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.
Jiang et al. (Thu,) studied this question.