Electronic structure and stability of hexagonal Ba₃Ti₂RuO₉

We investigated the electronic structure and stability of the hexagonal perovskite Ba₃Ti₂RuO₉ using high-resolution electron energy loss spectroscopy and first-principles band structure calculations. The comparison between experimental and theoretical results leads to a coherent picture of the electronic structure of this compound where both Ti and Ru ions are tetravalent, the first unoccupied states being of Ru 4d character. Structural relaxations performed on four variants of this compound allowed a detailed investigation of the influence of the Ru atoms location in the hexagonal unit cell and clarified the origin of the stabilization of this phase at room temperature. Two structures without inversion symmetry built, respectively, with two TiRuO₉ units (space group P6₃mc, 186) or with one Ru₂O₉ and one Ti₂O₉ unit (P6̅ m2,187) are found to be the most stable. The stabilization of the first structure occurs through the polarization of the partially filled Ru 4d bands, whereas a direct metal-metal bonding interaction taking place between the two Ru atoms in the Ru₂O₉ unit lowers the total energy of the second. The influence of the exchange correlation functional used to perform these calculations on the relative stability of the four variants is discussed.