[beta]-CuN3: The Overlooked Ground-State Polymorph of Copper Azide with Heterographene-Like Layers

• Published in: Angewandte Chemie International Edition 2015-02, Vol.54 (6), p.1954
• Main Authors: Liu, Xiaohui, George, Janine, Maintz, Stefan, Dronskowski, Richard
• Format: Article
• Language: English
• ISSN: 1433-7851; 1521-3773
• DOI: 10.1002/anie.201410987

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)Å3. The layered structure resembles graphite with an interlayer distance of 2.777(1)Å (=1/2c). 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 CuN bonds lengths of 1.91 and 2.00Å, and the N3- group is linear but with NN 1.14 and 1.20Å. Electronic-structure calculations and experimental thermochemistry show that the new polymorph termed [beta]-CuN3 is more stable than the established [alpha]-CuN3 phase. Also, [beta]-CuN3 is dynamically, and thus thermochemically, metastable according to the calculated phonon density of states. In addition, [beta]-CuN3 exhibits negative thermal expansion within the graphene-like layer.