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<p>Dense <i>sp</i><sup>3</sup> carbon networks are prime candidates for superhard materials, yet uniting mechanical supremacy, optical selectivity, and an experimentally credible synthesis in a single phase remains uncommon. Here we predict PNNN-C18, an orthorhombic layer-bridge <i>sp</i><sup>3</sup> framework discovered by CALYPSO and validated by first-principles calculations. The structure comprises stiff <i>σ</i>-bonded tetrahedral sub-layers periodically linked along <i>c</i>-axis by two elongated <i>σ</i> bridges embedded in otherwise short C–C bonds, i.e., a stiff-layer + compliant-bridge hierarchy. Phonons and AIMD verify its dynamical and thermal robustness. Discrete energetics place PNNN-C18 in the customary ambient metastable band, while its enthalpy decreases relative to graphite with pressure, becoming overtaking graphite near ~ 72 GPa, delineating a high-pressure synthesis possibility. Electronically, it is an indirect semiconductor, with <i>σ</i>-only localization and a PBE band gap of 2.623&#xa0;eV, which is corrected to 3.683&#xa0;eV by the HSE06 hybrid functional. Mechanically, it is superhard (<i>H</i><sub>V</sub> ≈ 76–77&#xa0;GPa) and directional, with a tension-hard axis and identifiable soft modes consistent with its topology. Optically, PNNN-C18 exhibits orientation-dependent deep-UV absorption together with low visible reflectance, enabling highly efficient UV shielding without mirror-like glare. By establishing “tetrahedral distortion and hierarchical <i>σ</i>-bonding in a pure-<i>sp</i><sup>3</sup> lattice” as a lever for tuning properties, without invoking <i>sp</i><sup>2</sup> content, this work delivers an experimentally approachable target and a transferable design principle for next-generation dense carbon allotropes.</p>

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Layer-bridge sp3 carbon framework: structural hierarchy and directional properties of PNNN-C18

  • 期刊论文

  • 发表日期：

    2026年4月29日

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  • Yang Yu,
  • Mingmei Tang,
  • Zihe Li,
  • Shuai Chen,
  • Hua Zhong,
  • Pan Ying

摘要

Dense sp3 carbon networks are prime candidates for superhard materials, yet uniting mechanical supremacy, optical selectivity, and an experimentally credible synthesis in a single phase remains uncommon. Here we predict PNNN-C18, an orthorhombic layer-bridge sp3 framework discovered by CALYPSO and validated by first-principles calculations. The structure comprises stiff σ-bonded tetrahedral sub-layers periodically linked along c-axis by two elongated σ bridges embedded in otherwise short C–C bonds, i.e., a stiff-layer + compliant-bridge hierarchy. Phonons and AIMD verify its dynamical and thermal robustness. Discrete energetics place PNNN-C18 in the customary ambient metastable band, while its enthalpy decreases relative to graphite with pressure, becoming overtaking graphite near ~ 72 GPa, delineating a high-pressure synthesis possibility. Electronically, it is an indirect semiconductor, with σ-only localization and a PBE band gap of 2.623 eV, which is corrected to 3.683 eV by the HSE06 hybrid functional. Mechanically, it is superhard (HV ≈ 76–77 GPa) and directional, with a tension-hard axis and identifiable soft modes consistent with its topology. Optically, PNNN-C18 exhibits orientation-dependent deep-UV absorption together with low visible reflectance, enabling highly efficient UV shielding without mirror-like glare. By establishing “tetrahedral distortion and hierarchical σ-bonding in a pure-sp3 lattice” as a lever for tuning properties, without invoking sp2 content, this work delivers an experimentally approachable target and a transferable design principle for next-generation dense carbon allotropes.