Sn 4+ /Sn 2+ 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 Sn 4+ with stereochemically active Sn 2+ in the α/β-LiSn 2 (PO 4 ) 3 parent structure, we successfully synthesized α/β-Li 2 SnP 2 O 7 pyrophosphates, achieving a remarkable 5.5 time enhancement in birefringence. This oxidation-state-driven transformation converts the symmetric [SnO 6 ] octahedra in α/β-LiSn 2 (PO 4 ) 3 into highly distorted [SnO 4 ] tetrahedra in α/β-Li 2 SnP 2 O 7 . 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 deg 2 to 1054.2/1122.1 deg 2 ), which triggers the [SnP 6 O 24 ] clusters into [SnP 2 O 9 ] 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.