Revealing the structure of nitrogen’s most complex polymorph

09-11-2018

Nitrogen exhibits an exceptional polymorphism under extreme conditions. Here, one of the most elusive phases of this model system has been resolved, revealing a crystalline structure with unexpected complexity.

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Nitrogen, the primary constituent of the air we breathe, exists as a simple diatomic molecule at ambient conditions. It may be surprising that nitrogen exhibits a diverse range of polymorphs in much the same way that water-ice is more famous for. The simplicity of the N2 molecule makes it a model system for exploring pressure and temperature-induced changes in structure and bonding. Understanding these pressure-induced phenomena is significant to many scientific fields varying from chemistry to astronomy, and the N2 molecule provides an ideal testing ground for experiment-theory comparison. As such, the nitrogen phase-diagram is a focus-point in current extreme conditions research and nitrogen’s high-degree of polymorphism has not been observed in other simple molecular systems such as hydrogen or oxygen.

One of the most elusive polymorphs of nitrogen is known as ι–N2 (pronounced ‘iota’–N2). ι–N2 is a molecular nitrogen polymorph which is only accessible via high-temperature high-pressure conditions. Despite attracting considerable attention, the crystal structure of ι–N2 has remained unknown since its original discovery over 15 years ago [1], and until now the ι–N2 polymorph has never been recreated in the laboratory. In this work, a single crystal of ι–N2 has been unequivocally synthesised in a resistively-heated diamond-anvil cell, which is a deceptively simple hand-held device for generating high-pressures comparable to those at the centre of the Earth.

A representation of the synthesis route to ι–N2 is shown in Figure 1. When compressed at ambient temperature, nitrogen solidifies into the β–N2 polymorph at a pressure of 2.4 GPa. β–N2 is characterised by tumbling N2 molecules centred on a hexagonal close packed lattice. With further compression, nitrogen eventually reaches the ε–N2 polymorph, which was heated at a pressure of 65 GPa to a temperature of 750 K for several minutes until it transformed into the elusive ι–N2 crystal.

Representation of the synthesis route to ι–N2

Figure 1. Representation of the synthesis route to ι–N2. The blue spheres in β-N2 represent tumbling N2 molecules. The blue spheres in εN2 and ι–N2 represent nitrogen atoms. εN2 is represented in the rhombohedral axis setting.

The ι–N2 crystal was recovered to ambient temperature and brought to the high-pressure diffraction beamline, ID15B. The single-crystal X-ray diffraction data revealed ι–N2 to have an unusually complex molecular crystal structure characterised by 48 N2 molecules arranged into an intriguing layered structure, making it the most complex molecular crystal ever observed in an elemental diatomic system. An example of a diffraction pattern from the ι–N2 crystal is shown in Figure 2.

Single crystal X-ray diffraction pattern of ι–N2 at 56 GPa

Figure 2. Single crystal X-ray diffraction pattern of ι–N2 at 56 GPa. The sample was rotated over a range of 60° in 0.5° increments. The very bright reflections originate from the diamond anvils.

In collaboration with theoreticians at the University of Edinburgh, it was possible to navigate the energetic landscape of this highly polymorphic system and explore the stability of the experimentally-determined ι–N2 structure. Ab initio simulations found the ι–N2 structure to display a surprising energetic favourability, while random structure searches did not generate any structures more favourable than ι–N2. Therefore, the uncovering of the ι–N2 structure has implications for the traditional nitrogen phase diagram.

The ESRF and the University of Edinburgh have been at the forefront of the discovery of complex high-pressure structures over the last 10 years, finding a number of fascinating polymorphs which contain hundreds of atoms in light alkali and alkaline earth metals [2-4]. This recent study, performed in collaboration with researchers based in China, pushes in a new direction, revealing for the first-time that simple molecular elements can also exhibit complex structures at high pressures. These findings raise intriguing questions of whether unusual phases should be more widely expected, and the results should prompt further investigations into the reasons for such complex structures appearing at high pressures in elemental molecular solids.

 

Principal publication and authors
Unusually complex phase of dense nitrogen at extreme conditions, R. Turnbull (a), M. Hanfland (b), J. Binns (c), M. Martinez-canales (a), M. Frost (a,d), M. Marques (a), R.T. Howie (c) & E. Gregoryanz (e,c,a), Nature Communications 9, 4717 (2018); doi: 10.1038/s41467-018-07074-4.
(a) Centre for Science at Extreme Conditions and School of Physics and Astronomy, University of Edinburgh (UK)
(b) ESRF
(c) Center for High Pressure Science & Technology Advanced Research, Shanghai (China)
(d) SLAC National Accelerator Laboratory, CA (USA)
(e) Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei (China)

 

References
[1] E. Gregoryanz, A.F. Goncharov, R.J. Hemley, H.-k. Mao, M. Somayazulu & S. Guoyin, Phys. Rev. B 66, 224108 (2002).
[2] C.L. Guillaume, E. Gregoryanz, O. Degtyareva, M.I. McMahon, & M. Hanfland, M. et al., Nat. Phys. 7, 211 (2011).
[3] E. Gregoryanz, L. F. Lundegaard, M.I. McMahon, C.L. Guillaume, R.J. Nelmes, & M. Mezouar, Science 320, 1054–1057 (2008).
[4] I. Loa, R.J. Nelmes, L.F. Lundegaard, & M.I. McMahon, Nat. Mater. 11, 627–632 (2012).