[Sn3OF]PO4vs. [Sn3F3]PO4: enhancing birefringence by breaking the R3 symmetry and realigning lone pairs

Tin(II)-containing phosphates are promising candidates for designing and studying materials with enhanced birefringence. Herein, we present a novel compound [Sn3OF]PO4 that exhibits significant birefringence (Delta n(obv.) = 0.117 at 546 nm), a five-fold improvement compared to that of the related compound [Sn3F3]PO4 discovered nearly 40 years ago. The remarkable enhancement in birefringence is primarily attributed to the anisotropic [SnO3F](5-) structure building unit. By simple regulation of the F/O ratio, the undesirable R3 symmetry in [Sn3F3]PO4 is disrupted. This regulation also realigns the lone pairs, resulting in substantial optical anisotropy.