Electronic Structure of the Bond Disproportionated Bismuthate Ag$_2$BiO$_3$

TL;DR

This study investigates the electronic structure of Ag$_2$BiO$_3$, revealing bond disproportionation, a band gap of about 1.25 eV, and the presence of Weyl nodal chains, with implications for topological states and similarities to BaBiO$_3$.

Contribution

It provides the first comprehensive experimental and theoretical analysis of Ag$_2$BiO$_3$, identifying its bond disproportionation, electronic band structure, and topological features such as Weyl nodal chains.

Findings

Bond disproportionation with holes on oxygen atoms confirmed.

Estimated band gap of approximately 1.25 eV.

Presence of Weyl nodal chains below the Fermi level.

Abstract

We present a comprehensive study on the silver bismuthate Ag$_2$BiO$_3$, synthesized under high-pressure high-temperature conditions, which has been the subject of recent theoretical work on topologically complex electronic states. We present X-ray photoelectron spectroscopy results showing two different bismuth states, and X-ray absorption spectroscopy results on the oxygen $K$-edge showing holes in the oxygen bands. These results support a bond disproportionated state with holes on the oxygen atoms for Ag$_2$BiO$_3$. We estimate a band gap of $\sim$1.25~eV for Ag$_2$BiO$_3$ from optical conductivity measurements, which matches the band gap in density functional calculations of the electronic band structure in the non-symmorphic space group $Pnn2$, which supports two inequivalent Bi sites. In our band structure calculations the disproportionated Ag$_2$BiO$_3$ is expected to host Weyl nodal chains, one of which is located $\sim$0.5~eV below the Fermi level. Furthermore, we highlight similarities between Ag$_2$BiO$_3$ and the well-known disproportionated bismuthate BaBiO$_3$, including breathing phonon modes with similar energy. In both compounds hybridization of Bi-$6s$ and O-$2p$ atomic orbitals is important in shaping the band structure, but in contrast to the Ba-$5p$ in BaBiO$_3$, the Ag-$4d$ bands in Ag$_2$BiO$_3$ extend up to the Fermi level.