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 mixt ures 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 c an 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 positi ons. From accurate Ru-O and Bi-O interatomic distances, a bond-valence stud y suggests an oxidation state lower than 4+ for Ru cations, which is consis tent with the presence of oxygen vacancies in the structure, and with the i ncrease of the lattice parameter with respect to stoichiometric Bi2Ru2O7 Wi th 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, th us, to the electronic delocalization across the solid, The control of the O ' content in Bi2Ru2O7-y (y depending on the final annealing conditions of t he material) may be, therefore, a clean way to tune the bandwidth of this t echnologically important electronic material.