Lewis Acidity-Governed Ion Exchange and Record Birefringence in A'[Ag

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 (∆nobv. = 0.101 versus ∆ncal. = 0.099 at 546 nm) among known fluorophosphates, doubling NaNH4PO3F·H2O's prior maximum (∆nobv. = 0.053 at 589 nm) and surpassing SnPO3F's lone-pair-driven anisotropy (∆ncal. = 0.082 at 546 nm) by 20%.