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

Abstract

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.