Crystal Engineering of Bi2WO6 to Polar Aurivillius-Phase Oxyhalides

Abstract

The Aurivillius phases of complex bismuth oxides have attracted considerable attention due to their lattice polarization (ferroelectricity) and photocatalytic activity. We report a first-principles exploration of Bi<sub>2</sub>WO<sub>6</sub> and the replacement of W<sup>6+</sup> by pentavalent (Nb<sup>5+</sup>, Ta<sup>5+</sup>) and tetravalent (Ti<sup>4+</sup>, Sn<sup>4+</sup>) ions, with charge neutrality maintained by the formation of a mixed-anion oxyhalide sublattice. We find that Bi<sub>2</sub>SnO<sub>4</sub>F<sub>2</sub> is thermodynamically unstable, in contrast to Bi<sub>2</sub>TaO<sub>5</sub>F, Bi<sub>2</sub>NbO<sub>5</sub>F and Bi<sub>2</sub>TiO<sub>4</sub>F<sub>2</sub>. The electric dipoles introduced by chemical substitutions in the parent compound are found to suppress the spontaneous polarization from 61.55 μC/cm<sup>2</sup> to below 15.50 μC/cm<sup>2</sup>. Analysis of the trends in electronic structure, surface structure, and ionization potentials are reported. This family of materials can be further extended with control of layer thicknesses and choice of compensating halide species.<br>

Anisotropic Electron Transport Limits Performance of Bi2WO6 Photoanodes

“…This coupling of Bi 6s and O 2p orbitals leads to an upward dispersion of the VB and a narrower band gap than would otherwise be expected. 65 Consistent with the results of a previous theoretical study screening Aurivillius oxides, 13 the conduction band (CB) minimum projects predominantly on W 5d states with some O 2p density. The CB exhibits a strong spatial anisotropy, illustrated by the iso-surface shown in Figure 3b.…”

“…The layered structure of these materials implies a vast library of possible sub-phases, each with a distinct set of optoelectronic properties, whilst studies of B site substitution demonstrate the possibility of band gap engineering to enhance solar harvesting. 12,13 Preliminary studies indicate that some members show promise in photocatalytic and photoelectrochemical applications. [14][15][16] However, relatively little is known about the factors which govern water oxidation efficiency in even the simplest examples of this diverse and complex family of materials.…”

Bi2CoO2F4─A Polar, Ferrimagnetic Aurivillius Oxide-Fluoride

“…Morita et al explored the change in polarization with Fcontent in the series Bi 2 (W,B)(O,F) 6 (B = Ta, Nb, and Ti) and noted that the polarization decreased noticeably with increasing F − content. 24 This is likely explained by reduced cation−anion hybridization for more ionic fluoride-based systems, 63 increasing the importance of geometric drivers for distortions over electronic (pseudo-Jahn−Teller effect) factors. 64 This is consistent with polar Γ 5 − distortions being the softest instability for Bi 2 WO 6 (and making similar contributions as X 3 + and X 2 + rotations to the ground state), 65 while the X 3 + rotations are dominant for Bi 2 CoO 2 F 4 and Γ 5 − distortions make a much diminished contribution (Table 3 and Supporting Information).…”

“…27 However, our work has also identified some challenges: while replacing O 2− by F − ions allows more magnetic ions to be accommodated by the perovskite-like layers (and gives similar magnetic behavior to the Ruddlesden−Popper oxides such as La 2 CoO 4 ), 52,67 the increased ionic character of the fluorides compared with oxides tends to reduce the stability of polar distortions, reducing the polarization. 24 It would be interesting to study the effect of concentrating F − ions in the equatorial sites in the perovskite-like layers, leaving the apical sites fully occupied by O 2− . 76 This may give similar energies for the polar distortions involving [Bi 2 O 2 ] 2+ layers to those observed in Bi 2 WO 6 (although the polarization contribution from the perovskite-like layers is likely to remain diminished).…”

“…Replacing some of the oxide ions by fluoride ions in n = 1 Aurivillius materials lowers the B-cation oxidation state required for charge balance 21−23 and may also change the relative significance of polar displacements and rotations of octahedra. 24 This has allowed magnetic n = 1 Aurivillius oxidefluorides Bi 2 BO 2 F 4 (B = Fe, Co, and Ni) to be prepared. 25−27 The report of long-range magnetic order in Bi 2 CoO 2 F 4 (with the possibility of a ferromagnetic component) 25 motivated us to investigate it further.…”