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

Hydrazoic acid (HN₃) 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 (HN₃)₄ 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, ..., A, B. We report here on a polycrystalline sample of HN₃ that retains the monoclinic structure between 55(5) and 180(5) K with nearly linear increase of the lattice parameters a and b, but with steeper, partly nonlinear increase for the lattice parameter c. Near the melting point additional reflections are observed in the diffractograms which may indicate structural stress in the planar layers.