Structural diversity of molecular nitrogen on approach to polymeric states

Abstract

Nitrogen represents an archetypal example of material exhibiting a pressure-driven transformation from molecular to polymeric state. Detailed investigations of such transformations are challenging because of a large kinetic barrier between molecular and polymeric structures, making the transformation largely dependent on kinetic stimuli. In the case of nitrogen, additional complications occur due to the rich polymorphism in the vicinity of the transition. Here, we report the observation of both molecular $(\ensuremath{\theta})$ and black phosphorus (bp) polymeric phases, crystallized upon temperature quenching of fluid nitrogen to room temperature at 97--114 GPa. Synchrotron single-crystal x-ray diffraction, Raman spectroscopy, and first-principles theoretical calculations have been used for diagnostics of the phases and determination of their structure and stability. The structure of $\ensuremath{\theta}\ensuremath{-}{\mathrm{N}}_{2}$ is determined as tetragonal, space group $P{4}_{2}{2}_{1}2$---one of the phases previously predicted theoretically above 9.5 GPa but never recognized experimentally. Molecular $\ensuremath{\theta}\ensuremath{-}{\mathrm{N}}_{2}$ is the most stable among molecular phases bordering the stability field of polymeric phases, partially settling a previously noted discrepancy between theory and experiment concerning the thermodynamic stability limit of molecular phases.