Unusually complex phase of dense nitrogen at extreme conditions

Robin Turnbull,Miguel Martinez-Canales,Jack Binns,Mungo Frost,Miriam Marqués,Ross T Howie,Michael Hanfland,Eugene Gregoryanz

doi:10.1038/s41467-018-07074-4

Abstract

Nitrogen exhibits an exceptional polymorphism under extreme conditions, making it unique amongst the elemental diatomics and a valuable testing system for experiment-theory comparison. Despite attracting considerable attention, the structures of many high-pressure nitrogen phases still require unambiguous determination. Here, we report the structure of the elusive high-pressure high-temperature polymorph ι–N2 at 56 GPa and ambient temperature, determined by single crystal X-ray diffraction, and investigate its properties using ab initio simulations. We find that ι–N2 is characterised by an extraordinarily large unit cell containing 48 N2 molecules. Geometry optimisation favours the experimentally determined structure and density functional theory calculations find ι–N2 to have the lowest enthalpy of the molecular nitrogen polymorphs that exist between 30 and 60 GPa. The results demonstrate that very complex structures, similar to those previously only observed in metallic elements, can become energetically favourable in molecular systems at extreme pressures and temperatures.

Highlights

Nitrogen exhibits an exceptional polymorphism under extreme conditions, making it unique amongst the elemental diatomics and a valuable testing system for experiment-theory comparison
Our study demonstrates that the appearance of complex structures under pressure is not limited only to metallic elements[1,2,3] and raises the intriguing question of whether unusual phases could be expected in other molecular systems at extreme conditions
Our Density functional theory (DFT) calculations find ι–N2 to have the lowest enthalpy of the molecular nitrogen polymorphs for which the structures are known between 30 and 60 GPa, which will have implications for the nitrogen phase diagram

Summary

Introduction

Nitrogen exhibits an exceptional polymorphism under extreme conditions, making it unique amongst the elemental diatomics and a valuable testing system for experiment-theory comparison. The transition is characterised by the loss of the ε–N2 ν1 vibrational mode, as shown in the Raman spectra across the synthesis P–T path in Supplementary Figure 1.

Results

Conclusion