First-principles study of three isomers of the large birefringent crystal Sb₄O₄F₄: electronic structure and optical properties

Abstract Metal oxyhalides are becoming an important branch of birefringent materials due to their rich structural types and stable physicochemical properties. In this work, the electronic structure and optical properties of three isomers of antimony oxyhalide Sb 4 O 4 F 4 ( I- Pna 2 1 ), Sb 4 O 4 F 4 ( II- Pnma ), and Sb 4 O 4 F 4 ( III- Pnma ) were investigated using first principles method. The results show that the band gap of I and III reaches the ultraviolet region (4.10 eV ( I ), 3.87 eV ( III )). In particular, I-III all exhibit large birefringence of 0.138–0.266 at 1064 nm. Born effective charge, and project density of states results show that the large birefringence of I-III is mainly from the stereochemically active lone pair electrons around the antimony atom. Finally, structure–property relationship studies of I-III indicate that different Sb-F bonds and structural arrangements are the key factors leading to the significantly different properties.