Structural transitions in nitrogen molecular clusters: Experiment and simulation

摘要

The structure of (N_2)_n clusters is investigated up to the size of several hundred molecules with means of electron diffraction experiments and numerical simulations. The comparison relies on the calculated diffraction patterns. The structures extracted numerically are obtained with a global optimization procedure. Two transformations are observed. The first one is the transition from polyicosahedral to multilayer icosahedral. It is seen from simulations to occur near the crossover size n~* approx 40 molecules, but is hardly seen in the experiment. The second one is the transition from icosahedral to cubic. In this case, both approaches indicate a crystalline crossover size n_c located near 150-200 molecules. However, while the structures resulting from global optimization clearly show the enhanced stability of perfect cubic structures at zero temperature above n_c, the information obtained from experimental diffraction functions at finite temperature and pressure exhibits a more progressive appearance of the crystalline features. This suggests the presence of significant defects in the real structure of nitrogen molecular clusters as they undergo their transformation toward the bulk face-centered-cubic geometry.