High Pressure Investigation of the S–N₂ System up to the Megabar Range: Synthesis and Characterization of the SN₂ Solid

Sulfur and nitrogen represent one of the most studied inorganic binary systems at ambient pressure on account of their large wealth of metastable exotic ring-like compounds. Under high pressure conditions, however, their behavior is unknown. Here, sulfur and nitrogen were compressed in a diamond anvil cell up to about 120 GPa and laser-heated at regular pressure intervals in an attempt to stabilize novel sulfur–nitrogen compounds. Above 64 GPa, an orthorhombic (space group Pnnm) SN₂ compound was synthesized and characterized by single-crystal and powder X-ray diffraction as well as Raman spectroscopy. It is shown to adopt a CaCl₂-type structurehence it is isostructural, isomassic, and isoelectronic to CaCl₂-type SiO₂comprised of SN₆ octahedra. Complementary theoretical calculations were performed to provide further insight into the physicochemical properties of SN₂, notably its equation of state, the bonding type between its constitutive elements, and its electronic density of states. This new solid is shown to be metastable down to about 20 GPa, after which it spontaneously decomposes into S and N₂. This investigation shows that despite the many metastable S–N compounds existing at ambient conditions, none of them are formed by pressure.