Role of acetylenic bonds in the mechanical, electronic and optical properties of yne-diamonds

Yne-diamonds are novel carbon allotropes designed by inserting acetylenic bonds into the framework of diamond. Varying the ratio of acetylenic bonds yields a new family of carbon allotropes consisting of sp- and sp ³-hybridized atoms. The study of these novel carbon frameworks is becoming a topic of increasing interest. Here, we report our systematic studies on the stability, mechanical, electronic and optical properties of yne-diamonds. Our calculations indicate that yne-diamonds are mechanically stable, although they are energetically less favorable than diamond due to sp-hybridized carbon atoms in the frameworks. In contrast to early estimation, yne-diamonds are unlikely superhard materials, but exhibit good ductility. The band gaps of yne-diamonds vary from 0.165 eV to 4.850 eV, as the ratio of acetylenic bonds increases. The energetically most favorable yne-diamond has a direct band gap of 2.916 eV. The optical properties of these carbon allotropes are also discussed.