Super‐hard hybrid ene‐C₅, yne‐C₆, and ene‐yne‐C₇ allotropes from crystal engineering and first principles

Abstract

Based on tetrahedral lonsdaleite‐like C4 inserted with C(sp2), C(sp1) and both C(sp2)‐ C(sp1) ultrahard hybrid allotropes ene‐C5, yne‐C6, and ene‐yne‐C7 were generated and identified from quantum density functional theory DFT calculations of ground states. The crystal density and the cohesive energy were found to decrease along the series C4‐C5‐C6‐C7 from the larger openness of the structures due to C insertions; such feature led to decrease the hardness throughout the series. The novel hybrid allotropes were found stable mechanically (elastic constants and their combinations) and dynamically (phonons band structures). From the electronic band structures, the inserted carbons were found to add up rigidly to the diamond‐like bands of C4 while being characterized by small band gap semi‐conductor to metallic‐like behavior. Modified diamonds such as the simple models herein considered exist (ex. nano‐diamond) and their investigations are of growing interest in materials sciences communities with applications as in electrochemistry.