n-Diamond: Dynamical stability of proposed structures

A number of experimental works, mainly under extreme conditions, report a carbon phase with the same reflections as diamond but showing additional reflections that are forbidden for diamond. The crystal structure of this phase, called n-diamond by Hirai and Kondo (H. Hirai and K.-i. Kondo, Science 253 (1991) 772–774), remains unclear. By means of ab initio calculations based on density functional perturbation theory, the dynamical stability of the structures proposed to be n-diamond has been investigated up to a pressure of 30GPa. According to the calculations, a tetragonal carbon allotrope, called glitter (M.J. Bucknum and R. Hoffman, J. Am. Chem. Soc. 116 (1994) 11456–11464), is the best candidate. The calculated electronic structure and bulk modulus of glitter are critically discussed.