Raman activity of $s{p}^3$ carbon allotropes under pressure: A density functional theory study

Raman spectroscopy is a powerful tool to study the intrinsic vibrational characteristics of crystals, and, therefore, it is an adequate technique to explore phase transitions of carbon under pressure. However, the diamond-anvil cell, which is used in experiments to apply pressure, appears as a broad intense feature in the spectra. This feature lies, unfortunately, in the same range as the principal modes of recently proposed $s{p}^3$ carbon structures. As these modes are hard to distinguish from the diamond cell background, we analyze all Raman-active modes present in the $s{p}^3$ carbon structures in order to find detectable fingerprint features for an experimental identification.

Figure 1

Carbon structures with the $s{p}^3$ tetrahedral bond studied in this work: cubic diamond, hexagonal diamond, $M$-carbon (monoclinic), $BcT$ carbon, $W$-carbon (orthorhombic), and $Z$-carbon (orthorhombic).

Figure 2

Calculated Raman-active modes of the $s{p}^3$ carbon structures studied in this work as a function of pressure: (a) cubic diamond, hexagonal diamond, and $BcT$ carbon; (b) $M$-carbon (monoclinic); (c) $W$-carbon (orthorhombic); and (d) $Z$-carbon (orthorhombic).

Figure 3

Raman spectra calculated for $s{p}^3$ carbon allotropes at 20 GPa. The green area represents the experimental evolution of the diamond peak under pressure. Only principal modes are labeled.