α-γ Transition in Solid Nitrogen and Carbon Monoxide at High Pressure

We have examined the stability of the experimentally observed γ phase of solid nitrogen in order to test the validity of various forms of the intermolecular potential for N2. It was found that a Lennard-Jones repulsive potential between nonbonded atoms fails to explain the stability of the tetragonal γ phase, whereas a parametric, shape-dependent, hard-core potential gives a satisfactory account of this high-pressure phase of solid N2. The behavior of solid nitrogen-15 is discussed briefly. On the basis of the hard-core potential one would predict that a γ phase also exists for carbon monoxide, although at a very much higher pressure.