Stereochemically Active Lone Pair Leads to Birefringence in the Vacancy Ordered Cs₃Sb₂Cl₉ Perovskite Single Crystals

Stereochemically active lone pair (SCALP) cations are attractive units for realizing optical anisotropy. Antimony(III) chloride perovskites with the SCALP have remained largely unknown to date. We synthesized a new vacancy ordered Cs 3 Sb 2 Cl 9 perovskite single crystals with SbCl 6 octahedral linkage containing the SCALP. Remarkably, all-inorganic halide perovskite Cs 3 Sb 2 Cl 9 single crystals exhibit an exceptional birefringence of 0.12 ± 0.01 at 550 nm. The SCALP brings a large local structural distortion of the SbCl 6 octahedra promoting birefringence optical responses in Cs 3 Sb 2 Cl 9 single crystals. Theoretical calculations reveal that the considerable hybridization of Sb 5s and 5p with Cl 3p states largely contribute to the SCALP. Furthermore, the change in the Sb–Cl–Sb bond angle creates distortion in the SbCl 6 octahedral arrangement in the apical and equatorial directions within the crystal structure incorporating the required anisotropy for the birefringence. This work explores pristine inorganic halide perovskite single crystals as a potential birefringent material with prospects in integrated optical devices.