Internal Vibrations and Phase Diagram for a Model of Condensed Nitrogen.
Properties of condensed molecular nitrogen in the high pressure solid, along the melting curve, and in the hot dense fluid were calculated using the constant -pressure deformable-shape Monte Carlo algorithm with a realistic intermolecular potential. A mean-field theory of the vibron frequency shift with pressure and temperature was derived and found capable of resolving the two modes in the high pressure delta phase. At P = 300 kbar, we find a transition into an orientationally ordered structure. The calculated lower vibron peak splits into four values as in the recent experiments. A similar transition was found at low temperature. The simulated delta solid at P = 100 kbar was found to superheat by 25 percent above the melting transition. Near melting, the calculated lower vibron mode is independent of temperature and nearly continuous into the fluid, as in the experiments. The local environment of the high pressure solid and fluid was investigated and the structure at the disk site in the solid, on average, closely resembles the fluid. This structure is found to be quite different from the structure of the fluid at low pressure. The packing at high pressure is dominated by the shape of the molecule (P and X orientations), while the packing at low pressure is favorable to the quadrupolar interaction energy (T orientations). The equation of state of the hot dense fluid was calculated in the range of 300 K < T < 3000 K and 0 < P < 300 kbar. The resulting PVT surface is represented quite accurately by a simple analytic function. The calculated speed of sound and heat capacities agree well with experiment at room temperature.
- Pub Date:
- Physics: Condensed Matter