Junjie Li

Two noncentrosymmetric selenides KGdSiSe ₄ and KGdGeSe ₄ were designed by combining rare-earth polyhedra and [MSe ₄ ] (M = Si, Ge) tetrahedra. KGdSiSe ₄ balances a 3.13 eV band gap and 0.8 × AGS SHG response, making it a promising IR NLO candidate.

Nonlinear optical (NLO) crystals are essential for laser frequency conversion, yet rational crystal design remains challenging due to intrinsic trade‐offs among key performance parameters. This review establishes a multi‐scale design paradigm to unify fragmented knowledge of NLO materials by correlating microscopic, mesoscopic, and macroscopic structural features, delivering a chemistry‐based framework to recognize cross‐scale constraints and direct NLO crystal exploration. At the microscopic level, the intrinsic optical responses of planar π‐conjugated units, non‐π‐conjugated tetrahedral motifs, and distorted polyhedra are systematically analyzed at the atomic scale, with emphasis on the chemical substitution, functional‐group modification, and multi‐unit assembly strategies that directly manipulate the microscopic electronic structures of these functional building blocks. At the mesoscopic scale, cation‐regulated coordination, dimensionality engineering, hydrogen bonding, domain manipulation, and interfacial coupling are discussed for ordered functional unit arrangement and constructive polarization accumulation. At the macroscopic scale, we discuss converting small‐scale designs into usable bulk crystals, focusing on mitigating layered growth via strengthened interlayer interactions to improve crystal dimension, quality, and device compatibility. Representative NLO crystal examples validate this coherent logic. This framework clarifies structure‐property correlations and supplies guidelines for new NLO materials, advancing this field from empirical screening toward rational, application‐driven design.

Nonlinear optical (NLO) crystals are essential for laser frequency conversion, yet rational crystal design remains challenging due to intrinsic trade‐offs among key performance parameters. This review establishes a multi‐scale design paradigm to unify fragmented knowledge of NLO materials by correlating microscopic, mesoscopic, and macroscopic structural features, delivering a chemistry‐based framework to recognize cross‐scale constraints and direct NLO crystal exploration. At the microscopic level, the intrinsic optical responses of planar π‐conjugated units, non‐π‐conjugated tetrahedral motifs, and distorted polyhedra are systematically analyzed at the atomic scale, with emphasis on the chemical substitution, functional‐group modification, and multi‐unit assembly strategies that directly manipulate the microscopic electronic structures of these functional building blocks. At the mesoscopic scale, cation‐regulated coordination, dimensionality engineering, hydrogen bonding, domain manipulation, and interfacial coupling are discussed for ordered functional unit arrangement and constructive polarization accumulation. At the macroscopic scale, we discuss converting small‐scale designs into usable bulk crystals, focusing on mitigating layered growth via strengthened interlayer interactions to improve crystal dimension, quality, and device compatibility. Representative NLO crystal examples validate this coherent logic. This framework clarifies structure‐property correlations and supplies guidelines for new NLO materials, advancing this field from empirical screening toward rational, application‐driven design.

Rare earth (RE) compounds offer an advantageous platform for developing advanced optoelectronic func-tional materials. In this work, a new Ce(III)-based selenide, Ce3MnGaSe7, was rationally designed via a polyhedral substitution strategy and synthesized through a high-temperature solid-state reaction. Ce3MnGaSe7 crystallizes in the polar space group P63. Its structure features [CeSe8] polyhedral, [MnSe6] octahedral, and [GaSe4] tetrahedral units, which are interconnected through a unique triple-connectivity pattern involving simultaneous vertex-, edge-, and face-sharing. It exhibits a strong phase-matching sec-ond-harmonic generation response (~2.0 × AgGaS2) and a high laser-induced damage threshold (~3.0 × AgGaS2), achieving a good balance for an infrared (IR) nonlinear optical (NLO) candidate. Moreover, density functional theory calculations reveal that Ce3MnGaSe7 is an indirect band gap compound, with the band gap determined by the polyhedral units. The results highlight the REIII3BIICIIIQVI7 family as an out-standing system for designing new IR NLO candidates, demonstrating that polyhedral substitution is an effective way for developing new RE functional materials.

Two new rare earth (RE) metal selenides KGdSiSe4 and KGdGeSe4 were synthesized by high-temperature solid-state reactions and structurally characterized by single-crystal X-ray diffractions. Both compounds crystallize in the noncentrosymmetric monoclinic P21 space group and feature layered anionic frameworks composed of NLO-active [GdSe7] polyhedra and [Si/GeSe4] tetrahedra. Optical measurements reveal strong second-harmonic generation (SHG) responses of approximately 1.1 × AGS for KGdGeSe4 and 0.8 × AgGaS2 (AGS) for KGdSiSe4. Notably, KGdSiSe4 exhibits a wide band gap (Eg) of 3.13 eV, which achieves a desirable balance for a promising infrared nonlinear optical (IR NLO) candidate. Theoretical calculations reveal that both the Eg and SHG responses mainly originate from the synergistic contributions of [Si/GeSe4] tetrahedra and [GdSe7] polyhedral units. The results demonstrate that alkali-metal RE metal selenides represent a promising platform for developing advanced IR NLO materials with balanced Eg and SHG performances.

Designing high‐performance birefringent materials that simultaneously achieve large birefringence, a wide transparency range, and favorable crystal growth habits remains a critical yet challenging objective in polarized optical devices. Herein, two new interhalogens AICl ₂ (A = NH ₄ , Rb), were rationally designed by coupling [NH ₄ ] ⁺ and alkali metal ions with linear dihalide units and fabricated by solution methods. Both compounds crystallize in the orthorhombic Pnma space group and comprise linear [ICl ₂ ] units. The well‐aligned [ICl ₂ ] units in the structures induce large birefringence: 0.75 for RbICl ₂ and 0.70 for NH ₄ ICl ₂ at 546 nm, respectively, which are 3.7/3.4 times larger than that of commercial birefringent crystal YVO ₄ (0.204@546 nm). A high‐quality RbICl ₂ single crystal (1.1 × 0.5 × 0.2 cm ³ ), exhibiting a wide transparency window extending from 0.50 to 25 µm, was grown via room‐temperature solution crystallization. Theoretical calculations reveal that the optical anisotropy is governed by the anisotropic electron density distribution within the [ICl ₂ ] unit and the ordered arrangement of these units, confirming it as an advantageous birefringence‐active motif. The results highlight the inorganic interhalogen as an emerging system for the design of high‐performance photoelectric functional materials, with RbICl ₂ as a promising birefringent material.

Arsenides represent an advantageous platform for designing long‐wavelength infrared (IR) nonlinear optical (NLO) materials; however, the intrinsic narrow bandgaps hinder their applications. In this work, by introducing As–As homoatomic bonds to eliminate the nonbonding electron pairs on As atoms, the first experimentally verified quaternary arsenide NLO material SrGa ₃ Si ₄ As ₁₁ was rationally designed within the As‐rich A ^II –B ^III –C ^IV –As system, and synthesized experimentally. The compound crystallizes in the uniaxial hexagonal system ( P 6 ₃ space group, No. 173) and is composed of [SrAs ₉ ] polyhedra, [GaAs ₄ ] and [SiAs ₄ ] tetrahedra with unique As–As homoatomic bonding. The introduction of As─As bonds simultaneously enhances the NLO response, bandgap, and birefringence, demonstrating a strong phase‐matching second‐order NLO response (∼8.0 × AgGaS ₂ ), a wide arsenide bandgap (∼1.59 eV), a high laser‐induced damage threshold (∼1.5 × ZnGeP ₂ and ∼4.0 × AgGaS ₂ ) and a large birefringence (Δ n = 0.10@2090 nm) in the compound. Both experimental and theoretical results confirm the positive contributions of As─As bonds to these superior properties. The results enrich the structural diversity of arsenides, and establish As‐rich arsenides incorporating homoatomic bonds as an emerging system for high‐performance IR NLO materials, with SrGa ₃ Si ₄ As ₁₁ serving as a promising candidate.

Arsenides represent an advantageous platform for designing long‐wavelength infrared (IR) nonlinear optical (NLO) materials; however, the intrinsic narrow bandgaps hinder their applications. In this work, by introducing As–As homoatomic bonds to eliminate the nonbonding electron pairs on As atoms, the first experimentally verified quaternary arsenide NLO material SrGa ₃ Si ₄ As ₁₁ was rationally designed within the As‐rich A ^II –B ^III –C ^IV –As system, and synthesized experimentally. The compound crystallizes in the uniaxial hexagonal system ( P 6 ₃ space group, No. 173) and is composed of [SrAs ₉ ] polyhedra, [GaAs ₄ ] and [SiAs ₄ ] tetrahedra with unique As–As homoatomic bonding. The introduction of As─As bonds simultaneously enhances the NLO response, bandgap, and birefringence, demonstrating a strong phase‐matching second‐order NLO response (∼8.0 × AgGaS ₂ ), a wide arsenide bandgap (∼1.59 eV), a high laser‐induced damage threshold (∼1.5 × ZnGeP ₂ and ∼4.0 × AgGaS ₂ ) and a large birefringence (Δ n = 0.10@2090 nm) in the compound. Both experimental and theoretical results confirm the positive contributions of As─As bonds to these superior properties. The results enrich the structural diversity of arsenides, and establish As‐rich arsenides incorporating homoatomic bonds as an emerging system for high‐performance IR NLO materials, with SrGa ₃ Si ₄ As ₁₁ serving as a promising candidate.

(CN ₄ H ₇ ) ₂ PO ₃ F·H ₂ O and (C ₄ N ₄ H ₁₁ ) ₂ PO ₃ F·H ₂ O exhibit large birefringence values of 0.116 and 0.165 at 546 nm, surpassing those of most previously reported monofluorophosphates/difluorophosphates.