Rigid‐Flexible Coupling Units Enable Robust Large Birefringence

Birefringent crystals are central to polarization control in laser technologies (e.g., wave plates and polarization beam splitters), yet combining large birefringence with robust optical stability remains rare. Herein, we demonstrate the rigid-flexible coupling strategy exemplified by a novel crystal, Sr₄(VO₄)₂S₃, constructed from rigid (VO₄)^3- tetrahedra and flexible linear (S₃)^2- units. Sr₄(VO₄)₂S₃ exhibits an exceptional birefringence (Δn = 0.52@550 nm), exceeding commercial YVO₄ by 249%. Remarkably, the birefringence remains robust over a broad temperature range (123-373 K) and after prolonged exposure to air and water. Structural analyses reveal that Sr-O/S polyhedra are interconnected with rigid (VO₄)^3- units to build the overall 3D framework that underpins robustness, which also facilitates the optimal alignment of flexible (S₃)^2- units. Theoretical calculations further indicate that (S₃)^2- units possess much higher bond flexibility than (VO₄)^3- tetrahedra (flexible index of S-S 4.33 vs. V-O 0.28), thereby governing the prominent optical birefringence. This work establishes rigid-flexible coupling as a general design principle to unite large birefringence with robustness for compact, reliable polarization optics.