The crystal structures of δ and δ* nitrogen

The crystal structures of the high-pressure delta and delta* phases of nitrogen have been investigated using single-crystal x-ray diffraction. The structure of the delta phase is very similar to isostructural gamma-O-2 and comprises spherically disordered molecules, with a preference for avoiding pointing along the cubic < 100 > directions, and disklike molecules with a uniform distribution of orientations. The structure of the delta* phase is tetragonal and the space group is identified unambiguously as P4(2)/ncm with unit cell parameters of a=8.603(5) angstrom and c=5.685(5) angstrom at 14.5 GPa. The orientations of the partially disordered molecules have been experimentally determined for the first time and are similar to those predicted on the basis of molecular dynamics simulations. (C) 2009 American Institute of Physics. [doi: 10.1063/1.3204074]