Crystal Structure and Electrical Properties of the Pyrochlore Ruthenate Bi2-xYxRu2O7

The bismuth yttrium pyrochlore ruthenates Bi2-xYxRu2O7 have been synthesized and characterized by X-ray Rietveld structure refinement and electrical conductivity measurement. Room temperature resistivities increase with x, and a change from metallic to semiconducting behavior is observed between x = 1.2 and 1.4. The distortion of the RuO6 octahedra and the bend in the RuO6 zig-zag chains increase with x. The transition from metallic to semiconducting state is correlated with the variation of the Ru-O-Ru angle from 139° at x = 0.0 to 129° at x = 2.0. The Ru-O bond length in the RuO6 octahedra increases in the interval 0 ≤ x ≤ 1.2. This variation is understood to result from the contribution of Bi orbitals to the conduction hand; in the metallic regime, 0 ≤ x ≤ 1.2, the electron transfer leads to short Ru-O bond length, whereas in the nonmetallic stale there is little Ru-O bond length variation because there is no contribution of the Bi electrons to the Ru 4d state.