Excellence in Research - Exploring Frontiers in Novel Material Synthesis at High Pressures: Synthesis and Recovery of Superhard and High Energy-Density Polynitrides

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Non-technical summary: Applying high pressure drastically changes physical and chemical properties of elements and compounds, which leads to the formation of fascinating new materials with unmatched properties. With support from NSF’s Office of Integrative Activities, researchers at Howard University and Carnegie Institution of Washington investigate nitrogen-rich metal compounds that form polymeric structures or large molecules (polynitrides) under high pressure. These materials show great promise as possible next-generation superhard or high-energy-density materials. The researchers explore the formation of specific alkali, alkali-earth and transition metal polynitrides at high-pressures and temperatures. Then they establish the material’s crystal chemistry and physical properties using a combination of laser-heated diamond anvil cells, synchrotron X-ray diffraction and Raman scattering experiments to tune the pressure-temperature conditions of synthesis and to probe the structure, physical properties, and chemical bonding of novel polynitrogen compounds. This research could aid and direct the functional design of new polynitrogen compounds with industrially relevant properties in the future. The project advances the Materials Science field in the search for new materials with potential energy applications as well as adjacent fields in Earth and Planetary sciences, which would benefit from better knowledge of crystal chemistry at extreme conditions. The project provides support and training to one postdoctoral associate at Howard University/Carnegie Institution for Science and advanced facilities (e.g. Advanced Photon Source, APS), as well as research opportunities for graduate and undergraduate students at Howard University. High school students will be engaged through a summer apprenticeship program.Technical summary: Material properties are greatly altered at high pressures, where unusual chemical bonding schemes and novel stoichiometries become thermodynamically stable. With support from NSF’s Office of Integrative Activities, researchers at Howard University and Carnegie Institution of Washington study the formation of alkali, alkali-earth and transition metal polynitrides (with focus on Li, K, Re, Os, W, Cr, Ru, Ti, Zr, Hf) at high-pressure high-temperature conditions, and their crystal chemistry and physical properties. The researchers exploit pressure as a tool to create a variety of unusual chemical bonding schemes and compositions, which become thermodynamically stable at high densities, but can be metastable at ambient pressure. Using a combination of X-ray diffraction and Raman experiments in laser-heated diamond anvil cell the researchers build a basis for further scaling-up of the synthesis, and reveal relations between synthesis conditions, chemical bonding, structure, and properties of polynitrides of different metals. This enables them to formulate crystal chemical principles governing the formation and behavior of polynitrides at extreme conditions. Furthermore, the researchers manipulate chemical composition, external strain, dimensionality, temperature, and chemical precursors as key variables that can be used to perform synthesis in the pressure-temperature range suitable for practical application and for metastable recovery of the materials.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.