Interpretation of charge transfer bands in Fe doped SrTiO3 crystals

The electronic structures of SrTiO₃ crystals doped with Fe³⁺, Fe⁴⁺ and Fe⁵⁺ ions have been investigated using the Xα cluster approach. The ground-state eigenvalues show the lower Fe acceptor level, of t_2g↓ symmetry, localized inside the SrTiO₃ band gap, respectively at 2.8 eV $\text{(}\text{Fe}{}^{\mathrm{3+}}\text{, S =}\text{5}\text{2}\text{), 1.6}\text{eV}\text{(}\text{Fe}{}^{\mathrm{4+}}\text{, S = 1)}\text{or}\text{1.1}\text{eV}\text{(}\text{Fe}{}^{\mathrm{4+}}\text{, S = 0)}\text{and}\text{1.1}\text{eV}\text{(}\text{Fe}{}^{\mathrm{5+}}\text{, S =}\text{3}\text{2}\text{)}$ above the valence band edge. Other acceptor levels, with eg↓ and eg↑ symmetries, appear inside the gap when the Fe nominal ionicity increases.The theoretical Xα excitation energies of O 2p-Fe 3d transitions confirms the experimental interpretations of acceptor charge transfer bands for the optical absorption spectra of SrTiO₃:Fe⁴⁺ and SrTiO₃:Fe⁵⁺ crystals.The large optical excitation energies compared with the thermal transitions are partly due to the O 2p band width.