Solid carbon dioxide and nitrogen exhibit rich phase diagrams at high pressure. The large number of viable packing motifs stems from their small size and weak, non-polar intermolecular interactions, which make many packing arrangements and orientations energetically competitive. Experimental observation and characterization of high-pressure poly- morphs have proved challenging, not only because of flat energy landscape, but also their kinetic path-dependence and hysteresis in the phase transitions. As a result, high-quality experimental data are difficult to obtain, leaving many high-pressure crystal structures of nitrogen to remain unknown over decades, or creating ambiguities in the nature of some carbon dioxide phases.
This thesis employs a combination of high-level fragment-based electronic structure method and Raman simulation to study high-pressure polymorphs in these systems. First, we investigate the nature of carbon dioxide phases III and VII. We provide evidence that the long-accepted structure of phase III is problematic from comparison of large-basis-set quasi-harmonic second-order Møller-Plesset and experimental data. The experimental phase III and VII structures both relax to the same phase VII structure. Furthermore, Raman spectra predicted for phase VII are in good agreement with those observed experimentally for both phase III and VII, while those for the purported phase III structure contradict experimental observations. Crystal structure prediction is employed to search for other potential structures which might account for phase III, but none are found. Together, these results suggest that phases III and VII are likely identical.
Second, we revisit nitrogen phase λ, one of the high-pressure solid nitrogen forms that was discovered by combining experimental monoclinic lattice parameters with atomic positions from an earlier, computationally predicted structure that had similar unit cell dimensions. Crystal structure prediction is performed to demonstrate that the reported P21/c structure is indeed the likeliest candidate for the λ phase. Furthermore, we provide further evidence for the structural assignment by demonstrating good agreement between its predicted and experimental structural parameters and Raman spectra. Finally, the thermodynamic stability of the λ phase relative to other phases has been uncertain, but the calculations do suggest that it may be the thermodynamically most stable phase for at least part of the pressure range over which it has been observed. Lastly, we perform crystal structure prediction using ab initio random structure searching and density functional theory to identify candidate structures for nitrogen phase ζ, the phase whose structure remains unknown decades after it was first observed spectroscopically, despite numerous experimental and theoretical investigations. The candidates are then analyzed for consistency with experiment in terms of their simulated x-ray diffraction patterns and Raman spectra. While none of the structures generated is a clear match for the phase ζ experimental data, several of the candidates do exhibit features in common with the experiments and could provide an interesting starting point for future studies. The techniques here also rule out several candidate ζ nitrogen structures that have been identified previously. Finally, one of the structures might be considered a candidate for phase κ, whose structure is also unknown.
