We have studied the Ti3+/Ti4+ mixed valence state in Mg-doped Ti2O3 using hard x-ray photoemission spectroscopy. The Ti 2p spectrum for the corundum-type Ti2O3 revealed the Ti3+ configuration with strong electronic coupling in the c-axis Ti-Ti pairs whereas the data for the ilmenite-type MgTiO3 confirmed the Mg2+-Ti4+ charge state in the c-axis cation pairs. In Mg0.29Ti1.71O3, the Ti 2p spectrum hardly showed the Ti4+ peak, which was in MgTiO3 indicating that the c-axis pairing of the Mg is with a Ti3+ ion rather than a Ti4+ and that the Ti3+/Ti4+ mixed valence state is materialized within the Ti-Ti c-axis pairs. In Mg0.63Ti1.37O3, we detected the presence of Mg2+-Ti4+ pairs. The results indicate the important role of hybridization within the c-axis pairs not only in the Ti3+-Ti3+ configuration, but also above all in the Ti3+/Ti4+ mixed valence state, superseding the Madelung energy gain of the Mg2+-Ti4+ c-axis pair formation.

Corundum oxide Ti2O3 shows the metal-insulator transition around 400-600 K accompanying the nearest Ti3+-Ti3+ bond (a1ga1g singlet state) formation along the c axis. In order to clarify the hole-doping effect for the a1ga1g singlet bond in Ti2O3, we investigated Ti 3d orbital anisotropy between corundum-type Ti2O3 and ilmenite-type MgTiO3 using linear dichroism of soft x-ray absorption spectroscopy of the Ti L2,3 edge. From the linear dichroic spectral weight in MgyTi2-yO3, we confirmed that the a1ga1g state is dominant not only in y = 0.01 (almost Ti2O3), but also in y = 0.29, indicating that the Ti-Ti bond survives against a certain level of hole doping. In y = 0.63 corresponding to 46% hole doping per Ti, the 3d orbital symmetry changes from a1g to e pi g.