Bond Disproportionation in the Silver Bismuthate Ag₂BiO₃

The stoichiometric silver bismuthate Ag₂BiO₃ is expected to contain bismuth in the nominal Bi⁺⁴ state, but the bismuth ions disproportionate into two distinct sites that render Ag₂BiO₃ insulating [1]. Recent theoretical predictions of metal-insulator transition and Weyl semimetal state in Ag₂BiO₃ have revived the interest in this material [2]. However, such novel properties are only expected in the absence of disproportionation of bismuth, where Ag₂BiO₃ is in the Pnna phase. Here, we examine nature of disproportionation in Ag₂BiO₃ via X-ray absorption and photoemission spectroscopies. We present an updated band structure for Ag₂BiO₃ in the Pnn2 (disproportionated) phase, where we have a band gap of 1 eV in agreement with our optical spectroscopy measurement. Furthermore, we present Raman spectroscopy results and highlight the extent of similarities to other disproportionated bismuthates despite the distinct crystal structure Ag₂BiO₃.

[1] C. B. M. Oberndorfer et al., Solid State Sciences 8, 267 (2006).
[2] J. He et al., Nature Communications 9, 492 (2018).