Nonlinear Optical Phosphide CuInSi₂P₄: The Inaugural Member of Diamond-Like Family I-III-IV₂-V₄ Inspired by ZnGeP₂

Noncentrosymmetric phosphides have garnered significant attention as promising systems of infrared (IR) nonlinear optical (NLO) materials. Herein, a new quaternary diamond-like phosphide family I-III-IV₂-V₄ and its inaugural member, namely, CuInSi₂P₄ (CISP), were successfully fabricated by isovalent and aliovalent substitution based on ZnGeP₂. First-principles calculations revealed that CISP has a large NLO coefficient (d₁₄ = 110.8 pm/V), which can be attributed to the well-aligned tetrahedral [CuP₄], [InP₄], and [SiP₄] units. Remarkably, the extremely small thermal expansion anisotropy (0.09) of CISP enables it to exhibit a considerable laser-induced damage threshold (LIDT, 5.0 × AgGaS₂@1.06 μm) despite the relatively narrow band gap (0.81 eV). This work improves the chemical diversity of inorganic phosphide and promotes the development of phosphide systems, which may provide valuable perspectives for future exploration of IR NLO materials.