Stereochemically Active Lone-pair Leads to Strong 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 SCALP have remained largely unknown till date. We synthesized vacancy ordered Cs3Sb2Cl9 perovskite single crystals with SbCl6 octahedral linkage containing SCALP. Remarkably, Cs3Sb2Cl9 single crystals exhibit an exceptional birefringence of 0.12 +(-) 0.01 at 550 nm, which is the largest among pristine all-inorganic halide perovskites. 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 with Sb 5p and 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.