Lewis Acidity‐Governed Ion Exchange and Record Birefringence in A′[Ag ₃ (PO ₃ F) ₂ ]

Four monofluorophosphates, A'[Ag₃(PO₃F)₂] (A' = Ag⁺, K⁺, Rb⁺, NH₄ ⁺), with layered sandwich structures were synthesized. Systematic characterization reveals that the Lewis acidity gradient of A' cations (Ag⁺ > Rb⁺ ≈ K⁺ > NH₄ ⁺) governs their distinct ion-exchange kinetics, where Ag⁺ substitution requires 7 days for Rb⁺/K⁺ but 14 days for NH₄ ⁺. The density functional theory (DFT) calculations demonstrate that optical anisotropy primarily stems from the alignment of P─F bonds in [PO₃F]^2- tetrahedra, as well as the alignment of dipole moment of [Ag(1)O₄]^7- tetrahedra. Compound with smaller angular deviations from the dielectric axis (α and β) yielding enhanced anisotropy. Remarkably, NH₄⁺-mediated hydrogen bonding introduces additional anisotropy, enabling NH₄[Ag₃(PO₃F)₂] to achieve record birefringence (∆n_obv. = 0.101 versus ∆n_cal. = 0.099 at 546 nm) among known fluorophosphates, doubling NaNH₄PO₃F·H₂O's prior maximum (∆n_obv. = 0.053 at 589 nm) and surpassing SnPO₃F's lone-pair-driven anisotropy (∆n_cal. = 0.082 at 546 nm) by 20%.