Lewis Acidity-Governed Ion Exchange and Record Birefringence in A'[Ag(POF)].

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