Characterization of the Isothermal Compression Behavior of LLM-172.

The high-pressure behavior of 3,4-bis(4-nitro-1,2,5-oxadiazol-3-yl)-1,2,5-oxadiazole (LLM-172) has been studied to 36 GPa by Raman spectroscopy and 50 GPa by X-ray diffraction. The Raman spectra and calculated unit-cell volumes at select pressures show reasonable qualitative agreement with first-principles density functional theory calculations. Raman peaks exhibit a gradual broadening and loss of intensity upon compression to near 20 GPa. Above 20 GPa, most Raman features disappear with the exception of modes associated with the skeletal ring modes. These modes were found to persist (although with low intensity) to 36 GPa. Because these modes exhibit very low compressibility over the pressure range studied, it is speculated that the ring structure is very stable. The X-ray diffraction suggests that while the crystal maintains an orthorhombic structure to near 40 GPa, it gradually undergoes a decomposition/amorphization beginning near 10 GPa. Analysis of the Raman results suggests that decomposition proceeds through isomerization, which leads to the formation of a C-O-N-O group rather than ring cleavage.

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.

Reactions of water molecules in silica-based network glasses.

Given that H(2)O dissolves minimally in quartz, the mechanism for the ubiquitous dissolution of H(2)O in silica glasses has been a long-standing puzzle. We report first-principles calculations in prototype silica glass networks and identify the ring topologies that allow the exothermic dissolution of H(2)O as geminate Si-O-H groups. The topological constraints of these reactions explain both the observed saturation of Si-O-H concentrations and the observed increase in the average Si-Si distance. In addition, calculations of H(2)O and Si-O-H dissociation account for the observed response to radiation by wet thermally grown SiO(2).