The crystal structures of δ and δ ∗ nitrogen

Abstract

The crystal structures of the high-pressure ␦ and ␦ ‫ء‬ phases of nitrogen have been investigated using single-crystal x-ray diffraction. The structure of the ␦ phase is very similar to isostructural ␥-O 2 and comprises spherically disordered molecules, with a preference for avoiding pointing along the cubic ͗100͘ directions, and disklike molecules with a uniform distribution of orientations. The structure of the ␦ ‫ء‬ phase is tetragonal and the space group is identified unambiguously as P4 2 / ncm with unit cell parameters of a = 8.603͑5͒ Å and c = 5.685͑5͒ 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.

Oxygen/noble gas binary phase diagrams at 296 K and high pressures

“…The diffraction pattern indicates a tetragonal distortion of the cubic phase. The situation is thus similar to what has been observed in pure solid nitrogen at the ␦-␦ Ã phase transition 19 but the P4 2 / ncm space group proposed for ␦ Ã does not fit all the diffraction lines. The present diffraction pattern is correctly indexed by a tetragonal unit cell with the lattice parameters a = 11.6606 Å and c = 11.1970 Å.…”

(N2)6Ne7: A High Pressure van der Waals Inse

“…If we correct these distances by assuming a rigid libration of the molecules, the distances change to 1.093 and 1.099 Å, respectively. These distances are in good agreement with previously measured values: 1.098 Å in the gas phase [30], 1.0 to 1.06 Å in the δ-N 2 phase [16], and 1.097 Å in the ðN 2 Þ 11 He compound [13].…”

“…It was shown that for the N 2 molecules, the presence of tightly bonded core electrons overshadow the changes in scattering due to the covalent bond affecting the valence electrons [27]. In the situation of spherical disorder for the N 2 molecule, a specific scattering form factor was used [16,28]. In our study, treating the molecule as isolated atoms was satisfactory, resulting in small residual electron density between −0.26 e=Å 3 and þ0.22 e=Å 3 .…”

“…3 Table I, surrounded by six N 2 molecules in a benzenelike structure and (b) thirteen Ne atoms surrounded by twelve N 2 molecules. Interestingly, the N 2 molecules were found to be oriented at 8.0 GPa and 296 K, unlike in pure δ-nitrogen in which N 2 molecules present spherelike or disklike disorder [16].…”

“…The quadrupole-quadrupole interaction, proportional to 1=r 5 (r being the nearest-neighbor intermolecular distance), which therefore increases under pressure, is responsible for the rich polymorphism of pure solid N 2 under pressure [16].…”