A Metallic Tetrahexagonal NiN₂ Monolayer Realized via Full Stone-Wales Reconstruction: A First-Principles Prediction

We propose a novel two-dimensional (2D) tetrahexagonal NiN₂ (tetrahex-NiN₂) monolayer, derived through a full Stone-Wales transformation of penta-NiN₂, which converts an all-pentagonal lattice into a network of four- and six-membered rings. First-principles calculations confirm that this new phase is dynamically and thermally stable. Tetrahex-NiN₂ exhibits pronounced mechanical anisotropy and shortened N-N bonds, indicating significant structural reorganization. Hybrid functional calculations reveal metallic character, with strong hybridization between Ni-d and N-p orbitals near the Fermi level. Furthermore, the material demonstrates broadband and anisotropic optical absorption across the visible spectrum, suggesting promising potential for optoelectronic and photothermal applications. These findings position tetrahex-NiN₂ as a stable and tunable 2D nitride and highlight the effectiveness of topological reconstruction as a strategy for engineering novel 2D quantum phases.