Ene-C7 (sp3-sp2) and yne-C8 (sp3-sp1): Novel ultra-hard trigonal hybrid diamonds from crystal engineering and first principles

Abstract

Hybrid trigonal carbon allotropes labeled ene-C7 (“ene” for ethene-like C=C double bond) and yne‑C8 (“yne” for ethyne-like C≡C triple bond) are identified from crystal engineering by inserting sp2 and sp1-carbons respectively into rhombohedral diamond-like C6 (C-sp3). Density Functional Theory (DFT) calculations reveal ground state mechanically (elastic properties) and dynamically (phonons) stable hybrid allotropes characterized by large Vickers hardness letting qualify them as ultra-hard. The electronic band structures show insulating pristine C6, metallic ene-C7 and semi-conducting yne-C8. Such simple formulae hybrid allotropes may be considered as models enabling further assessing nanodiamonds properties in the field of electronics.