Electron doping in single crystalline BaBiO$_3$: BaBiO$_{3-x}$F$_{x}$

Topological insulators are a new class of insulators with conducting surface state. Most of the topological insulators are chalcogenides, where a tiny amount of chalcogen vacancy destroys the predicted bulk insulating state and results in a metallic or semimetallic bulk electrical transport. BaBiO$3$ (BBO) is an interesting large bandgap (0.7 eV) insulator that upon hole doping becomes a superconductor and is theoretically predicted to show a topological insulating state under electron doping. We have explored electron doping through the chemical substitution of fluorine atoms at the oxygen site. The single crystals of BBO and fluorine doped BBO were synthesized via a one-step solid-state technique. The single crystals of pure BBO and 10 % F -doped BBO (BaBiO${2.7}$F$_{0.3}$) are chemically single-phase samples and crystallize in monoclinic I2/m crystal structure. The core level and valence band X-ray photoelectron spectra confirm electron doping in the 10% fluorine-doped BBO. 20 % F-doped BBO appears to be a multiphase sample, confirmed by back-scattered electron (BSE) imaging and X-ray diffraction. This article reports on the successful growth of pure and F-doped BBO using a one-step solid-state technique and discusses the effect of F-doping on structural and electronic properties.

💡 Research Summary

The paper investigates electron doping of the wide‑band‑gap oxide BaBiO₃ (BBO) by substituting fluorine for oxygen, aiming to realize the theoretically predicted topological insulating (TI) phase. BBO possesses an indirect band gap of ~0.7 eV, considerably larger than that of most known TIs, making it an attractive platform where surface states could dominate transport if the bulk remains insulating. Theory suggests that shifting the Fermi level upward by ~2 eV—equivalent to adding roughly one electron per formula unit—would place the Dirac‑like surface states at the Fermi energy, turning BBO into a TI.

To achieve electron doping, the authors employed a one‑step solid‑state synthesis: stoichiometric mixtures of BaO, Bi₂O₃, and BiF₃ were ground, placed in an alumina crucible, heated to 1100 °C for 24 h, then slowly cooled (1 °C h⁻¹) to 1015 °C before furnace cooling. This route avoids a pre‑sintered polycrystalline stage that often introduces secondary phases during subsequent doping. Crystals obtained from the melt were mechanically extracted for analysis.

Structural characterization by powder X‑ray diffraction (PXRD) and Rietveld refinement showed that both undoped BaBiO₃ and the 10 % fluorine‑doped composition (nominally BaBiO₂.₇F₀.₃) crystallize in the monoclinic I2/m space group, identical to the parent perovskite. Lattice parameters change only marginally (Δa ≈ –0.001 Å, Δb ≈ –0.010 Å, Δc ≈ –0.008 Å), indicating that fluorine, having an ionic radius close to that of oxygen, does not significantly distort the framework. Back‑scattered electron (BSE) imaging corroborates the single‑phase nature of the 10 % doped crystals, displaying uniform contrast. In contrast, the 20 % fluorine‑doped sample exhibits alternating bright and dark bands in BSE images, and its PXRD pattern contains extra reflections indexed to BaF₂, confirming a multiphase mixture. Energy‑dispersive X‑ray spectroscopy (EDS) on the contrasting regions shows that the darker areas contain higher fluorine content, while the lighter ones are fluorine‑poor, establishing that the solubility limit of fluorine in bulk BBO under these conditions is around 10 %.

Electronic effects of fluorine substitution were probed by synchrotron X‑ray photoelectron spectroscopy (XPS). The Bi 4f core‑level spectrum of the doped crystal displays an additional component at lower binding energy, attributed to Bi–F bonding, and the main Bi 4f peaks shift by ~0.3 eV toward lower binding energy relative to the undoped sample. This shift is a hallmark of electron donation to the Bi 6s/6p states. The O 1s spectrum shows a reduced intensity of the lattice‑oxygen component (528.9 eV) and a relative increase of the higher‑binding‑energy component (≈531 eV), consistent with partial O→F substitution. Valence‑band spectra reveal that the valence‑band maximum (VBM) moves closer to the Fermi level by ~0.2 eV in the doped crystal, further confirming an upward shift of the Fermi level due to electron doping.

Collectively, these observations demonstrate that fluorine substitution successfully introduces electrons into the BaBiO₃ lattice while preserving the host crystal structure up to ~10 % nominal fluorine content. The achieved electron concentration is sufficient to move the Fermi level substantially, bringing the system closer to the regime where the predicted topological surface states would intersect the Fermi energy. However, attempts to increase the fluorine concentration beyond this solubility limit lead to phase separation (BaF₂ formation), which would likely disrupt any topological surface conduction.

The study thus provides the first bulk single‑crystal realization of electron‑doped BaBiO₃ via a straightforward solid‑state route, establishes the practical fluorine solubility limit, and supplies spectroscopic evidence of electron doping. These results lay the experimental groundwork for future investigations—such as angle‑resolved photoemission spectroscopy (ARPES), low‑temperature transport, and scanning tunneling microscopy—to directly detect the topological surface states and to assess whether BaBiO₃ can indeed serve as a large‑gap oxide topological insulator. Further work may explore alternative electron donors, high‑pressure synthesis, or epitaxial strain to surpass the 10 % fluorine limit while maintaining phase purity.