  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.
