Shared Holes Trapped by Charge Defects in SrTiO3

Using the techniques of EPR and optical irradiation in conjunction with optical-absorption measurements, we have gained useful information about the nature of some hole centers in SrTi${\mathrm{O}}_3$. Primarily, we have investigated, at temperatures near 77°K, single crystals doped with aluminum. Two principal centers have been explored: (1) the Al-${\mathrm{O}}^{-}$ center—a hole shared among the oxygens which surround ${\mathrm{Al}}^{3+}$, and (2) the $X$-${\mathrm{O}}^{-}$ center—a hole shared in a similar fashion, but more deeply trapped by a charge defect of unknown origin. The Al-${\mathrm{O}}^{-}$ center arises after band-gap irradiation and is characterized at 77°K by the following $g$ values and hyperfine constants: $g_{\mathrm{II}}=2.0137\mathrm{}$, $g_{\perp }=2.0124\mathrm{}$, $A_{\mathrm{II}}=8.3\mathrm{}\times {10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$, and $A_{\perp }=7.6\mathrm{}\times {10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$. The $X$-${\mathrm{O}}^{-}$ center is present before optical irradiation. No hyperfine structure is observed, but the isotropic $g$ value is 2.0130 at all temperatures from 4.2 to 300°K. An 800-nm absorption band arising after band-gap excitation has been correlated with the Al-${\mathrm{O}}^{-}$ center. 430- and 600-nm absorption bands have been correlated with the absence of ${\mathrm{Fe}}^{3+}$ in the EPR spectrum, and a 500-nm band has also been observed. In addition, the role of iron in the photochromic processes of SrTi${\mathrm{O}}_3$ is presented. Finally, theoretical work utilizing the molecular orbital $\sigma$ and $\pi$ states in $O_h$ symmetry has provided a firm basis for the sharing model. The experimental $g$ values and hyperfine constants are discussed in light of this model and are found to be in good agreement.