4.3 Properties

We also computed the mechanical and optical properties (see Supplementary Materials). Similar to previous theoretical investigations ^{25; 87; 88; 236; 237; 238; 239; 240}, all of these candidate allotropes exhibit high hardnesses ³³ and bulk moduli, which are comparable with those of diamond. Fig. 5.3 shows the calculated total and partial electronic densities of states in both systems. The DFT band gaps of M-carbon, X-carbon, bct-C, and Y-carbon are 3.6, 3.8, 2.7 and 2.9 eV, and we should bear in mind that DFT always underestimate the band gaps - so the real gaps are larger, and all of these -allotropes should be transparent colorless insulators.

Figure 8.11: Enthalpies of various carbon structures relative to graphite-2H.