ISSN print edition: 0366-6352 ISSN electronic edition: 1336-9075 Registr. No.: MK SR 9/7 Published monthly

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

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

School of Electronic Information and Automation, Hefei University, Hefei, China

E-mail: shuaichenys@163.com

Received: 8 December 2025  Accepted: 3 April 2026

Abstract:

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 (H_V ≈ 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.

Keywords: Carbon allotrope; First-principles calculations; Hierarchical structure; Superhardness; Anisotropy; Deep UV

Full paper is available at www.springerlink.com.

DOI: 10.1007/s11696-026-04882-7

Chemical Papers 80 (7) 8201–8211 (2026)