High-pressure static and dynamic properties of theR3¯cphase of solid nitrogen

The zero-temperature equilibrium structure and orientations of solid ${\mathrm{N}}_2$ are calculated at pressures 20≤P≤415 kbar by optimizing the lattice energy of an eight-molecule unit cell with respect to a rhombohedral distortion of the crystal from the cubic Pm3n structure. The resulting equilibrium structures have R3¯c($C_{3v}^6$) symmetry, upon which the pressure-volume relation and the libron, vibron, and lattice phonon frequencies at zone center are calculated using lattice dynamics. These quantities are compared with experiment and the symmetry assignments of the calculated and observed modes are given. Also calculated are the second virial coefficients, the α-${\mathrm{N}}_2$ molar volume and sublimation energy, and the α- and γ-${\mathrm{N}}_2$ vibron frequencies, versus pressure.