Solid nitrogen has been studied intensely, with over 13 phases having been reported. Despite decades of studies, several nitrogen crystal structures have remained unsolved. Here we present an approach to predict the unresolved crystal structures using density functional theory and higher-level fragment-based second-order Møller-Plesset perturbation theory (MP2) and/or coupled cluster singles, doubles, and perturbative triples (CCSD(T)). Looking first at the lambda phase, we performed crystal structure prediction and simulated Raman spectroscopy to ascertain the crystal structure. We found that the previously proposed P21/c structure is indeed the likeliest candidate based on both energetic stability and spectroscopic properties. Next, we applied the same approach to predict the crystal structure of the zeta phase. Unlike phase lambda, the predicted crystal energy landscape for the zeta phase is more difficult to interpret in the context of existing experimental data. We identify one possible crystal structure with P21212, symmetry that is somewhat consistent with reported lattice constants, x-ray powder pattern, and Raman spectrum, though the evidence for this structural assignment is not clear cut. We will discuss the implications of this research toward resolving the long-unsolved structure of the zeta phase.

Combining structure prediction and spectroscopy to investigate challenging phases of solid nitrogen

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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