Precompression engineering of metal-insulator transition and magnetism in designed breathing kagome systems

Kagome materials featuring dispersive Dirac cones and topological flat bands exhibit unique electronic and magnetic properties. However, kagome compounds with tunable electrical conductivity remain scarce, which severely impedes their device applications. Here, based on density functional theory and Boltzmann transport theory, we introduce the breathing effect into kagome materials Nb₃XCl₇ (X = F, Cl, Br, I) via chemical precompression, thereby inducing a metal-insulator transition and magnetic variation. We determine that the band structures, optical absorption spectra and magnetic ground states agree well with experimental results at the effective correlation strength U_eff = 2 eV. The calculated conductivity and magnetic properties reveal that monolayer Nb₃Nb₈ and Nb₃XNb₃ undergo transitions from paramagnetic metals to Slater insulators at U_eff = 1 eV and t_out/t_in = 0.6674, respectively. Our detailed analysis establishes that the stronger breathing effect corresponds to enhanced chemical precompression, which reduces the regions of free-electron gas between intercell Nb atoms and facilitates the metal-insulator transition. Finally, we propose several viable synthesis routes for Nb₃FNb₃, Nb₃BrNb₃, and Nb₃INb₃, providing predictive guidance for experimental studies. Our study establishes a practical framework for investigating the breathing effect in correlated kagome systems and yields valuable insights into the mechanisms underlying metal-insulator transition in real breathing kagome materials.