β‐CuN3: The Overlooked Ground‐State Polymorph of Copper Azide with Heterographene‐Like Layers.

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) A and V=199.34(7) A3. The layered structure resembles graphite with an interlayer distance of 2.777(1) A (=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 CuN bonds lengths of 1.91 and 2.00 A, and the N3− group is linear but with NN 1.14 and 1.20 A. 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.