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Description
Diboron trioxide (B$_2$O$_3$) represents a peculiar case among polymorphic oxides, for its vitrified state presents superstructural units – planar ring-shaped boroxol groups, B$_3$O$_6$ – that are totally absent in its crystalline polymorphs. In fact, the crystallization of B2O3 can occur only under applied pressure, where the boroxol groups can be disrupted and the triangular BO3 building blocks can be arranged in a crystalline unit cell. To date, 2D and 3D crystalline polymorphs that incorporate boroxol groups are only predicted theoretically, although their formation in ambient pressure is crucial to rationalize the B$_2$O$_3$ ability to vitrify. Here we present the synthesis of a two-dimensional (2D) B2O3 polymorph constituted by boroxol groups bridged by oxygen atoms and arranged in an atomically thin honeycomb lattice. By means of surface science experimental techniques, as well as ab initio calculations [1], we characterized the regular nanoporosity over the mesoscale, the peculiar softness upon isotropic strain, the very weak electronic interaction with the substrate used for growth, Pt(111), and the large band gap in the electronic structure. This discovery adds one member to the family of 2D materials, proves the existence of boroxol-based B$_2$O$_3$ crystalline polymorphs and enables the atomic-scale tracking of individual structural units that can be exploited for in-depth studies on the crystalline-vitreous transition of trioxides.
[1] T. Zio et al., Science 390, 95-99 (2025)