Static disorder from lone-pair electrons in Bi2-xMxRu2O7-y (M=Cu,Co; x=0,0.4) pyrochlores

The crystal structures of Bi2Ru2O7 and Bi1.6M0.4Ru2O7 (M=Cu,Co) pyrochlores are determined by Rietveld refinement of neutron powder diffraction data collected between 300 and 12 K. An appreciable oxygen non-stoichiometry, with no vacancy ordering, is found only in undoped Bi2Ru2O7. In all three compounds, static displacive disorder of both the Bi and Oʹ sites is observed. This type of disorder has not been reported previously for Bi2Ru2O7, and is proposed to be a common feature of A2B2O7 pyrochlores having a lone electronic pair on the A-site cation. The electronic properties of Bi2Ru2O7 are discussed in terms of calculated electronic band structure, local ruthenium coordination, and the static bismuth displacement.