Band Gap Narrowing by Suppressed Lone-Pair Activity of Bi 3 .

Post-transition metal cations with a lone pair ( n s 2 n p 0 ) electronic configuration such as Pb 2+ and Bi 3+ are important components of materials for solar-to-energy conversion. As in molecules like NH 3 , the lone pair is often stereochemically active in crystals, associated with distorted coordination environments of these cations. In the present study, we demonstrate that suppressed lone pair stereochemical activity can be used as a tool to enhance visible light absorption. Based on an orbital interaction model, we predict that a centrosymmetric environment of the cations limits the orbital interactions with anions, deactivates the lone pair, and narrows the band gap. A high-symmetry Bi 3+ site is realized by isovalent substitutions with Y 3+ by considering its similar ionic radius and absence of a lone pair. The quaternary photocatalyst Bi 2 YO 4 X is singled out as a candidate for Bi substitution from a survey of the coordination environments in Y-O compounds. The introduction of Bi 3+ to the undistorted Y 3+ site in Bi 2 YO 4 X results in a narrowed band gap, as predicted theoretically and confirmed experimentally. The orbital interaction controlled by site symmetry engineering offers a pathway for the further development of post-transition metal compounds for optoelectronic applications.