title: Direct Observation of Pressure-Driven Valence Electron Transfer in Ba3BiRu2O9, Ba3BiIr2O9, and Ba4BiIr3O12.
abstract: The hexagonal perovskites Ba3BiIr2O9, Ba3BiRu2O9, and Ba4BiIr3O12 all undergo pressure-induced 1% volume collapses above 5 GPa. These first-order transitions have been ascribed to internal transfer of valence electrons between bismuth and iridium/ruthenium, which is driven by external applied pressure because the reduction in volume achieved by emptying the 6s shell of bismuth upon oxidation to Bi(5+) is greater in magnitude than the increase in volume by reducing iridium or ruthenium. Here, we report direct observation of these valence transfers for the first time, using high-pressure X-ray absorption near-edge spectroscopy (XANES) measurements. Our data also support the highly unusual "4+" nominal oxidation state of bismuth in these compounds, although the possibility of local disproportionation into Bi(3+)/Bi(5+) cannot be definitively ruled out. Ab initio calculations reproduce the transition, support its interpretation as a valence electron transfer from Bi to Ir/Ru, and suggest that the high-pressure phase may show metallic behavior (in contrast to the insulating ambient-pressure phase).
authors.affiliation: Canadian Light Source , 44 Innovation Boulevard, Saskatoon, Saskatchewan S7N 2V3, Canada.
authors.forename: Peter E R
authors.initials: PE
authors.lastname: Blanchard
authors.affiliation: X-ray Science Division, Advanced Photon Source, Argonne National Laboratory , Argonne, Illinois 60439, United States.
authors.forename: Karena W
authors.initials: KW
authors.lastname: Chapman
authors.affiliation: X-ray Science Division, Advanced Photon Source, Argonne National Laboratory , Argonne, Illinois 60439, United States.
authors.forename: Steve M
authors.initials: SM
authors.lastname: Heald
authors.affiliation: Institute Laue Langevin , 71 avenue des Martyrs, Grenoble 38042, France.
authors.forename: Mohamed
authors.initials: M
authors.lastname: Zbiri
authors.affiliation: Institute Laue Langevin , 71 avenue des Martyrs, Grenoble 38042, France.
authors.forename: Mark R
authors.initials: MR
authors.lastname: Johnson
authors.affiliation: School of Chemistry, The University of Sydney , Sydney 2006, Australia.
authors.forename: Brendan J
authors.initials: BJ
authors.lastname: Kennedy
authors.affiliation: School of Chemistry, The University of Sydney , Sydney 2006, Australia.
authors.forename: Chris D
authors.initials: CD
authors.lastname: Ling