Band Gap Narrowing by Tailored Lone-Pair Orbital Interactions in Bi³⁺ Materials

Post-transition metal cations such as Sn, Sb, Pb, and Bi are important components for solar-to-energy conversion systems including photovoltaics and photocatalysis.¹ Their unique structural and electronic features are derived from the lone pair (<i>n</i>s²<i>n</i>p⁰) configuration. Often, the stereochemical activity of the lone pair results in a distorted local atomic environment of these cations in crystals.<br/>Our question is “what happens if the cationic lone pair is confined to an un-distorted site in a crystal”. Based on the underlying orbital interactions, we predict that such a confined lone pair significantly narrows the band gap and enhances visible light absorption. Validation is performed for Bi³⁺ using a cation substitution approach. To realize a high-symmetry Bi³⁺ site, we focus on isovalent substitutions with Y³⁺ because of its similar ionic radius and absence of a lone pair. Through the survey of the coordination environments, Bi₂YO₄X with un-distorted Y³⁺ is singled out as a candidate for Bi substitution.<br/>DFT calculation supported that introducing Bi³⁺ to the Y³⁺ in Bi₂YO₄X results in a redshift in the band gap. The key component is the anti-bonding Bi 6s – O 2p combination and Bi 6p. The Bi 6s and – O 2p interactions create filled bonding and antibonding combination, the latter contributing to the upper valence band. When Bi is at an asymmetric environment, the anti-bonding combination can interact with Bi 6p, accompanied by the stabilization of its energy. On the other hand, when Bi is on Y site, the anti-bonding combination is not stabilized by Bi 6p because of the non-net orbital interaction owing to the symmetry difference. This results in the negative shift of the upper valence band and the narrowed band gap. The band gap narrowing was also demonstrated experimentally through the Bi³⁺ doping to Y³⁺ by using solid-state reaction synthesis. The present lone pair engineering offers a strategy for controlling the optoelectronic structure of the post-transition metal compounds for the intended application beyond the limits of known materials.²<br/>[1] A. Walsh, D. J. Payne, R. G. Egdell and G. W. Watson, <i>Chem. Soc. Rev.</i>, 2011, <b>40</b>, 4455.<br/>[2] K. Ogawa, R. Abe and A. Walsh, <i>J. Am. Chem. Soc.</i>, 2024, <b>146</b>, 5806–5810.