Modeling of Extended N-H Solids at High Pressures.

The formation of nitrogen-hydrogen networked compounds is a promising approach for obtaining high energy density materials. Multiple experimental reports indicate that the synthesis pressure and temperature of high-energy nitrogen networked compounds significantly decrease when adding hydrogen to nitrogen. One- and two-dimensional structures of nitrogen-hydrogen mixtures are reported to form during synthesis and have also been observed with simulations; however, the structures are not thoroughly established or well understood. Here, we present results of calculations of nitrogen-hydrogen mixtures at pressures up to 50 GPa and predict their structural transformations upon applying and releasing pressure using density functional theory and evolutionary algorithms. Improvements in the computational procedure resulted in efficient on-the-fly elimination of slowly converging structures during the geometry optimization process. This enabled the continuation of long evolution simulations of the nitrogen-hydrogen structures with N/H ratios of 3:1, 4:1, and 9:1 at high pressures (10-50 GPa). New stable crystalline structures with high symmetry and covalent bonds are predicted that have (i) infinite chains and (ii) two-dimensional sheets of nitrogen-hydrogens. The structure with N/H ratio of 4:1 is found to be metallic at 50 GPa. Some crystalline phases stabilized by high pressure may exist as metastable structures with high symmetry and high mass density after lowering the pressure from 50 GPa down to 10 GPa. Vibration modes of calculated Raman and IR spectra are in agreement with published experimental data.