Cs₄ Zn₅ P₆ S₁₈ I₂ : the Largest Birefringence in Chalcohalide Achieved by Highly Polarizable Nonlinear Optical...

Chalcohalides not only keep the balance between the nonlinear optical (NLO) coefficient and wide band gap, but also provide a promising solution to achieve sufficient birefringence for phase-matching ability in NLO crystals. In this study, a novel chalcohalide, Cs₄ Zn₅ P₆ S₁₈ I₂ (1) is successfully synthesized, by incorporating the highly electropositive Cs and the large electronegative I element into the zinc thiophosphate. Its 3D open framework features an edge-shared by distorted [ZnS₄ ], ethanol-like [P₂ S₆ ], and unusual [ZnS₂ I₂ ] polyhedrons, which is inconsistent with the soft-hard-acids-bases theory. Remarkably, compound 1 simultaneously exhibits the large second-harmonic generation (SHG, 1.1×AgGaS₂ , @1.3 µm) and a wide band gap (3.75 eV) toward a high laser-induced damage threshold (16.7×AgGaS₂ , @1.06 µm), satisfying the rigorous requirements for a prominent infrared NLO material with concurrent SHG intensity (≥0.5×AGS) and band gap (≥3.5 eV). Moreover, to the best of the knowledge, the experimental result shows that phase 1 has the largest birefringence (0.108, @546 nm) in chalcohalide and meets phase-matching behavior demand originating from the polarizable anisotropy of NLO-functional motifs. This finding may provide great opportunities for designing birefringent chalcohalides.