Pentaheptite diamond: a new carbon allotrope.

The search for sp 3 -hybridized carbon allotropes other than diamond has attracted extensive interest because of their fascinating properties. In this paper, an orthorhombic carbon phase in sp 3 bonding, named pentaheptite diamond, by combining the particle swarm optimization method with first-principles calculations has been predicted. The phonon spectra, total energy and elastic constants calculations of the pentaheptite diamond confirm its dynamical, thermal and mechanical stability at zero pressure, respectively. It possesses a high bulk modulus of 385 GPa and Vickers hardness of 72.6 GPa, comparable to diamond. Electronic band structure calculations show that the pentaheptite diamond has a direct band gap of 4.18 eV.