Nitrogen under Extreme Conditions: From Fundamental Physics to Novel Functional Materials

Technological advancements over the last 400 years have largely stemmed from a deeper understanding of the fundamental behaviour of matter, which has been harnessed to create breakthrough functional materials. In modern societies, two of the most important classes of materials are high energy density materials (HEDMs) and superhard solids. HEDMs are used in mining explosives, with hundreds of millions of tons annually employed to extract essential minerals, as well as in rocket fuels for space exploration and travel. Intense research is focused on developing higher-performance, non-polluting alternatives. Superhard materials, on the other hand, represent a growing multibillion-dollar international market and are indispensable across a wide range of applications, including machining tools, aerospace, optics, and even jewellery. While diamond is considered the sovereign superhard material, its low abrasiveness and chemical stability limit its use in some applications, necessitating the search for substitutes. The remarkable properties of nitrogen make it an ideal candidate for next-generation HEDMs and superhard materials. Nitrogen-nitrogen single covalent bonds are among the most energetic, storing and releasing nearly ten times more energy than the current best HEDMs. Additionally, nitrogen solids as HEDMs are entirely eco-friendly, releasing inoffensive molecular nitrogen (N2) into the atmosphere. Nitrogen's single bonds are also extremely stiff, enabling the formation of superhard solids. Despite exceptional potential of nitrogen solids, classical synthesis approaches have proven inadequate in producing attractive nitrogen-rich compounds. Recently, high-pressure synthesis has emerged as a leading method for achieving the sought-after nitrogen-based materials. Compression to millions of times atmospheric pressure dramatically alters matter’s behaviour, facilitating new atomic arrangements conducive to high-energy and ultra-stiff nitrogen covalent bonds. Building on the success of our original FLF project, this research aims to exploit high-pressure techniques to unlock nitrogen’s vast potential for creating new technological materials. First, a novel synthesis method utilizing athermal energy transfer will be developed to scale-up the production of materials discovered in the first three years of this FLF that show significant potential as HEDMs and superhard solids. Next, the search for new nitrides will expand to three-element systems, significantly increasing complexity but also offering greater promise for discovering next-generation materials. In collaboration with theorists, these compounds will aid in training artificial intelligence models under high-pressure conditions. Finally, pure nitrogen will be studied at unprecedentedly high pressures, exploring a regime relevant for planetary sciences and for uncovering novel high energy density nitrogen phases, which can later be targeted at lower pressures using alternative methods. This work can only be successfully achieved by exploiting a recently developed technique: synchrotron single-crystal X-ray diffraction (SCXRD) from polycrystalline samples. The research will take place at the Centre for Science at Extreme Conditions at the University of Edinburgh, a world-leading institution in high-pressure sciences, equipped with the necessary tools and expertise for this project’s successful realization. The pressure parameter holds the key to unlocking nitrogen’s full potential. By exploiting novel experimental methods, this project will push the boundaries of our understanding of matter under extreme conditions, ushering a new era in the design of functional materials. The solids discovered in this research will undoubtedly play a pivotal role in technological breakthroughs in the decades to come.