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

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).