W0422

Thermal Expansion of Ammonium Dinitramide - A Variable-Temperature Crystallographic Study. Richard L. Speer, Jr., A. Alan Pinkerton, Dept. of Chemistry, Univ. of Toledo, Toledo, Ohio 43606 USA.

The crystal structure of ammonium dinitramide (ADN), NH₄.N₃O₄, has been investigated in the temperature range 20 – 298K using single crystal X-ray diffraction data. The structure is monoclinic, crystallizing in the space group P2₁/c. Upon cooling, both the a and b axes decrease, the b-axis being more dramatic, while the c-axis increases slightly. The volume was also found to decrease upon cooling. The data collected allowed the determination of the second-rank thermal expansion tensor. This tensor, found to be significantly anisotropic, indicates that the most extensive thermal expansion of ADN is directed perpendicular to the plane of the anion. There is a small thermal contraction also present, perpendicular to the direction of expansion and perpendicular to the b-axis. The expansion and contraction of the unit cell implies that the atomic thermal motion is anharmonic. The libration and translation second-rank tensors were also used to perform a rigid-body analysis of the molecular thermal motion.