Chemical Reactivity of Tetrasulfur Tetranitride: Synthesis, Physical Properties, and Structural Characterization of the Amorphous Phase Cu 7 S 4 N 4

Tetrasulfur tetranitride, S 4 N 4 , reacts with elemental Cu within inert solvents to a black‐blue material of approximate composition Cu 7 S 4 N 4 which is totally amorphous to X‐rays and which cannot be made crystalline by either thermal treatment or electron radiation. Cu 7 S 4 N 4 explodes if heated above 234 °C or when subjected to mechanical shock to eventually yield copper(I) sulfide; this together with the characteristic infrared spectrum of Cu 7 S 4 N 4 indicates the presence of molecular S 4 N 4 units inside the amorphous phase. The metastable nature of Cu 7 S 4 N 4 is also mirrored by electron microscopy which furthermore allows the structural characterization of its degradation products. Based on experimental EXAFS data offering characteristic Cu—N and Cu—S distances, a theoretical crystalline approximant of Cu 7 S 4 N 4 was suggested and structurally optimized by density‐functional total‐energy calculations including periodic boundary conditions. This model incorporates a central S 4 N 4 unit bonded to three shells of Cu atoms of different functionalities; in addition, a partial rupture of the S 4 N 4 unit is likely to allow for a lowering of the total energy of the metastable phase. The latter observation supports the impossibility to make Cu 7 S 4 N 4 crystallize using 4 N 4 crystallize using whatever kind of measures.