High-pressure routes to novel nitrogen-containing materials characterized through single-crystal X-ray diffraction of polycrystalline samples

Nitrogen is an exceptional element. It can store and release the largest quantities of chemical energy and produce extremely strong covalent bonds. Nitrogen compounds are naturally sought-after for environmentally friendly energy storage, superhard materials, but also for their electronic properties. However, at ambient conditions, severe difficulties related to their synthesis hinder their investigation, thereby limiting their applicability. In recent years, high-pressure high-temperature synthetic approaches, combined with the development of a new methodology for in-situ experimental determination of crystal structures, enabled overcoming these obstacles.

In this talk, we will first present the novel experimental approach enabling to extract single-crystal X-ray diffraction data from micron-sized polycrystals confined in a diamond anvil cell. Then, the recent progress and discoveries in high-pressure high-temperature stabilized binary nitrogen-based materials will be presented, including new energetic polynitrogen anions as well as strongly covalent superhard solids. Strategies to expand the (meta)stability pressure range of these solids to allow their recovery to ambient conditions will also be discussed. In this context, the formation of the first C₃N₄ polymorphs with fully sp³-hybridized carbon atoms, superhard and stable upon full pressure release, will be discussed. Expanding on this result, the strong covalent C-N bonds formed in ternary carbon-nitrogen systems will be highlighted as a promising pathway to stabilizing energetic polynitrogen species to lower pressures. This will be exemplified by presenting first results on various ternary carbon-nitrogen systems, including rare earth elements- as well as transition metal-carbon-nitrogen systems.