Bond-length-driven magnetic transition in quasi-one-dimensional CrSb X ₃ ( X =S, Se)

Using ab initio calculations, we investigate the magnetic ground states of quasi-one-dimensional insulating CrSb X 3 ( X =S, Se) with infinite double-rutile chains. Within conventional band theory, without explicit Coulomb correlations ( U ), we obtain band gaps in close agreement with experiment. Remarkably, we find that the magnetic order is highly sensitive to the Cr-Cr bond length d Cr-Cr : increasing the bond length induces a transition from antiferromagnetic to ferromagnetic order at a critical distance d c Cr-Cr ≈3.53(±0.05) Å. Accordingly, CrSbS 3 lies near the transition boundary, whereas CrSbSe 3 is robustly ferromagnetic, in good agreement with experiment. Analysis of the exchange interactions reveals that the first-order phase transition is dominated by a sign reversal of the intrachain nearest-neighbor superexchange J 1 mediated by chalcogen ions, while the intrachain direct exchange J 2 remains ferromagnetic and changes only gradually. This behavior reflects an emergent Bethe-Slater-like behavior driven by competing exchange pathways in a quasi-1D transition-metal system, where the competition between J 1 and J 2 dictates the magnetic ground state. Besides, the electronic structures of the ground states of each compound are investigated.