Structural studies of the metal-nonmetal transition in Ru pyrochlores

A combination of powder neutron and X-ray diffraction measurements were carried out in the pyrochlore type oxides Bi2-xNdxRu2O7-y, 0 < x < 2 in order to probe the structural changes that occur on going from the metallic oxides, x < 0.75, to the semiconducting oxides, x > 0.75. The relative small change in the cubic lattice parameter across the series (0.4%) provides clear evidence that the lengthening of the Ru-O bond lengths in the nonmetallic oxides and the contraction of the Ru-O-Ru angle is a result of the change in the electronic properties of the oxides. The studies also illustrate the importance of oxide vacancies for metallic conductivity in these oxides. Metallic conductivity occurs in oxides where the Ru-O-Ru angle is greater than 132.5 degrees and there is a measurable oxygen nonstoichiometry, (C) 2000 Academic Press.