect. Since MCD is related to absorption, this identification provides evidence for the attribution of the broad 2.6-eV absorption band, specific to sillenites, to ${\mathrm{Bi}}_{\mathit{M}}$.