Structural studies of the metal-nonmetal transition in Ru pyrochlores

A combination of powder neutron and X-ray diffraction measurements were car ried 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 oxide s, x < 0.75, to the semiconducting oxides, x > 0.75. The relative small cha nge 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 o xides 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 i mportance of oxide vacancies for metallic conductivity in these oxides. Met allic 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.