Partial congener substitution induced wide band gap witnessed in diamond-like chalcogenide ZnHg₂P₂S₈ and its second-order nonlinear optical properties

Infrared nonlinear optical (IR NLO) materials play a pivotal role in laser technology, yet achieving a balance between a wide band gap and a strong second-harmonic generation (SHG) response remains challenging. Here, we report the rational design and synthesis of ZnHg₂P₂S₈, a novel diamond-like (DL) chalcogenide developed through a partial congener substitution strategy. The precise partial substitution of Zn²⁺ for Hg²⁺ in the normal [Hg1S₄] tetrahedra, while preserving the structurally distorted [Hg2S₄] units critical for NLO effects, simultaneously enhances the band gap to 2.91 eV (compared to 2.77 eV in the parent compound Hg₃P₂S₈) and maintains a strong SHG response of 1.5 × AgGaS₂. First-principles calculations elucidate the structure-property relationship, showing that the preserved [Hg2S₄] motifs dominate the SHG effect, while the introduction of Zn enlarges the band gap. This work not only presents an exceptional IR NLO candidate overcoming the band gap-NLO efficiency paradox, but also establishes a general design strategy for developing high-performance IR NLO materials through targeted structural engineering.