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.
Highlights
Transition-metal oxides include many novel classes of materials that host a wide range of structures and electronic properties
We used x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), and x-ray absorption spectroscopy (XAS) measurements to demonstrate that Ag2BiO3 in the Pnn2 phase is a bond disproportionated bismuthate
We have demonstrated that despite the use of a high pressure of 6 GPa, Ag2BiO3 samples crystallize in a structure consistent with the Pnn2 phase as evidenced by the XPS data for Bi and the measured band gap
Summary
Transition-metal oxides include many novel classes of materials that host a wide range of structures and electronic properties. The disproportionation in BaBiO3 is reflected in the existence of two distinct sites for Bi, but the nature of this disproportionation is debated It was emphasized in a following neutron diffraction study that Ag2BiO3 has two distinct bismuth sites and crystalizes in the Pnn space group [10], and the two sites were explained with the disproportionation of Bi4+ into Bi3+ and Bi5+ states. We highlight the extent of similarities between Ag2BiO3 and BaBiO3; similar breathing mode peaks are observed in Raman spectra, and we identify similarities in the band structure of these compounds when considering the oxygen molecular orbitals in these disproportionated bismuthates. Raman spectra and XRD for the previously reported structural transition to a Pn phase at low temperature [10]
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