The crystal structures of δ and δ ∗ nitrogen

The crystal structures of the high-pressure δ and δ ∗ phases of nitrogen have been investigated using single-crystal x-ray diffraction. The structure of the δ phase is very similar to isostructural γ -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 δ ∗ phase is tetragonal and the space group is identified unambiguously as P 4 2 / n c m with unit cell parameters of a = 8.603 ( 5 ) Å and c = 5.685 ( 5 ) Å 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.