A molecular-dynamics simulation study of the β phase of nitrogen

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Abstract
The structural characteristics of the β phase of solid nitrogen have been studied for large samples by the molecular-dynamics simulation technique. An effective interatomic potential appropriate to this phase was developed including nonbonded and quadrupole terms. Several properties of the crystal structure have been studied as functions of both temperature and pressure. In particular, a detailed investigation of the molecular orientational order has been made as a function of these parameters. At high temperatures the simulation sample is found to have a hexagonal structure with randomly oriented molecules. As the temperature decreases an orientational ordering transition is found to occur at ≊35 K. The effects of increasing pressure are found to be analogous to those of reducing the temperature. The nature of the local transition in the sample is investigated by calculating correlation parameters which specify twinning and tetrahedral ordering. The variation of the latter indicates that tetrahedral ordering increases strongly with decreasing temperature or increasing pressure, suggesting that the low-temperature phase is a condensation of this arrangement.