Energetic materials: alpha-NTO crystallizes as a four-component triclinic twin

The prevalent polymorph of the energetic material 5-nitro-2,4-dihydro-1,2,4,-triazol-3-one, alpha-NTO, crystallizes as a four-component twin with triclinic symmetry (space group P\bar 1). All crystals under investigation were fourlings, i.e. they contained each of the four possible twin components. Complete data sets were collected for two crystals, one with a predominant amount of one individual component (55%) and one with approximately equal volumes of each component. In both cases the fourling components are related by the twofold axes inherent in the holohedral symmetry of a pseudo-orthorhombic superlattice with a(o) = a(t), b(o) = b(t) and c(o) = a(t) + b(t) + 2c(t). The triclinic unit cell contains four crystallographically independent planar molecules in the asymmetric unit, each of which forms a hydrogen-bonded flat chain parallel to a(t). Pairs of chains are combined into planar ribbons by additional hydrogen bonds. Thus, two independent ribbons extend parallel to a(t), creating a dihedral angle of approximately 70 degrees. The origin of the twinning is derived from consideration of the crystal packing and the hydrogen-bonding scheme.