Intra c -axis dimer hybridization and mixed valency in Mg-doped Ti2O3

Abstract

We have studied the ${\mathrm{Ti}}^{3+}/{\mathrm{Ti}}^{4+}$ mixed valence state in Mg-doped ${\mathrm{Ti}}_{2}{\mathrm{O}}_{3}$ using hard x-ray photoemission spectroscopy. The Ti $2p$ spectrum for the corundum-type ${\mathrm{Ti}}_{2}{\mathrm{O}}_{3}$ revealed the ${\mathrm{Ti}}^{3+}$ configuration with strong electronic coupling in the $c$-axis Ti-Ti pairs whereas the data for the ilmenite-type ${\mathrm{MgTiO}}_{3}$ confirmed the ${\mathrm{Mg}}^{2+}\text{\ensuremath{-}}{\mathrm{Ti}}^{4+}$ charge state in the $c$-axis cation pairs. In ${\mathrm{Mg}}_{0.29}{\mathrm{Ti}}_{1.71}{\mathrm{O}}_{3}$, the Ti $2p$ spectrum hardly showed the ${\mathrm{Ti}}^{4+}$ peak, which was in ${\mathrm{MgTiO}}_{3}$ indicating that the $c$-axis pairing of the Mg is with a ${\mathrm{Ti}}^{3+}$ ion rather than a ${\mathrm{Ti}}^{4+}$ and that the ${\mathrm{Ti}}^{3+}/{\mathrm{Ti}}^{4+}$ mixed valence state is materialized within the Ti-Ti $c$-axis pairs. In ${\mathrm{Mg}}_{0.63}{\mathrm{Ti}}_{1.37}{\mathrm{O}}_{3}$, we detected the presence of ${\mathrm{Mg}}^{2+}\text{\ensuremath{-}}{\mathrm{Ti}}^{4+}$ pairs. The results indicate the important role of hybridization within the $c$-axis pairs not only in the ${\mathrm{Ti}}^{3+}\text{\ensuremath{-}}{\mathrm{Ti}}^{3+}$ configuration, but also above all in the ${\mathrm{Ti}}^{3+}/{\mathrm{Ti}}^{4+}$ mixed valence state, superseding the Madelung energy gain of the ${\mathrm{Mg}}^{2+}\text{\ensuremath{-}}{\mathrm{Ti}}^{4+} c$-axis pair formation.