Structural diversity of molecular nitrogen on approach to polymeric states

TL;DR

This study investigates the pressure-induced transformation of nitrogen from molecular to polymeric states, revealing new phases and clarifying stability limits through experimental and theoretical methods.

Contribution

It reports the observation of both molecular and polymeric nitrogen phases at high pressures, combining experimental diagnostics with first-principles calculations to resolve previous stability discrepancies.

Findings

Identification of molecular nd polymeric BP phases at 97-114 GPa

Molecular nd polymeric phases coexist under specific conditions

Clarification of the thermodynamic stability limit of molecular nitrogen

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 (\theta) and polymeric (BP) 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. Molecular \theta-nitrogen 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.