Sn₂B₅O₉Br as an Outstanding Bifunctional Material with Strong Second‐Harmonic Generation Effect and Large Birefringence

Abstract The first tin (II) borate bromide, Sn₂B₅O₉Br (SBOB), as an outstanding bifunctional material with excellent linear and nonlinear optical (NLO) performance is synthesized successfully. As far as it is known, it exhibits the largest birefringence among the reported borates (0.439 at 546 nm), even over famous birefringent materials YVO₄ and TiO₂. Meanwhile, it also owns a strong second harmonic generation (SHG) response of 2.4 times that of KH₂PO₄. Theoretical calculations display that its incredible optical properties mainly originate from the extremely intensive stereochemical activity lone pair (SCALP) of [SnO₇Br₂] polyhedra, which results in the enhancement of birefringence and SHG response, increasing to 2.6‐ and 4.8‐fold, respectively, from Sn₂B₅O₉Cl (SBOC) to SBOB. These perceptions will guide and accelerate the exploration of excellent birefringence and NLO materials in tin (II)‐based compound.