Enhanced Birefringence in Organic–Inorganic Hybrid Antimony Halides via Partial Halogen Substitution

In this study, two novel organic–inorganic hybrid antimony halides, (C8H7N2O)SbF4 (I) and (C8H7N2O)2Sb2FCl7 (II), were synthesized. Compared with the single-halide compound I (0.21 at 546 nm), compound II exhibits an enhanced birefringence of 0.30 at 546 nm, corresponding to a 43% increase achieved through a partial halogen substitution strategy. This strategic introduction of Cl not only effectively increases optical anisotropy but also allows II to maintain a favorable band gap of 3.85 eV, while retaining good ultraviolet transparency. Structural and theoretical analyses reveal that the birefringence enhancement in II originates from the Cl-induced transition to a more coparallel alignment of organic [C8H7N2O]+ cations and the superior polarizability of mixed-halide units. These results demonstrate the effectiveness of partial halogen substitution for regulating birefringence in hybrid antimony halides.