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)angstrom and V=199.34(7)angstrom(3). The layered structure resembles graphite with an interlayer distance of 2.777(1)angstrom (=1/2c). Within a single layer, considering N-3(-) 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 angstrom, and the N-3(-) group is linear but with NN 1.14 and 1.20 angstrom. Electronic-structure calculations and experimental thermochemistry show that the new polymorph termed -CuN3 is more stable than the established -CuN3 phase. Also, -CuN3 is dynamically, and thus thermochemically, metastable according to the calculated phonon density of states. In addition, -CuN3 exhibits negative thermal expansion within the graphene-like layer.