Carbon structures and defect planes in diamond at high pressure

We performed a systematic structural search of high-pressure carbon allotropes for unit cells containing from 6 to 24 atoms using the minima hopping method. We discovered a series of new structures that are consistently lower in enthalpy than the ones previously reported. Most of these include ($5+7$)- or ($4+8$)-membered rings and can therefore be placed in the families proposed by H. Niu et al. [Phys. Rev. Lett. 108, 135501 (2012)]. However, we also found three more families with competitive enthalpies that contain ($5+5+8$)-membered rings, $s{p}^2$ motives, or buckled hexagons. These structures are likely to play an important role in dislocation planes and structural defects of diamond and hexagonal diamond.

Figure 1

Enthalpies (at 20 GPa) of the most interesting structures found in this work as a function of the number of atoms in the unit cell.

Figure 2

Calculated enthalpy difference per atom with respect to graphite for different structures of carbon as a function of applied pressure.

Figure 3

(left) Structure 24A exhibiting a $5+7$ line defect. (right) Structure 20C exhibiting a defect plane composed entirely by buckled hexagons.

Figure 4

Defect planes in hexagonal and cubic diamond (HD and CD, respectively).

Figure 5

X-ray diffraction patterns of carbon polytypes under pressure. The blue line (top) represents the acquired experimental spectrum of compressed graphite at 24 GPa from Ref. 6. Black lines show the simulated spectra ($\lambda$ $=$ 0.3329 Å) of different structures of carbon at 20 GPa. The yellow areas serve as a guide for the eye to show the principal changes found in the experimental diffraction.