γ-FOX-7:: Structure of a high energy density material immediately prior to decomposition

1,1-Diamino-2,2-dinitroethene, C2H4N4O4 (FOX-7), is a novel high energy density material with low friction and impact sensitivity and a high activation barrier to detonation. In this study, the previously unknown crystal structure of the gamma-polymorph of trimorphic FOX-7 is reported. gamma-FOX-7 is stable from similar to 435 K until the compound decomposes just above 504 K. A single crystal of alpha-FOX-7 (P2(1)/n, Z=4, a=694.67(7) pm, b= 668.87(9) pm, c = 1135.1(1) pm, P = 90.14(1)degrees, T=373 K) was first transformed into a single crystal of beta-FOX-7 (P2(1)2(1)2(1), Z=4, a=698.6(1) pm, b = 668.6(2) pm, c = 1168.7(3) pm, T=423 K) and then into a single crystal of gamma-FOX-7 at 450 K. The gamma-FOX-7 crystal was then subsequently quenched to 200 K. The structure of gamma-FOX-7 (P2(1)/n, Z=8, a=1335.4(3) pm, b=689.5(1) pm, c=1205.0(2) pm, beta=111.102(8)degrees, T=200K) consists of four planar layers, each containing two crystallographically independent FOX-7 molecules found in the asymmetric unit.