Pressure-induced high-energy-density and superhard Be–C–O compounds

Light element compounds under high pressure display intriguing properties and applications, owing to their diverse bonding patterns and crystalline structures. However, the system of ternary Be–C–O compounds under high pressure, as the lightest representative of the IIA–IVA–VIA family, remains largely unexplored. Using a machine-learning-accelerated crystal structure search and first-principles calculations, Be–C–O phase diagrams are investigated at pressures ranging from 0 to 100 GPa. Four ternary compounds are proposed to be stable at corresponding pressures: BeCO3, Be2CO4, Be2C4O3, and BeC4O2. Analyses of electronic structure and chemical bonding further reveal how the structural diversity of these compounds is induced. Remarkably, Be2C4O3 and BeC4O2 are recoverable to ambient conditions and possess both high energy density and high hardness. The volumetric energy densities of Be2C4O3 and BeC4O2 could approach 9.03 and 7.94 kJ/cm3, respectively. The Vickers hardnesses of Be2C4O3 and BeC4O2 are found to be close to 39.58 and 51.57 GPa, respectively. These findings demonstrate the structural and functional diversity of Be–C–O compounds under high pressure, providing guidance for further exploration of the IIA–IVA–VIA compounds.