Electronic band structure of the pyrochlore ruthenium oxides A(2)Ru(2)O(7)(A=Bi, Tl and Y)

A local-density electronic-band-structure calculation was performed for pyr ochlore 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 R u 4d t(2g) and O 2p orbitals near the Fermi energy (Er) analogous to the la yered 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 pyroc hlores. Specifically in Tl2Ru2O7, bands which consist of antibonding states of T1 6s and O 2p orliials exist with nearly half-fillings. A tight-bindin g model shows an energy-gap formation of the Tl-O bands with an orthorhombi c lattice distortion. The calculated result suggests importance of coupling between electronic and lattice instabilities. which may be a clue to under stand metal-insulator transition observed in Tl2RU2O7.