Novel Non-Diamond-like Isoelectronic B-C-O Metastable Phases under High Pressure: B₂CO₅ and B₃CO₃

Due to the strong covalent bonds that can readily form among the light elements B, C, and O, B-C-O compounds are considered one of the ideal systems for exploring and designing novel superhard materials. In this work, the CALYPSO structure prediction method, combined with first-principles calculations, was employed to systematically investigate the stability of ternary B-C-O compounds under high pressure. Two novel metastable compounds, B₂CO₅ and B₃CO₃, were identified. Among them, the formation energy of B₂CO₅ is only 6 meV/atom above the thermodynamic convex hull. Both B₂CO₅ and B₃CO₃ are non-diamond isoelectronic compounds and exhibit novel bonding configurations. The crystal structure of B₂CO₅ consists of sp²-hybridized planar triangular CO₃ units and sp₃-hybridized BO₄ tetrahedra, which are interconnected through shared O atoms. In contrast, the crystal structure of B₃CO₃ contains sp³-hybridized BCO₃ tetrahedral units. Mechanical property analysis reveals that B₂CO₅ exhibits pronounced elastic anisotropy along different crystallographic planes, whereas B₃CO₃maintains elastic isotropy on the (001) plane. Hardness calculations indicate that B₃CO₃ has a Vickers hardness of approximately 36.4 GPa at 0 GPa, suggesting its potential for applications as a superhard material. Electronic structure calculations show that both B₂CO₅ and B₃CO₃ exhibit wide-band-gap semiconducting characteristics at 25 GPa, with PBE-calculated band gaps of 4.49 and 5.46 eV, respectively, while the corresponding values obtained using the HSE06 hybrid functional are 6.77 and 7.07 eV. This study enriches the diversity of bonding configurations in B-C-O compounds and provides theoretical insights and data for the design of novel superhard materials under high-pressure conditions.