Energetic materials:: variable-temperature crystal structures of γ- and ε-HNIW polymorphs

Crystals of gamma- and epsilon-hexanitrohexaazaisowurtzitane (gamma- and epsilon-HNIW, space group P2(1)/n for both crystals) have been investigated in the 100 - 298 K temperature range using single-crystal X-ray diffraction techniques. Temperature-dependent changes of their crystal lattices have been evaluated from the second-rank thermal expansion tensors. Both lattices undergo anisotropic thermal expansion, that of gamma-HNIW being more anisotropic than that of the epsilon phase. Comparison with previously reported predictions from molecular dynamics calculations indicates significant differences. Although there are many short ( less than van der Waals) intermolecular interactions in both polymorphs, there is no obvious relationship between the short distances and the difference in thermal expansion behavior. Non-linear temperature dependence of the atomic displacement parameters is indicative of anharmonicity of the crystal mean field potential.