Electronic band structure of the pyrochlore ruthenium oxides A2Ru2O7 (A=Bi, Tl and Y)

A local-density electronic-band-structure calculation was performed for pyrochlore ruthenium oxides A(2)Ru(2)O(7) (A=Bi,T1 and Y). Asa general feature. these pyrochlores have band structure composed of antibonding states of Ru 4d t(2g) and O 2p orbitals near the Fermi energy (Er) analogous to the layered perovskite superconductor Sr2RuO4. It was round that effects of A ion's orbitals to the band structure near E-F are quite important in the pyrochlores. Specifically in Tl2Ru2O7, bands which consist of antibonding states of T1 6s and O 2p orliials exist with nearly half-fillings. A tight-binding model shows an energy-gap formation of the Tl-O bands with an orthorhombic lattice distortion. The calculated result suggests importance of coupling between electronic and lattice instabilities. which may be a clue to understand metal-insulator transition observed in Tl2RU2O7.