title: β-CuN3: the overlooked ground-state polymorph of copper azide with heterographene-like layers.
abstract: An unexpected polymorph of the highly energetic phase CuN3 has been synthesized and crystallizes in the orthorhombic space group Cmcm with a=3.3635(7), b=10.669(2), c=5.5547(11) Å and V=199.34(7) Å(3). The layered structure resembles graphite with an interlayer distance of 2.777(1) Å (=1/2 c). Within a single layer, considering N3(-) as one structural unit, there are 10-membered almost hexagonal rings with a heterographene-like motif. Copper and nitrogen atoms are covalently bonded with Cu-N bonds lengths of 1.91 and 2.00 Å, and the N3(-) group is linear but with N-N 1.14 and 1.20 Å. Electronic-structure calculations and experimental thermochemistry show that the new polymorph termed β-CuN3 is more stable than the established α-CuN3 phase. Also, β-CuN3 is dynamically, and thus thermochemically, metastable according to the calculated phonon density of states. In addition, β-CuN3 exhibits negative thermal expansion within the graphene-like layer.
authors.affiliation: Institut für Anorganische Chemie, Lehrstuhl für Festkörper- und Quantenchemie, RWTH Aachen, Landoltweg 1, 52056 Aachen (Germany).
authors.forename: Xiaohui
authors.initials: X
authors.lastname: Liu
authors.forename: Janine
authors.initials: J
authors.lastname: George
authors.forename: Stefan
authors.initials: S
authors.lastname: Maintz
authors.forename: Richard
authors.initials: R
authors.lastname: Dronskowski
keywords: azides; density functional calculations; electronic structure; phase transitions; thermochemistry