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 Cs₃Sb₂Cl₉ perovskite single crystals with SbCl₆ octahedral linkage containing the SCALP. Remarkably, all-inorganic halide perovskite Cs₃Sb₂Cl₉ single crystals exhibit an exceptional birefringence of 0.12 ± 0.01 at 550 nm. The SCALP brings a large local structural distortion of the SbCl₆ octahedra promoting birefringence optical responses in Cs₃Sb₂Cl₉ 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 SbCl₆ 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.