Ab initio structure determination of n-diamond

Abstract

A systematic computational study on the crystal structure of n-diamond has been performed using first-principle methods. A novel carbon allotrope with hexagonal symmetry R32 space group has been predicted. We name it as HR-carbon. HR-carbon composed of lonsdaleite layers and unique C 3 isosceles triangle rings, is stable over graphite phase above 14.2GPa. The simulated x-ray diffraction pattern, Raman, and energy-loss near-edge spectrum can match the experimental results very well, indicating that HR-carbon is a likely candidate structure for n-diamond. HR-carbon has an incompressible atomic arrangement because of unique C 3 isosceles triangle rings. The hardness and bulk modulus of HR-carbon are calculated to be 80GPa and 427GPa, respectively, which are comparable to those of diamond. C 3 isosceles triangle rings are very important for the stability and hardness of HR-carbon.