Oxygen vacancy control in the defect Bi2Ru2O7-y pyrochlore:: a way to tune the electronic bandwidth

An oxygen deficient Bi2Ru2O7-y pyrochlore has been prepared by heating mixtures of Bi2O3 and RuO2 at 1000 degrees C in air and a subsequent treatment under Ar atmosphere. This material is metallic between 4 and 290 K, showing an increased resistivity, by one order of magnitude at 300 K, with respect to stoichiometric Bi2Ru2O7 The structure is cubic, a = 10.2991(7) A, and can be described in the space group Fd (3) over bar m. A profile refinement of neutron diffraction data indicates a significant oxygen vacancy content of y = 0.20(2), with the vacancies distributed at random over the 8a positions. From accurate Ru-O and Bi-O interatomic distances, a bond-valence study suggests an oxidation state lower than 4+ for Ru cations, which is consistent with the presence of oxygen vacancies in the structure, and with the increase of the lattice parameter with respect to stoichiometric Bi2Ru2O7 With respect to other ruthenate pyrochlores, the very short bonds between Bi3+ and the special O' oxygens, in which the 6s(2) electron lone pair of Bi3+ are probably involved, account for the comparatively weak remaining six Bi-O bonds, giving rise to a shift of the O positions and indirectly opening the Ru-O-Ru angles. These angles control the transfer integral between Ru t(2g) and O 2p orbitals, strongly related to the tie-block bandwidth and, thus, to the electronic delocalization across the solid, The control of the O' content in Bi2Ru2O7-y (y depending on the final annealing conditions of the material) may be, therefore, a clean way to tune the bandwidth of this technologically important electronic material.