Abstract
The quaternary pyrochlore ruthenates, Bi2-xLnxRu2O7 (Ln = Pr-Lu), have been synthesized and characterized by X-ray Rietveld structure refinement and electrical conductivity measurements. Room temperature resistivities increase with x, and a change from metallic to semiconducting behavior is observed between x = 1.2 and 1.4. In this composition region, smooth change from metallic to semiconducting behavior was observed at 40-80 K for the Pr-Sm systems, whereas no change was observed for the Dy and Y systems. Lattice parameters increase with x in the systems with larger rare earth cations (Pr, Nd), and decrease with x in the systems with smaller rare earth cations (Sm, Dy). However, the room-temperature structure analysis indicated similar structural changes in these solid solutions; the Ru-O bond length in the RuO6 octahedra increases, the distortion of the RuO6 octahedra increases, and the bend in the RuO6 zig-zag chains increases with increasing Ln contents. This corresponds to the change from metallic to semiconducting behavior around x = 1.2-1.4. The RuO6 octahedra distortion and the bend in the RuO6 zig-zag chains increase in the order Lu → Pr. The semiconductor-metal transition is discussed from the structural changes.
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