Understanding shear-induced s p 2 -to- s p 3 phase transitions in glassy carbon at low pressure using first-principles calculations

Glassy carbon is a disordered carbon allotrope consisting of $s{p}^{2}$-hybridized bonds, which can be transformed to mixed $s{p}^{2}\text{\ensuremath{-}}s{p}^{3}$ forms with completely different mechanical and electrical properties. The transformation from an $s{p}^{2}$-rich to $s{p}^{3}$-rich structure under pressure has been extensively studied, while the effect of shear strain on the transformation remains unexplored. In this work, we use a first-principles calculation method to study the phase transitions of glassy carbon under both shear strain and pressure. We find that shear strain can significantly reduce the external pressure needed to transform the structure from $s{p}^{2}$ rich to $s{p}^{3}$ rich. Compared with the initial $s{p}^{2}$-rich structure, $s{p}^{3}$-rich structures recovered to ambient condition have a much higher mechanical strength and lower electronic density of states near the Fermi level. Our results demonstrate that applying large shear strain is a promising approach for industrial production of superhard amorphous carbon under lower pressures.