Improved Birefringence Activated by Tetrahedra Decorated with a Single Linear Unit

Abstract Performance enhancement induced by structural modification has long been the target in materials science fields. Direct evidence to witness the effectivity of one strategy is challenging and necessary. In this work, a tetrahedra‐decoration strategy was proposed to improve the birefringent performance sharply, namely decorating the tetrahedra with a single linear [S 2 ] unit. The strategy was verified by comprehensive characterization of two thiogermanates K 2 BaGeS 4 and K 2 BaGeS 5 , which crystallize in the same space group, have similar unit cells and the same units arrangements. Theoretical characterization verified that the [GeS 5 ] group has much larger polarization anisotropy than [GeS 4 ], further demonstrated that the linear [S 2 ] led to the sharp birefringence enlargement of K 2 BaGeS 5 (0.19 vs 0.03 of K 2 BaGeS 4 ). This work provides a new guiding thought to improve the birefringence performance.