Mixed-anion square-pyramid [SbS3I2] units causing strong second-harmonic generation intensity and large birefringence

Enhancement of the nonlinear optical (NLO) output power of lasers requires urgent development of an NLO crystal with a significant second-harmonic generation (SHG) response and sufficient birefringence for phase-matching capability; however, simultaneously optimizing these two key parameters remains a great challenge. In contrast to traditional single-anion units, the stereochemically-active lone pair Sb3+ ion is coordinated by S2− and I− ions to yield the mixed-anionic SbSI chalcohalide that can enhance hyperpolarizability and anisotropic polarizability concurrently. As anticipated, SbSI exhibited the largest SHG response (5.7 × AgGaS2@1.91 μm) among phase-matching Sb-based sulfides, the favorable laser-induced damage threshold (LIDT, 2.3 × AgGaS2@2.09 μm), and the giant calculated birefringence (0.62@1.91 μm). Structural analysis and computational simulations indicate that the highly polarizable mixed anion determine the enormous SHG response and birefringence.