Carbon clathrates C24, C28 and CA6: Structure formation and properties

V.A. Greshnyakov, V.V. Pavlik, M.E. Belenkov, E.A. Kulakova show affiliations and emails

Received 21 November 2025; Accepted 21 December 2025;

Citation: V.A. Greshnyakov, V.V. Pavlik, M.E. Belenkov, E.A. Kulakova. Carbon clathrates C24, C28 and CA6: Structure formation and properties. Lett. Mater., 2026, 16(1) 23-29

BibTex https://doi.org/10.48612/letters/2026-1-23-29

Abstract

The structures of two novel carbon clathrates, whose properties differ significantly from those of diamond, were predicted. The most likely precursors and methods for producing certain types of clathrates were identified.

The theoretical research of the potential existence and synthesizability of novel nanostructured diamond-like carbon clathrates has been carried out. Within the framework of density functional theory calculations, two previously unknown orthorhombic clathrate structures, C24 and C28, have been identified. The crystal lattices of these diamond-like compounds belong to the spaсe group Pmmm and are characterized by five crystallographically inequivalent atomic positions. The C24 clathrate, composed of polyhedral blocks C20, C24, and C28, should be stable up to at least 500 K, whereas the C28 clathrate is stable only at low temperatures or high pressure. The cohesive energy of the most stable clathrate (C24) is less than the corresponding energy of diamond by 0.299 eV / atom. The densities of the new clathrates and their bulk modulus are lower than those of diamond by at least 14 % and 24 %, respectively. The orthorhombic clathrates are expected to be wide-bandgap semiconductors with bandgap in the range from 3.7 to 4.0 eV. The research has revealed that ultrathin hybrid carbon layers, consisting of sp2 and sp3 hybridized atoms, are the most plausible clathrate precursors. For the first time, it has been established that the structural variant of clathrate CA6 (carbon sodalite) can be obtained via high-pressure compression of crystals composed of layered or molecular precursors. In particular, CA6 clathrate can be formed by the process of strong compression of graphite consisting of L4‑6‑12 graphene perpendicular to the planes of the graphene layers when the pressure reaches ≈29 GPa. The pressure required for complete structural transformation of precursors into this diamond-like phase can be significantly reduced to 1.1 GPa if hydrostatic compression of dense fullerite composed of C24 clusters is applied. The experimental identification of the new carbon clathrates can be accomplished using calculated powder X-ray diffraction patterns and Raman spectra.

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Funding

1. The Russian Science Foundation (RSF) - 25-72-31032