Crystal Engineering of Bi2WO6 to Polar Aurivillius-Phase Oxyhalides

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 Bi₂WO₆ and the replacement of W⁶⁺ by pentavalent (Nb⁵⁺, Ta⁵⁺) and tetravalent (Ti⁴⁺, Sn⁴⁺) ions, with charge neutrality maintained by the formation of a mixed-anion oxyhalide sublattice. We find that Bi₂SnO₄F₂ is thermodynamically unstable, in contrast to Bi₂TaO₅F, Bi₂NbO₅F and Bi₂TiO₄F₂. The electric dipoles introduced by chemical substitutions in the parent compound are found to suppress the spontaneous polarization from 61.55 μC/cm² to below 15.50 μC/cm². 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.