Direct Observation of Pressure-Driven Valence Electron Transfer in Ba3BiRu2O9, Ba3BiIr2O9, and Ba4BiIr3O12

• Published in: Inorganic chemistry Vol. 55; no. 11; pp. 5649 - 5654
• Main Authors: Blanchard, Peter E.R, Chapman, Karena W, Heald, Steve M, Zbiri, Mohamed, Johnson, Mark R, Kennedy, Brendan J, Ling, Chris D
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 06.06.2016
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/acs.inorgchem.6b00718

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 Bi5+ 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 Bi3+/Bi5+ 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).