Enhancing birefringence of non-π-conjugated sulfate systems through rare-earth metal-centered polyhedra

By the introduction of trivalent rare-earth metal-centered polyhedra [LnO6F4] (Ln = La, Gd) into alkali metal sulfates, two novel sulfate fluorides, NaLn2(SO4)2F3 (Ln = La, Gd), were successfully synthesized via a high-temperature solution method in sealed quartz tubes. The two compounds are isomorphic and feature a structure composed of [LnO6F4] (Ln = La, Gd) polyhedra, [NaO4F3] polyhedra, and [SO4] groups. The UV-vis-NIR diffuse reflection spectra, infrared spectra, and thermal stability are also presented. Meanwhile, theoretical calculations reveal that NaLa2(SO4)2F3 has a moderate birefringence (0.041 at 1064 nm) which is significantly improved compared with Na2SO4. And the enhancement of birefringence can be ascribed to the introduction of rare-earth metal-centered polyhedra by a real space atom-cutting method. This work shows that the introduction of trivalent rare-earth metal-centered polyhedra is one of the effective strategies for bolstering the birefringence performance of non-π-conjugated sulfate systems.