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Exploration of phase stability and low-pressure synthesis of solid nitrogen by means of atomic scake computer simulations and experiments

Subject Area Solid State and Surface Chemistry, Material Synthesis
Term from 2005 to 2015
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 16359709
 
Final Report Year 2016

Final Report Abstract

Molecular nitrogen can be transformed under high compression into a variety of chemically bonded solid phases. This transformation to non-molecular phases is of fundamental interest for understanding the physics of molecular solids and the chemistry of nitrogen, but also of interest for applications in energy technology, since non-molecular nitrogen can potentially be used as high energy density material. At the beginning of this project it was well established that crystals with cubic gauche (cg) structure can be synthesized at high temperatures (T > 2000 K) and pressures above 110 GPa, while many other polymeric phases of nitrogen were theoretically proposed. In our studies, we initially explored different ways for the polymerization of nitrogen and found additional evidence for the existence of polymeric nitrogen by studying X-ray diffraction patterns in a wide range of diffraction angles both for polycrystalline and single-crystalline samples. We could show that at high pressures and high temperatures, molecular nitrogen transforms into monoatomic cg-N through a disordered transitional state. This transformation between two solids, the molecular and cg-N phases of nitrogen, occurs during continuous temperature increases at constant pressure. Theoretically, a comparative study of all relevant phases was carried out. We systematically investigated various solid nitrogen phases under pressure by means of ab initio calculations based on density-functional theory and showed that the previously suggested nitrogen phases such as chaired web, layered boat, and arsenic-7 are most likely of no relevance to the experiments. Moreover, we predicted a layered Pba2 and helical tunnel P21 21 21 structure to be stable in the pressure ranges of 188–320 GPa, and above 320 GPa, respectively, ruling out the low-temperature stability of the previously proposed black phosphorus structure. We then continued with investigating various possibilities to stabilize nitrogen-based high energy density materials by the high-pressure synthesis of ammonium and lithium azides. High pressure Raman studies of ammonium azides revealed a polymorph phase transition in the simple molecular ionic crystal NH4 N3 at pressures < 3 GPa, which was not predicted theoretically. Also studies on the high-pressure behavior of metals azides were carried out in order to gain an understanding of the mechanism of pressure-induced rearrangement of azide ions and phase transitions that might result in the formation of polymeric nitrogen. In this context we investigated lithium azide (LiN3 ) by X-ray diffraction and Raman spectroscopy at hydrostatic compression up to pressures above 60 GPa at room temperature. The results of X-ray diffraction reveal the stability of the ambient-pressure C 2/m crystal structure up to the highest pressure. This phase stability of LiN3 is in contrast to that of sodium azide (which is isostructural at ambient pressure), for which a set of phase transitions has been reported at pressures below 50 GPa. We then followed the idea that addition of hydrogen might reduce the pressure needed for the synthesis of new energetic materials, and make these more stable in comparison to pure nitrogen polymers. Our results revealed that nitrogen and hydrogen directly react at room temperature and pressures of 35 GPa forming chains of single-bonded nitrogen atom with the rest of the bonds terminated with hydrogen atoms - as identified by IR absorption, Raman, X-ray diffraction experiments and theoretical calculations. At releasing pressures below 10 GPa, the product transforms into hydrazine. Thus, our findings might open a way for the practical synthesis of these extremely high energetic materials as the formation of nitrogen-hydrogen compounds is favorable already at pressures above 2 GPa according to theoretical calculations. Finally, we extended our research towards other high-pressure phases. We examined both experimentally and theoretically the properties of NF3 at large compression and found that this compound undergoes three solid-solid phase transitions up to 55 GPa while retaining its molecular structure. Periodic DFT calculations indicate that, in fact, even at 100 GPa NF3 should remain molecular, in contrast to ammonia which transforms to NH4 + NH2 at large compression. Further, our first-principles calculations allowed us to identify a boron oxynitride with composition B6N4O3 and low enthalpy of formation at 20 GPa, which was explaining experimental results obtained in the SPP 1299. Eventually, we could show that molecular H2 CO3 appears to be an integral component of CO2/H2O mixtures at and above pressures of 2.4 GPa at elevated temperature. The relative abundance of H2CO3 and its dissociation products HCO−_3 and CO2−_3 in an aqueous solution of CO2 is largely determined by the dielectric constant (ε) and the autodissociation constant Kw of its host solvent H2O, and both are complex functions of pressure (p) and temperature (T). In summary, our findings might open a way for the practical synthesis of these extremely high energetic nitrogen compounds.

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