Optimization of Functional Building Blocks Generates a Substantial Improvement in Birefringence from Sn₂OSO₄ to Sn₃O₂(OH)(HSO₄)

In this work, two tin(II)-based sulfates, Sn₂OSO₄ and Sn₃O₂(OH)(HSO₄), were synthesized via the mild hydrothermal method. Both compounds employ the Sn²⁺ cation with stereochemically active lone pair (SCALP) electrons and non-π-conjugated tetrahedral anionic groups SO₄ as the functional structural blocks. Interestingly, the experimental birefringence of Sn₃O₂(OH)(HSO₄) is 0.169@546 nm, approximately 42 times larger than that of Sn₂OSO₄, which is 0.004@546 nm. Detailed structural analysis and theoretical calculations suggest that this significant birefringence difference arises from the optimization of functional building blocks in coordination environments and spatial arrangements. Furthermore, both compounds exhibit ultraviolet absorption edges at 308 and 307 nm, respectively. This indicates that Sn₃O₂(OH)(HSO₄) has the potential to be a candidate for an ultraviolet (UV) birefringent crystal. This study offers inspiration for further exploration of tin(II)-based compounds with excellent comprehensive properties.