High-Pressure Synthesis of the First Thermodynamically Stable Silver Nitride, AgN ₅

Being a noble metal, silver is known for its chemical inertness. Molecular nitrogen, due to its extremely strong covalent triple bond, is also typically considered unreactive. It is thus unsurprising that no credible report on the formation of a thermodynamically stable silver and nitrogen compound exists. In this study, we report the synthesis of silver pentazolate (AgN₅), achieved through the direct reaction of elemental silver with molecular nitrogen at a pressure of 118(3) GPa and a temperature of 2000(200) K. The crystal structure of AgN₅ was determined from synchrotron single-crystal X-ray diffraction (SCXRD) data, revealing it to be comprised of <i>cyclo</i>-N₅ ^- anions. Remarkably, this solid's structure does not correspond to any of the silver nitrides previously predicted. Moreover, density functional theory (DFT)-based enthalpy convex hull calculations demonstrate that this AgN₅ compound is the only thermodynamically stable Ag-N solid between 10 and 120 GPa while also providing information on its phonon and electron band structures, including its electronic band gap. Both DFT calculations and SCXRD experimental data yield insights into the stability pressure range of AgN₅ upon decompression. This study provides yet another example of the capability of high pressure and high temperature to facilitate unprecedented chemical reactions between elements often assumed to be inert, in turn enabling the formation of novel nitrogen-rich compounds.