Jahn–Teller Effect Dominates the Left‐Hand Rule: Magneto‐Optics in Metal Halide Lattice

Symmetry operation during molecular packing in the crystalline phase determines diverse unique features. Inducing asymmetry in hybrid inorganic–organic metal‐halide typically relies on importing chiral A‐site cations or exploiting hydrogen bonding to distort the lattice. However, achieving precise spatial control over asymmetrical molecules using stereo‐chemical templating strategy remains challenging. Here, we demonstrate that two highly symmetric single crystals, DMA 4 InCl 7 :Sb 3+ and DETABiCl 6 :Sb 3+ , containing isolated SbCl 6 octahedra with idealized octahedral ( O h ) geometry, exhibit a unique case of a static Jahn–Teller effect (JTE) with the interplay of strong spin‐orbit coupling interaction. This inherently handed property drives the spontaneous lowering of symmetry from O h to D 4h , accompanied by structural deformation in the excited states. Under an external magnetic field (B from 0 to 45 T) applied parallel or anti‐parallel to the (100) direction, the A 2u level in the upper position is preferentially populated during relaxation, resulting in a dominant left‐circularly polarized (σ–) emission. Rather than forming an intrinsically asymmetric lattice at equilibrium, chiral preference emerges from a Jahn–Teller‐amplified transient‐state process, yielding a highly effective g m value of up to 8.9×10 −2 . This mechanism will be broadly applicable to d ‐ and p ‐block metal‐activated hybrid metal halides across various dimensionalities.