Sn9O4Br9X (X = Cl, Br): Two new Sn(II) oxyhalides exhibiting large birefringence derived from highly distortive polyhedra of lone pair cations

The combination of Sn2+ cations with stereochemically active lone pair and the smaller electronegativity of Cl- and Br- anions generates two new birefringent crystal materials, Sn9O4Br10 and Sn9O4ClBr9. The first-principles calculations reveal that the two crystals possess a large birefringence, 0.273 and 0.258@1064 nm, respectively, deriving from the extremely intensive stereochemically active lone pair of highly distorted Sn-O/X (X = Cl, Br) polyhedra. And it is worth noting that the birefringence of Sn9O4Br10 is the largest among the tin(II)-containing oxyhalides reported so far. This work provides an approach for the future exploration of new birefringent crystal materials.