Linear dichroic x-ray absorption response of Ti-Ti dimers along the \(c\) axis in Ti\(_2\)O\(_3\) upon Mg substitution

Corundum oxide Ti\(_2\)O\(_3\) shows the metal-insulator transition around 400-600 K accompanying the nearest Ti\(^{3+}\)-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 Ti\(_2\)O\(_3\), we investigated Ti \(3d\) orbital anisotropy between corundum-type Ti\(_2\)O\(_3\) and ilmenite-type 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 Mg\(_y\)Ti\(_{2-y}\)O\(_3\), we confirmed that the \(a_{1g}a_{1g}\) state is dominant not only in \(y=0.01\) (almost Ti\(_2\)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 \(e_g^{\pi}\).