Structures and phase diagrams of N2 and CO to 13 GPa by x-ray diffraction
Abstract
The structures and phase transitions of N2 and CO were studied by powder x-ray diffraction from 100 to 300 K and 4 to 13 GPa. Three solid phases, β, δ, and ɛ, were observed in each material. The known β and δ solids were confirmed to have hexagonal space group P63/mmc and cubic space group Pm3n, respectively. From refinements using photographic x-ray intensities, the new ɛ-N2 and ɛ-CO structures were determined to be rhombohedral R3¯c. There are eight ordered molecules in the ɛ-N2 unit cell with a=5.928 Å and α=85.14° at 110 K and 7.8 GPa, and eight ordered molecules in the ɛ-CO unit cell with a=6.059 Å and α=85.73° at 100 K and 5.5 GPa. The CO molecules are randomly oriented head to tail. The δ-ɛ transition takes place through an ordering and small displacement of the N2 and CO molecules, accompanied by a slight extension of the lattice along a cube diagonal. Molar volumes are presented over an expanded P-T region. Recent theoretical calculations using lattice energies, molecular dynamics, and symmetry correlations correctly predict features in the N2 and CO phase diagrams.
- Publication:
-
Journal of Chemical Physics
- Pub Date:
- March 1986
- DOI:
- 10.1063/1.450310
- Bibcode:
- 1986JChPh..84.2837M