Metal-insulator Crossover in Pb-Ru Based Oxides with Pyrochlore-type Structure

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Synthesis and physical properties of the Pb-Ru based pyrochlores Pb_2−xM_xRu₂O_7−δ (M = Eu and Ca) have been studied. For Pb-Eu solid solution Pb_2−xEu_xRu₂O_7−δ, single phases were successfully synthesized by the solid-state reaction. Magnetic susceptibility measurements indicated the magnetic transition due to spin-glass freezing in the samples with x ≥ 0.8. From electric resistivity measurements, a crossover from metallic to semiconducting behaviors was observed in the range of 0.6 ≤ x ≤ 0.8. In the metallic region, normalized electric resistivity was found to be almost proportional to T², indicating the strongly-correlated electron behaviors. The crossover between insulating and metallic phases can be caused by the strong electron-electron couplings between the itinerant 4d electrons. For Pb-Ca solid solution Pb_2−xCa_xRu₂O_7−δ, single phases were obtained for x ≤ 0.6 samples by heat treatments in air. Pb_2−xCa_xRu₂O_7−δ samples with 0.8 ≤ x ≤ 1.6 were successfully synthesized under high oxygen pressures. Lattice parameter of Pb_2−xCa_xRu₂O_7−δ compounds was found to decrease with Ca content x, resulting in the positive chemical pressure. As a result of electric resistivity measurements, the samples with 0.0 ≤ x ≤ 1.0 showed the metallic behaviors.