Structures and phase diagrams of N₂ and CO to 13 GPa by x‐ray diffraction

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 ε, 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 ε‐N₂ and ε‐CO structures were determined to be rhombohedral R3̄c. There are eight ordered molecules in the ε‐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 ε‐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–ε 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.