High-Pressure Synthesis of Pnictogen Nitrides

ConspectusNitrides represent a class of chemical compounds of high scientific and technological relevance. Nevertheless, due to the challenging synthetic conditions, essentially related to the stability of the N₂ molecule, nitrides have remained largely unexplored compared with the corresponding oxides.The laser-heated diamond anvil cell (LH-DAC), providing access to the GPa pressure range and temperatures as high as several thousands of K, has dramatically changed the rules of the game, unveiling thermodynamic conditions in which N₂ becomes unstable and polymerizes into extended crystalline phases. A variety of N-compounds have been indeed synthesized by laser-heating the corresponding elements in a N₂ environment under high-pressure (HP) conditions.Nevertheless, only recently group 15 elements heavier than N have been targeted by this method, and excluding α-P₃N₅ and γ-P₃N₅, crystalline pnictogen nitrides have remained essentially unknown.Since the discovery of phosphorene, while the quest for 2D materials has raised the interest for group 15 xenes and their N-doping as a key development, the inherent tendency of pnictogens to adopt crystalline layered structures persisting at high pressure has increased focus on binary N-compounds with heavier pnictogens, with implications for fundamental chemistry and potential applications.In this scenario, the discovery of the pseudo simple-cubic (p-sc) structure in the phase diagram of P, and, a few years later, the high-pressure-high-temperature (HP-HT) synthesis of PH₃ from the elements and the discovery of the crystalline van der Waals (vdW) compound (PH₃)₂H₂ have further reconnected the HP behavior of P to that of lighter N.These experimental studies, highlighting consistency in the structural and reactive properties of group 15 elements at high pressure, together with additional theoretical and computational insights, have opened new perspectives and motivated further investigations about the existence of crystalline pnictogen nitrides.Indeed, not only α- and γ-P₃N₅, but also three other crystalline polymorphs of phosphorus nitride (δ-P₃N₅, PN₂, and α'-P₃N₅), have been synthesized by the direct HP-HT chemical reaction of P and N₂ in a LH-DAC.Moving down in group 15, the same method has led to the discovery of the first crystalline nitrides of arsenic (AsN) and antimony (Sb₃N₅), whose existence has always represented an open question in inorganic chemistry, and to the structural characterization of two crystalline polymorphs of bismuth nitride (BiN).This Account provides an overview of the recent progress in the high-pressure and high-temperature synthesis of crystalline pnictogen nitrides, demonstrating the effective activation of a direct chemistry between N and heavier pnictogens. The presented results mark fundamental advancements in the chemistry of group 15 elements and pioneer the discovery of new advanced pnictogen-based materials of energetic and technological relevance, potentially recoverable under ambient conditions as stable or metastable systems.