Λ‐Shaped Dimers Enables Polar Antiperovskites Through A‐Site Displacement Mechanism

Polar non‐centrosymmetric materials are essential for high‐tech applications, yet their rational design remains a formidable challenge. “Λ‐shaped” dimers with bent geometries intrinsically favorable for symmetry breaking; however, their non‐thermodynamic preference and steric effect often result in centrosymmetric structures. Herein, inspired by the A‐site displacement mechanism in classical perovskites, we developed an anionic‐group engineering strategy for antiperovskites by the ordered assembly of Λ‐shaped dimers within tailored cage‐like frameworks, resulting in a polar antiperovskite, [CsCd 2 ]Cl[V 2 O 7 ]. In the structure, the cooperative alignment of intrinsically polar Λ‐shaped [V 2 O 7 ] 4− dimers generates pronounced macroscopic polarization, giving rise to the strongest second‐harmonic generation (SHG) response (17 × KH 2 PO 4 @1064 nm) among all reported vanadates without additional SHG‐active chromophores. Furthermore, the geometrically flexible V–O–V linkage impart a unique temperature‐dependent SHG switching behavior over a broad temperature window (300–773 K) with a giant switching contrast ∼300, surpassing all previously reported all‐inorganic materials. Temperature‐dependent structural analysis and first‐principles calculations confirm that the SHG switching originates from continuous dipole modulation driven by reversible geometric evolution of the Λ‐shaped dimers. This work establishes anionic‐group engineering as an effective strategy for constructing polar antiperovskites, uncovers an A‐site displacement mechanism driven by asymmetric anionic groups, and extends its functional properties.