Re. Carbonio et al., Oxygen vacancy control in the defect Bi2Ru2O7-y pyrochlore: a way to tune the electronic bandwidth, J PHYS-COND, 11(2), 1999, pp. 361-369
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.