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

Abstract Four monofluorophosphates, A′[Ag 3 (PO 3 F) 2 ] (A′ = Ag + , K + , Rb + , NH 4 + ), with layered sandwich structures were synthesized. Systematic characterization reveals that the Lewis acidity gradient of A′ cations (Ag + > Rb + ≈ K + > NH 4 + ) governs their distinct ion‐exchange kinetics, where Ag + substitution requires 7 days for Rb + /K + but 14 days for NH 4 + . The density functional theory (DFT) calculations demonstrate that optical anisotropy primarily stems from the alignment of P─F bonds in [PO 3 F] 2− tetrahedra, as well as the alignment of dipole moment of [Ag(1)O 4 ] 7− tetrahedra. Compound with smaller angular deviations from the dielectric axis ( α and β ) yielding enhanced anisotropy. Remarkably, NH 4 ⁺‐mediated hydrogen bonding introduces additional anisotropy, enabling NH 4 [Ag 3 (PO 3 F) 2 ] to achieve record birefringence (∆ n obv. = 0.101 versus ∆n cal. = 0.099 at 546 nm) among known fluorophosphates, doubling NaNH 4 PO 3 F·H 2 O's prior maximum ( ∆n obv. = 0.053 at 589 nm) and surpassing SnPO 3 F's lone‐pair‐driven anisotropy ( ∆n cal. = 0.082 at 546 nm) by 20%.