Lattice Geometry Modulation of Heisenberg Superexchange in B/C/N-Substituted MXenes

The 2D architecture of MXenes offers a scaffold for high-density magnetic moments. Yet the microscopic magneto-structural coupling remains elusive. Here, combining first-principles calculations with crystal orbital Hamilton population (COHP) analysis, we conduct a qualitative trend study to unravel the lattice geometry modulation in B/C/N-substituted MXenes. We reveal that the increased population of antibonding states by nitrogen's excess electrons weakens the covalent network. This "lattice softening", a reduction in structural rigidity, facilitates spontaneous bond contractions driven by nitrogen's smaller intrinsic radius. To maximize the magnetic exchange energy gain, the lattice undergoes geometric relaxation. Via Harrison's relation, we qualitatively rationalize that this lattice contraction modulates the exchange interaction. A simplified d-7 scaling is employed strictly as a qualitative descriptor to illustrate the strong sensitivity of the magnetic coupling to distance variations. Our findings demonstrate that, under Goodenough-Kanamori constraints adapted for these systems, lattice geometry plays an important role in modulating magnetic exchange. This establishes MXenes as a platform for rational magneto-structural engineering in next-generation spintronics.