Metal-metal bonding in the KSbO₃-type oxides La₄Ru₆O₁₉ and La₃Ru₃O₁₁: A mechanism for band gap formation in t_2g states

The electronic structures of La₃Ru₃O₁₁ and La₄Ru₆O₁₉ were calculated using a LMTO-ASA-TB method. Although both compounds have similar conduction networks, La₃Ru₃O₁₁ was found to have a very normal band structure while La₄Ru₆O₁₉ showed many striking features. The t_2g band is split into three portions of clearly defined orbital origin. The two highest-energy portions are extremely narrow (∼0.15 eV), and each have a capacity of only 1 electron per Ru atom. The highest portion of the t_2g band is separated from the remainder of the band by a gap of 0.5 eV. All these features occur in the absence of Jahn-Teller distortions. The origin of the gap is instead found to lie in metal-metal bonding due to the unusually short Ru-Ru distance in La₄Ru₆O₁₉ (2.5 Å vs 3.0 Å for La₃Ru₃O₁₁).