Large Out-of-plane Polarization in Two-dimensional Bi2O3 Monolayers Predicted by an AI-driven Inverse-design Workflow
Two-dimensional (2D) polar oxides with built-in electric fields offer promising opportunities for nanoscale electronics, optoelectronics, and surface charge engineering. However, their discovery remains challenging due to the enormous structural complexity of nonlayered oxide networks. Here, we combine inverse structural design with a machine-learning interatomic potential and first-principles calculations to explore 2D oxide configurations. Using Bi2O3 as a representative system, we identify two previously unreported polar monolayers with tetragonal and orthorhombic symmetries. Both structures exhibit vertically separated Bi and O sublayers, giving rise to pronounced intrinsic polarity with Berry-phase 2D polarizations of 34.12 and 37.15 pC/m, respectively. Notably, the o-Bi2O3 monolayer exhibits one of the largest reported out-of-plane polarizations among 2D oxides. Both monolayers exhibit dynamical stability, finite-temperature structural integrity, and wide band gaps (~3.8 eV). The large polarization originates from dimensionality-induced reconstruction of the nonlayered Bi-O network, which breaks local coordination symmetry and produces out-of-plane charge redistribution, while the stereochemically active Bi 6s2 lone pair further favors asymmetric Bi-O bonding. These findings suggest dimensionality-induced local-coordination symmetry breaking as a promising design motif for high-polarization 2D oxides, and demonstrate the utility of combining inverse design with machine-learning-assisted screening to explore unconventional polar phases in nonlayered oxide networks.