Calculation of the Phase Diagrams For the Fluid-Solid and Solid-Solid (Delta-Delta(Loc)-Epsilon) Transitions in Molecular Nitrogen By Using Mean Field Model

Abstract

The temperature-pressure (T-P) phase diagrams are calculated using the mean field models for the solid-fluid (first order) and, the solid-solid transitions of the delta-delta(loc) (second order) and delta(loc)-epsilon (first order) in nitrogen. By expanding the free energy in terms of the order parameters, the phase line equations are derived and they are fitted to the experimental data from the literature for this molecular crystal. For the fitting procedure, the coefficients in the free energy expansion are assumed to be the temperature and pressure dependent. The mean field models presented here can explain the observed T-P phase diagram of nitrogen. The critical behavior of some thermodynamics quantities can also be predicted by the mean field models close to the solid-fluid and solid-solid transitions in this molecular crystal.