Penta-AgN₈ Monolayer: Flat Bands Driving Magnetic and Topological Transitions from Real Chern Insulator to Double-Weyl Metal

Inspired by the rich physics of honeycomb-kagome (HK) lattices and flat-band magnetism, we predict a stable two-dimensional (2D) penta-AgN₈ monolayer through comprehensive tight-binding (TB) model analysis and first-principles calculations. This novel material integrates pentagonal AgN₄ building blocks into an effective HK superstructure, exhibiting a unique planar hexagonal geometry with hypercoordinated Ag atoms. We demonstrate that penta-AgN₈ is intrinsically a bipolar magnetic semiconductor (BMS) and, more notably, a magnetic real Chern insulator (MRCI) protected by <i>C</i>_2<i>z</i><i>T</i> symmetry, featuring spin-polarized flat bands near the Fermi level, intrinsic in-plane ferromagnetic ordering, and observable corner states. A key finding is the exceptional strain-tunability of these flat bands, which allows for precise engineering of its electronic and topological properties. Under biaxial strain, penta-AgN₈ undergoes transitions from a BMS to a half-semiconductor (HSC) and subsequently to a half-metal (HM). Concurrently, its topological properties evolve transition from an MRCI to a double-Weyl metal phase, featuring quadratically dispersing Weyl points characterized by a Chern number of |<i>C</i>| = 1. The predicted thermodynamic, dynamic, mechanical, and thermal stability, combined with advances in synthesizing nitrogen chains and metal nitrides, suggests high feasibility for experimental realization. This work not only introduces a new member to the penta-structured 2D material family with unprecedented planar Ag hypercoordination but also offers a versatile platform for developing multifunctional spintronic devices leveraging strain-modulated topological states.