K2(BeS)O4F2: a novel fluorosulfate with unprecedented 1D [(BeO3F)–(SO3F)]∞ chains exhibiting large birefringence

Abstract

A novel fluorosulfate, K₂(BeS)O₄F₂, is successfully synthesized via introducing the non-π-conjugated polar tetrahedra [(BeS)O₃F]. Its structure features unprecedented 1D isolated [(BeO₃F)–(SO₃F)]_∞ chains connected by KO₂F₃ pyramids. Owing to the polar [(BeS)O₃F] tetrahedra and 1D isolated [(BeO₃F)–(SO₃F)]_∞ chains, K₂(BeS)O₄F₂ exhibits a large birefringence of 0.056 at 546.1 nm in experiments. Also, its UV-Vis diffuse reflectance spectra demonstrate a short absorption edge of 227 nm and thermal analysis reveals high thermodynamic stability. It is the first reported fluorosulfate that features 1D isolated zigzag chains from a polar NLO-active unit in the area of nonlinear optics. This work has not only provided a novel NLO material but also a new strategy to obtain NLO crystals with large birefringence.