Through a systematic structural search we found an allotrope of carbon with Cmmm symmetry which we predict to be more stable than graphite for pressures above 10 GPa. This material, which we refer to as Z-carbon, is formed by pure sp3 bonds and is the only carbon allotrope which provides an excellent match to unexplained features in experimental X-ray diffraction and Raman spectra of graphite under pressure. The transition from graphite to Z-carbon can occur through simple sliding and buckling of graphene sheets. Our calculations predict that Z-carbon is a transparent wide band gap semiconductor with a hardness comparable to diamond.
@article{arxiv.1109.1158,
title = {The crystal structure of cold compressed graphite},
author = {Maximilian Amsler and José A. Flores-Livas and Lauri Lehtovaara and Felix Balima and S. Alireza Ghasemi and Denis Machon and Stéphane Pailhès and Alexander Willand and Damien Caliste and Silvana Botti and Alfonso San Miguel and Stefan Goedecker and Miguel A. L. Marques},
journal= {arXiv preprint arXiv:1109.1158},
year = {2012}
}