Scientists Unlock the Secrets of Nitrogen’s Unique ζ-N2 Solid Phase

The crystal structure of the exotic solid molecular nitrogen phase ζ-N2 has finally been solved and sheds light onto nitrogen’s unique progressive molecular-to-polymeric transformation.

In a ground-breaking study led by University of Edinburgh researchers, the enigmatic world of nitrogen’s solid phases was unravelled, shedding light on its complex behaviour. Their findings, published in the journal Nature Communications, provide unprecedented insights into the gradual molecular-to-polymeric transformation of nitrogen and the formation of amorphous nitrogen. Colleagues from the Universities of Bayreuth and Linköping were also involved in this study.

Phases of Nitrogen under pressure

At ambient pressure and temperature, nitrogen is gas and is found in the form of an N₂ molecule (N≡N) composed of an extremely strong triple-bond. When extreme pressures are applied to molecular gaseous nitrogen, it first becomes liquid and then a solid at 2.54 gigapascal (GPa; 25,400 times the atmospheric pressure). And for over a century, scientists have delved into these solid phases of molecular nitrogen, and although a seemingly simple diatomic element, it features an astonishingly intricate phase diagram boasting 16 different solid phases. Of particular interest is nitrogen’s behaviour from 80 GPa, as previous work revealed the onset of molecular nitrogen’s most unique transformation: the gradual rupture of its triple-bond, which culminates at about 160 GPa into a polymeric amorphous form composed of a three-dimensional network of single-bonded nitrogen atoms. As far as we know, nitrogen is the only diatomic solid that undergoes a progressive polymerisation. Knowledge of the chemico-physical mechanisms underpinning this transformation is vital in testing and refining theories of solid-state sciences.

Crystal structure of ζ-N₂

The ζ-N₂ phase of nitrogen, existing between 60 and 115 GPa, is a critical piece of the puzzle for understanding nitrogen’s molecular to polymeric transition. However, despite a large number of investigations, its crystal structure (i.e.the nitrogen molecules’ arrangement) was yet unknown—and key to deciphering nitrogen’s odd behaviour. The research team led by Dominique Laniel and co-authors employed a newly developed experimental methodological approach to successfully determine the crystal structure of ζ-N₂.

To accomplish this feat, molecular nitrogen was squeezed to extreme pressures between 60 and 85 GPa within diamond anvil cells. By applying laser heating, they were able to recrystallize high-quality submicrometer ζ-N₂ crystallites. From these crystallites, a complete structure model (see figure below) was derived using synchrotron single-crystal X-ray diffraction data.

Further insights

With these experimental findings in hand, density functional theory calculations were performed by the University of Linköping (Sweden), providing further insights into nitrogen’s unique polymerization process. The calculations revealed a remarkable gradual shift of electron density from the molecules’ triple-bond to intermolecular spaces. At 130 GPa, electronic density bridges formed, connecting three-molecule-long units. This phenomenon progressed to a 1D percolation at 150 GPa, offering a compelling explanation for the formation of the single-bonded amorphous phase of nitrogen.

The implications of this research extend beyond nitrogen itself, offering a deeper understanding of molecular transformations under extreme conditions. The findings pave the way for advancements in materials science and high-pressure physics.