Molecular Structure of Hydrazoic Acid from 55 K to Close to the Melting Point Determined with Synchrotron Radiation.

Hydrazoic acid (HN) is the simplest covalent azide, potentially explosive, and strongly toxic with both a low boiling and a low melting point (309 and 193 K, respectively). The monoclinic structure, recently solved by X-ray single-crystal diffraction at 100(2) K, is built up by tetramers (HN) in unique pseudotetragonal layers with N-H···N hydrogen bonds, but with only weak van der Waals bonds between them. As also observed in 2H-graphite, nearly planar layers are stacked parallel to (001) with the sequence A, B, .

More Than 50 Years after Its Discovery in SiO Octahedral Coordination Has Also Been Established in SiS at High Pressure.

SiO exhibits a high-pressure-high-temperature polymorphism, leading to an increase in silicon coordination number and density. However, for the related compound SiS such pressure-induced behavior has not been observed with tetrahedral coordination yet. All four crystal structures of SiS known so far contain silicon with tetrahedral coordination.

Two high-pressure phases of SiS2 as missing links between the extremes of only edge-sharing and only corner-sharing tetrahedra.

The ambient pressure phase of silicon disulfide (NP-SiS2), published in 1935, is orthorhombic and contains chains of distorted, edge-sharing SiS4 tetrahedra. The first high pressure phase, HP3-SiS2, published in 1965 and quenchable to ambient conditions, is tetragonal and contains distorted corner-sharing SiS4 tetrahedra. Here, we report on the crystal structures of two monoclinic phases, HP1-SiS2 and HP2-SiS2, which can be considered as missing links between the orthorhombic and the tetragonal phase.

Molecular structure of hydrazoic acid with hydrogen-bonded tetramers in nearly planar layers.

Hydrazoic acid (HN(3))--potentially explosive, highly toxic, and very hygroscopic--is the simplest covalent azide and contains 97.7 wt % nitrogen. Although its molecular structure was established decades ago, its crystal structure has now been solved by X-ray diffraction for the first time.

alpha- and beta-FOX-7, polymorphs of a high energy density material, studied by X-ray single crystal and powder investigations in the temperature range from 200 to 423 K.

The alpha-beta phase transition in the novel energetic material 1,1-diamino-2,2-dinitroethylene, C2H4N4O4 (FOX-7), has been studied by single-crystal X-ray investigations at five different temperatures over the 200-393 K range. In these investigations, the positions of the hydrogen atoms were experimentally determined without any geometric constraints. In addition, X-ray powder investigations using the Guinier technique have been performed to characterize the beta-phase up to 423 K.