beta -CuN sub(3): The Overlooked Ground-State Polymorph of Copper Azide with Heterographene-Like Layers

An unexpected polymorph of the highly energetic phase CuN sub(3) has been synthesized and crystallizes in the orthorhombic space group Cmcm with a=3.3635(7), b=10.669(2), c=5.5547(11)Aa and V=199.34(7)Aa super(3). The layered structure resembles graphite with an interlayer distance of 2.777(1)Aa (=1/2c). Within a single layer, considering N sub(3) super(-) 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 2.00Aa, and the N sub(3) super(-) group is linear but with N--N 1.14 and 1.20Aa. Electronic-structure calculations and experimental thermochemistry show that the new polymorph termed beta -CuN sub(3) is more stable than the established alpha -CuN sub(3) phase. Also, beta -CuN sub(3) is dynamically, and thus thermochemically, metastable according to the calculated phonon density of states. In addition, beta -CuN sub(3) exhibits negative thermal expansion within the graphene-like layer. Are there others azide from me? A new CuN sub(3) polymorph with a heterographene-like motif was synthesized and characterized. Experimental and theoretical investigations give insight to its thermochemistry and its electronic and vibrational properties. Not only is beta -CuN sub(3) identified as the ground-state structure, the compound exhibits negative thermal expansion in the abplane, similar to, but more pronounced, than in graphite.