Band Gap Narrowing by Suppressed Lone-Pair Activity of Bi3+

Post-transition metal cations with a lone pair (ns2np0) electronic configuration such as Pb2+ and Bi3+ are important components of materials for solar-to-energy conversion. As in molecules like NH3, 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 Bi3+ site is realized by isovalent substitutions with Y3+ by considering its similar ionic radius and absence of a lone pair. The quaternary photocatalyst Bi2YO4X is singled out as a candidate for Bi substitution from a survey of the coordination environments in Y–O compounds. The introduction of Bi3+ to the undistorted Y3+ site in Bi2YO4X 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.

The dielectric constants were calculated by density functional perturbation theory (DFPT).Electronic band structure diagrams were generated using the sumo package. 6 calculate the vibration frequencies, a 4 × 4 × 2 supercell was built and force constants were calculated by the method of small displacements implemented in Phonopy. 7The Helmholtz free energy calculation in the quasi-harmonic approximation was also made using the Phonopy software. 8

Synthesis
Bi2BixY1-xO4X (x = 0 -0.5) were prepared by prepared by solid-state reactions.Bi2O3 (FUJIFILM Wako Pure Chemical Corporation, 99.99%), Y2O3 (FUJIFILM Wako Pure Chemical Corporation, 99.99%), and BiOX were mixed at the ratio of 1+x: 1-x: 2, heated in an evacuated silica tube at 1073 K (1023 K for Bi2YO4I and Bi2Bi0.6Y1-0.6O4I ) for 10h.BiOCl was purchased from FUJIFILM Wako Pure Chemical Corporation, while BiOI was synthesized by a soft liquid deposition method; 9 5 mmol of Bi(NO3)3•5H2O (FUJIFILM Wako Pure Chemical Corporation) was dispersed in 30 mL of ethanol and mixed with the solution of 5 mmol of KI (FUJIFILM Wako Pure Chemical Corporation) dissolved in 10 mL of pure water.
After 5 h stirring at room temperature, the precipitate was collected by centrifugation, washed several times with water and ethanol, and finally dried in air at 333 K.
High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and annular bright-field scanning transmission electron microscopy (ABF-STEM) images were collected using a JEM-ARM200CF (JEOL Ltd., Tokyo, Japan) operating at an accelerating voltage of 200 kV and equipped with a cold field emission gun and a Cs corrector to observe atomic columns.Elemental analysis was carried out using JEM-ARM200CF equipped with energy-dispersive X-ray spectroscopy (EDX).Samples were prepared by grinding the material and depositing a few drops of the suspension onto a holey copper grid covered with a thin carbon film.Synchrotron X-ray diffraction (SXRD) patterns were collected at the BL02B2 in SPring-8, Japan (λ = 0.419432 Å).The collected X-ray diffraction data were analyzed using RIETAN-FP 10 with Bi2YO4X (X = Cl, I) (P4/mmm) structure model. 11The ionization energy was measured by photoelectron yield spectroscopy (PYS; BIP-KV201, Bunkoukeiki) in a vacuum (< 5 ×         Free energies of Bi2BiO4I with respect to volume at temperatures from 0 to 400 K with 100 K steps are depicted by filled circles and the values are fit by the solid curves.The red line connects the energy minima of the respective curves.(g) Volumetric thermal expansion and (g) its coefficient of Bi2BiO4I.Bi2BiO4I (P4/mmm) is dynamically stable at a low temperature.However, a dynamic instability emerges for expanded volumes at high temperatures above 400°C (d), where the produced imaginary mode is related to the movement of the symmetric Bi2.From these results, the Bi 3+ at the symmetric site is not dynamically stable at a high temperature, which results in the formation of the impurity phase having a distorted Bi site during the synthesis (Fig. S10c).

Fig. S1 .
Fig. S1.(a) The coordination environment of Y 3+ compounds including oxygen.(b) Histogram of the CSM value of each coordination environment.The meaning of each symbol is described in the appendix.

Fig. S3 .
Fig. S3.(a) The coordination environment of Bi 3+ compounds including oxygen.(b) Histogram of the CSM value of each coordination environment.The meaning of each symbol is described in the appendix.

Fig
Fig. S6.(a-c) Crystal structure of Bi-based compounds with the smallest CSM value in C:8 environment.

Fig. S17 .
Fig. S17.(a, b) Diffuse reflectance spectra of Bi2BixY1-xO4X (X = (a) Cl, (b) I).(c, d) The Urbach energies (EU) estimated with the following equation, α(hν) = α0exp((hν-E1)/EU), where α0 and E1 are fitting parameters and EU is the Urbach energy.12,13The EU can be estimated from the slope of ln(α) vs hν, where dln(α)/d(hν) = 1/EU.The Urbach Energy quantifies the broadness of the onset of absorption near the band edge, where a higher Urbach Energy represents a broader density of state.Judging from the absence of the additional peak in the absorption spectra, the density of the state formed by Bi introduced on the Y site overlaps with the original VBM of Bi2YO4X.As the Bi introduced on the Y site increases, the density of state of the extended VBM increases, which is supported by the decreased Urbach energy with the increase of the introduced Bi.

Table S2 .
Born effective charges of Bi2MO4X (M = Y, Bi; X = Cl, I) calculated from density functional perturbation theory.