As an important family of potential mid-IR nonlinear optical (NLO) crystals, halides exhibit superiority in large optical band gaps and wide IR transparency. However, their second-harmonic generation (SHG) effects are generally small, limiting their frequency conversion efficiency in NLO processes. Diamond-like structures, due to the intrinsic polarization-parallel alignment of tetrahedral frameworks, enable sufficiently large SHG effects while maintaining balanced NLO performance in the mid-IR spectral region. Accordingly, in this study, we focused on the NLO performance mining and assembly of tetrahedral polar motifs in halides with unique diamond-like structures. We screened almost all diamond-like structures in inorganic halide systems and investigated in detail how their balanced NLO performances evolve with ideal and defective diamond-like structures. We predicted that the three existing diamond-like iodides, CuGaI4, Ag2CdI4, and Ag2ZnI4, can satisfy the balanced requirement of strong SHG effects (>3.9 pm/V) and large optical band gaps (>3 eV) by analyzing the microstructural NLO origin. Preliminary experiments also verified our predictions with Ag2CdI4 (1.4 × AgGaS2 and 3.3 eV) and Ag2HgI4 (2.2 × AgGaS2 and 2.4 eV). More importantly, a map with the diamond-like halide family tree was illuminated, unlocking dozens of diamond-like structures with balanced NLO properties, from ternary AgCdI3, Ag2Cd3I8, Ag4CdI6, and Aglnl4 to quaternary AgCuCdI4, Ag8HgCd3I_(16), Ag3HgGaI8, AgCd2InI8, and Ag2CdI2Br2. As long as following this map, one may design and discover potentially promising mid-IR NLO halide crystals with diamond-like structures.
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