Bayreuth Scientists Unlock the Secrets of Nitrogen’s Solid Phase

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 about 2.5 GPa (i.e. 25,000 times the atmospheric pressure). For over a century, scientists have delved into solid phases of molecular nitrogen, asnowledge of the chemico-physical mechanisms underpinning transformations in nitrogen is vital in testing and refining theories of solid-state sciences.

 The Zeta-N₂phase 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 hitherto unknown—and key to deciphering nitrogen’s odd behaviour. The research team, led by Dominique Laniel (University of Edinburgh) and Natalia Dubrovinskaia and Leonid Dubrovinsky (the University of Bayreuth) employed an experimental technique newly developed in Bayreuth to successfully determine the crystal structure of Zeta-N₂. The researchers squeezed molecular nitrogen in diamond anvil cells to extreme pressures between 60 and 85 GPa, such as those prevailing in the Earth's mantle. By applying laser heating up to 2000 degrees Celsius, they were able to recrystallize high-quality submicrometer-size grains of Zeta-N₂. Their crystal structure (see Figure) was solved and refined from synchrotron single-crystal X-ray diffraction. With these experimental findings in hand, theoreticians at the University of Linköping (Sweden) gained further insights into nitrogen’s unique polymerization process.