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

Abstract

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 (Δn_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.