Sn4+/Sn2+ Aliovalent Substitution: A Strategy for Local Structure Engineering to Enhance Birefringence and Trigger Structural Symmetry Transition

Aliovalent substitution involving cations with lone pair electrons is a powerful strategy for designing novel optical materials with enhanced properties. Herein, by substituting Sn4+ with stereochemically active Sn2+ in the α/β-LiSn2(PO4)3 parent structure, we successfully synthesized α/β-Li2SnP2O7 pyrophosphates, achieving a remarkable 5.5 time enhancement in birefringence. This oxidation-state-driven transformation converts the symmetric [SnO6] octahedra in α/β-LiSn2(PO4)3 into highly distorted [SnO4] tetrahedra in α/β-Li2SnP2O7. This conversion leads to the bond length distortion index boosting (from 9.42/9.95 × 10–3 to 47.17/49.21 × 10–3) and the bond angle varying (from 26.97/29.65 deg2 to 1054.2/1122.1 deg2), which triggers the [SnP6O24] clusters into [SnP2O9] three-membered rings, repectively. The increased distortion of Sn–O groups and the decreased symmetry of Sn–P–O blocks enhance the optical anisotropy of the compounds effectively. This aliovalent substitution strategy offers a valuable gudience for amplifying the intrinsically low birefringence of phosphates.