Determination of nitrogen's phase diagram up to 400 GPa

As an archetypal diatomic molecule, the high-pressure allotropes of molecular nitrogen are an excellent testbed for refining solid-state physics models. Above 110 GPa, the physicochemical properties of nitrogen radically change as it transforms into a 3D polymeric single-bonded solid. While ab-initio calculations predict a straightforward phase diagram for the polymeric phases—with only three polymeric solids up to 2100 GPa—recent experimental investigations suggest a much greater complexity. As it stands, four polymeric nitrogen phases have been synthesized below 245 GPa, with only one matching theoretical predictions. We here aim to uncover nitrogen's phase diagram between 220 and 400 GPa, discover new polymeric phases, solve their structure with single-crystal X-ray diffraction (SCXRD) and determine their stability range. These experiments will challenge current theoretical models, providing a rare example of the behavior of simple homoatomic systems at ultrahigh pressures.