Pressure-Induced Interlayer Bonding in Arsenic Revealed by Single-Crystal Synchrotron X-ray Diffraction in a Diamond Anvil Cell

Arsenic was studied by single-crystal synchrotron X-ray diffraction during room T compression up to 37.8 GPa across the rhombohedral (As–I, A7) to simple cubic (As–II, sc) phase transition, using a diamond anvil cell for pressure generation and He as a pressure-transmitting medium. The pressure dependence of the rhombohedral lattice parameter a, angle α, atomic position parameter u, and first (d1) and second (d2) neighbor distances provides experimental evidence for a two-step pressure-induced interlayer bond formation mechanism, underlying the transition from the layered A7 structure to the nonlayered simple cubic one. Consistent with phosphorus, our data demonstrate the existence of an intermediate pseudo simple-cubic (p-sc) structure between 26.5 and 31.8 GPa. Within this pressure interval, where α = 60° and u < 0.250, the progressive reduction of the interlayer distances (d1) and the corresponding elongation of the intralayer ones (d2) reveal a pressure-induced trans-influence effect, underpinning the transition from trigonal pyramidal to octahedral atomic coordination and relating the formation of the p-sc structure to the persistence of s-p orbital mixing and stereochemically active lone pair electron density. While solving a long-standing structural issue in As, this study provides in-depth chemical insights concerning bond theory in pnictogens and the synthesis of innovative As-based materials.