Synthesis, Crystal Structure, and Electrical Properties of the Pyrochlores Pb 2- xLn xRu 2O 7- y( Ln = Nd, Gd)

Abstract

The lead-lanthanoid pyrochlores Pb 2- x Ln x Ru 2O 7- y ( Ln = Nd, Gd) have been synthesized and characterized by X-ray diffraction, neutron diffraction, and electrical resistivity measurements. The solid solutions synthesized from metal nitrates and ruthenium oxide showed two monophasic regions in both neodymium and gadolinium systems: an oxygen-vacancy-ordered phase with F4̄3m symmetry for the lead-rich region, 0 ≤ x ≤ 0.2, and a vacancy-disordered phase with Fd3̄ m symmetry for the lanthanoid-rich region, 1.0 ≤ x ≤ 2.0. Electrical resistivity measurements showed a metallic behavior for the lead-rich region of the solid solutions (0 ≤ x ≤ 0.2). In the lanthanoid-rich region, a change from metallic to semiconducting behavior was observed at 1.0 < x < 1.2 and 1.2 < x < 1.5 in Pb 2- x Nd x Ru 2O 7- y and Pb 2- x Gd x RU 2O 7- y , respectively. From x = 1.0 to 2.0, the lattice parameters increase in Pb 2- x Nd x Ru 2O 7- y and decrease in Pb 2- x Gd x Ru 2O 7- y . However, Rietveld analysis of Pb 2- x Ln x Ru 2O 7- y at room temperature indicates similar structural changes in both systems; the Ru-O(1) bond length decreases and the bend in RuO 6 zigzag chains increases with increasing x. The analysis of neutron diffraction data for PbNdRu 2O 7- y indicates the existence of oxygen vacancies, y = 0.188(8), with no evidence for long-range ordering. The relationship between the structural changes and the electrical properties is discussed.