Linear dichroic x-ray absorption response of Ti-Ti dimers along the c axis in Ti2O3 upon Mg substitution

Corundum oxide ${\mathrm{Ti}}_{2}{\mathrm{O}}_{3}$ shows the metal-insulator transition around 400--600 K accompanying the nearest ${\mathrm{Ti}}^{3+}\text{\ensuremath{-}}{\mathrm{Ti}}^{3+}$ bond (${a}_{1g}{a}_{1g}$ singlet state) formation along the $c$ axis. In order to clarify the hole-doping effect for the ${a}_{1g}{a}_{1g}$ singlet bond in ${\mathrm{Ti}}_{2}{\mathrm{O}}_{3}$, we investigated Ti $3d$ orbital anisotropy between corundum-type ${\mathrm{Ti}}_{2}{\mathrm{O}}_{3}$ and ilmenite-type ${\mathrm{MgTiO}}_{3}$ using linear dichroism of soft x-ray absorption spectroscopy of the Ti ${L}_{2,3}$ edge. From the linear dichroic spectral weight in ${\mathrm{Mg}}_{y}{\mathrm{Ti}}_{2\ensuremath{-}y}{\mathrm{O}}_{3}$, we confirmed that the ${a}_{1g}{a}_{1g}$ state is dominant not only in $y=0.01$ (almost ${\mathrm{Ti}}_{2}{\mathrm{O}}_{3}$), 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 ${a}_{1g}$ to ${e}_{g}^{\ensuremath{\pi}}$.