Monte Carlo simulations of the gamma-beta, alpha-gamma, and alpha-beta phase transitions of nitrogen

We have performed Monte Carlo simulations in order to determine the gamma-beta, alpha-gamma, and the alpha-beta phase transition lines of nitrogen with a recently developed nitrogen-nitrogen potential, and to examine the driving forces for these transitions. We have shown that it is possible to obtain the alpha-gamma phase transition line by starting, at higher temperature, with the hexagonal representation of the disordered fcc structure. The transition line was found about O.4 GPa higher than the experimental line. The orientational order-disorder behavior of the gamma-beta and the alpha-beta phase transitions could be observed, but the structural changes, fcc-hcp, did not occur, probably due to a potential barrier. It is also possible that the potential model causes the fcc structure to be stable with respect to the hcp structure. The orientational entropy of the various phases has been compared qualitatively by introducing the orientational order parameters. It is suggested that a small difference in translational, entropy, due to a difference in the c/a ratio, stabilizes the hcp structure (beta phase) with respect to the fcc structure at zero pressure. In contrast with previous work, our simulations reveal that not all the layers of the ordered hcp structure at low temperature have the same orientational order. This might be the reason that the ordered fcc structure is stable at low temperature. (C) 1997 American Institute of Physics.