Ferroelectricity in Two-Dimensional Antimony Oxides

Three different polymorphs of antimony (IV) oxide, namely, γ-Sb₂O₄, δ-Sb₂O₄, and ε-Sb₂O₄, are predicted by using the evolutionary algorithm combined with the first-principles density functional theory calculations. The γ-Sb₂O₄ and δ-Sb₂O₄ phases are layered, which makes them the first two-dimensional (2D) materials of this stoichiometry. Out-of-plane ferroelectricity is predicted in the 2D γ-Sb₂O₄ phase, while in-plane ferroelectricity is predicted in the 2D δ-Sb₂O₄ phase. Meanwhile, the ε-Sb₂O₄ phase is not layered. All three phases are predicted to be indirect bandgap semiconductors, with the gap ranging from 2.25 eV to 5.51 eV, depending upon the Sb-O bonding configurations and their dimensionalities. Also, the anisotropy in optical properties is observed, which is closely related to their anisotropic crystal structures and the unique bonding mechanisms between Sb and O atoms. Analysis of Raman spectra along with the vibrational modes is also performed to pave the way for the experimental investigation of the predicted structures. The existence of both in-plane and out-of-plane 2D ferroelectricity and the large band gaps are notable features of these new Sb₂O₄ phases, which may be of interest for potential applications in electronics and optoelectronics.