Solution-Grown Cs2HgI4 Bulk Crystals: Achieving Strong SHG and High Damage Threshold Synergistically for IR Nonlinear Optics

Cs 2 HgI 4 is demonstrated to be a promising infrared nonlinear optical (IR NLO) crystal through a combined experimental and theoretical investigation. Phase-pure Cs 2 HgI 4 was synthesized via solution evaporation in acetone, where the formation of [HgI 4 ] 2– complexes served as a critical switch to solubilize CsI through a positive-feedback coupling mechanism, representing a fundamental advancement from aqueous heterogeneous precipitation to organic-phase homogeneous reaction. Guided by precisely determined solubility and metastable zone width in methanol, a bulk single crystal with dimensions of 5 × 5 × 2 mm 3 was grown via the slow cooling method. PXRD, high-resolution X-ray rocking curve, XPS, TGA/DSC, and long-term air-exposure tests confirmed high phase purity, crystalline perfection, chemical stability, and thermal robustness. Optical measurements revealed a direct bandgap of 2.82 eV and an ultrabroad transmission window spanning from 0.43 to 79.1 μm. Powder SHG measurements demonstrated phase-matching behavior with an effective nonlinear coefficient d eff of 13.49 pm/V (∼1.36 × AgGaS 2 ) and a high LIDT of 6.01 MW cm –2 (1.9 × AgGaS 2 ). DFT calculations further showed that the superior performance originates from the dual-orbital hybridization mechanism of Hg–I bonds within [HgI 4 ] tetrahedra, while the incorporation of Cs + widens the bandgap through enhanced ionic character. This synergistic interplay between covalent and ionic motifs establishes Cs 2 HgI 4 as a competitive candidate for IR NLO applications, and the synthetic strategy developed herein provides a new reaction engineering paradigm for solution-based growth of metal halide NLO crystals.