(CN4H7)2SO4·H2O: high-performance metal-free ultraviolet birefringent crystal with KBBF-like configuration

Abstract

The advancement of high-quality ultraviolet (UV) birefringent crystalline materials is pivotal in advancing optoelectronic functional crystal technology. The outstanding birefringent fundamental group is indispensable for synthesizing target crystals that meet high-performance optical requirements. This study selected the [CN₄H₇]⁺ group with a wide HOMO–LUMO gap and substantial polarizability anisotropy. Furthermore, by modifying the KBe₂BO₃F₂ (KBBF) template structure at the molecular level, the first metal-free sulfate (CN₄H₇)₂SO₄·H₂O was successfully synthesized. This crystal effectively balanced the short UV cut-off edge (212 nm) and large birefringence (0.132@546.1 nm). Theoretical calculations indicated that [CN₄H₇]⁺ group and its favorable arrangement were primarily responsible for the large birefringence. Our study reveals that coupling the [CN₄H₇]⁺ group to tetrahedral frameworks serves as an effective approach to engineering UV birefringent crystals with enhanced optical anisotropy.