Hypercoordinated PN₄ with Nonclassical N-N Bonds: An sp²⁺ Hybridized Framework Stabilized by d-Orbital Activation

Nitrogen (N), despite being adjacent to carbon in the periodic table, struggles to form noninteger hybridizations due to its unique electronic structure and inherent thermodynamic stability. Investigating the specific bonding modes of nitrides not only enhances our understanding of classical bonding theory but also provides valuable guidance for the development of novel materials. Here, using particle swarm optimization methods, we predict a hypercoordinated phase, P3̅1c-PN₄, comprising PN₆ octahedra interconnected by N₄ units, in which the strong ligand field of nitrogen atoms activates P 3d orbitals, stabilizing this six-coordinate framework. Remarkably, the N₄ units exhibit nonclassical bonding with intermediate N-N bond lengths (1.35 Å) and sp²⁺ hybridization, balancing covalent rigidity with structural flexibility. Furthermore, P3̅1c-PN₄ displays superhardness (45.26 GPa), high energy density (5.42 kJ/g), and pressure-induced blue-shifted absorption behavior. Comparative studies on MN₄ compounds (M = P, As, Sb) reveal a central atomic-size-dependent attenuation of the nitrogen ligand field, underscoring the synergy of nitrogen ligand fields and d-orbital activation in hypercoordinated systems. This work establishes a blueprint for the design of tunable, multifunctional, and nonclassical nitrides through orbital engineering under high pressure.