SYNTHESIS, CRYSTAL-STRUCTURE, AND ELECTRICAL-PROPERTIES OF THE PYROCHLORES PB(2-X)LN(X)RU(2)O(7-Y)(LN=ND, GD)

The lead-lanthanoid pyrochlores Pb(2-x)Ln(x)Ru(2)O(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 F (4) over bar 3m symmetry for the lead-rich region, 0 less than or equal to x less than or equal to 0.2, and a vacancy-disordered phase with Fd (3) over bar m symmetry for the lanthanoid-rich region, 1.0 less than or equal to x less than or equal to 2.0. Electrical resistivity measurements showed a metallic behavior for the lead-rich region of the solid solutions (0 less than or equal to x less than or equal to 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 Pb2-xNdxRu2O7-y and Pb2-xGdxRu2O7-y, respectiveIy. From x = 1.0 to 2.0, the lattice parameters increase in Pb2-xNdxRu2O7-y and decrease in Pb2-xGdxRuxO7-y. However, Rietveld analysis of Pb(2-x)Ln(x)Ru(x)O(7-y) at room temperature indicates similar structural changes in both systems; the Ru-O(1) bond length decreases and the bend in RuO6 zigzag chains increases with increasing x. The analysis of neutron diffraction data for PbNdRu2O7-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. (C) 1995 Academic Press, Inc.