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

The technological developments seen in the last 400 years largely rest on incremental increases of the understanding of the fundamental behaviour of matter, in turn exploited to enable the formation of breakthrough functional materials. In our modern day societies, two of the most important class of materials are high energy density and superhard solids. High energy density materials (HEDMs) are employed as mining explosives-with hundreds of millions of tons annually used to extract essential minerals from the Earth-and as rocket fuel. They are the subject of intense research to find higher performance and non-polluting alternatives. On the other hand, superhard materials represent a growing multibillion international market and are indispensable for a wide range of applications, from machining tools to medical prosthetics, passing through aerospace, optics and even jewellery. While diamond is considered the sovereign superhard material, it low abrasiveness and chemical stability makes it inadequate for a number of applications for which an alternative must be found.

The remarkable characteristics of nitrogen make it the ideal element for new breakthrough HEDMs and superhard materials. Indeed, covalent nitrogen-nitrogen single bonds are the most energetic bonds among all elements, storing and releasing close to ten times more energy than the current best HEDMs. Nitrogen HEDMs are entirely eco-friendly. At the same time, nitrogen covalent bonds are also ultra-stiff, enabling the formation of superhard solids. While the exceptional potential of nitrogen for technological materials has been known for decades, classical approaches have proven inadequate in producing attractive nitrogen-rich compounds. In recent years, a new parameter for material synthesis has established itself as a top contender for achieving the sought-after nitrogen-based industrial materials: pressure. Indeed, compression to millions of times the atmospheric pressure radically changes the behaviour of matter and favours new and exotic atomic arrangements that are predominantly inaccessible otherwise, such as the greatly desirable high energy and ultra-stiff nitrogen covalent bonds.

This research project aims at exploiting the high pressure approach to harness the tremendous potential of nitrogen to produce new technological materials. As a crucial first step, the physico-chemical forces governing the high pressure behaviour of molecular nitrogen (N2) will be experimentally investigated. Then, in a collaborative effort with theorists, a new theoretical framework will be elaborated to address the shortcomings of first-principles predictions and significantly increase their accuracy-essential for engineering novel nitrogen-based functional materials. In a second step, the most promising nitrogen binary systems for forming high energy density as well as superhard solids will be experimentally studied. All pressure-produced compounds will be characterized to determine their exact nature and properties, as well as to establish their potential use as industrial materials. This work can only be successfully achieved by exploiting a recently developed technique: synchrotron single-crystal X-ray diffraction from polycrystalline samples (SC-XRDp). This research project will take place at the Centre for Science at Extreme Conditions (CSEC) of the University of Edinburgh-a world-renowned institution in the field of high pressure sciences with the necessary tools and expertise required for the successful realization of this research project.

The pressure parameter promises to be key to finally unravel the full potential of nitrogen. Exploiting a novel experimental method, the boundaries of our understanding of matter under extreme conditions will be pushed further back than ever before; ushering a new era for the design of novel functional materials. The discovered solids will undoubtedly play a pivotal role in the upcoming decades' technological breakthroughts.