Design and fabrication of new infrared (IR) nonlinear optical (NLO) materials with balanced properties are urgently needed since commercial chalcopyrite‐like (CL) NLO crystals are suffering from their intrinsic drawbacks. Herein, the first defect‐CL (DCL) alkali‐earth metal (AEM) selenide IR NLO material, DCL‐MgGa2Se4, has been rationally designed and fabricated by a structure prediction and experiment combined strategy. The introduction of AEM tetrahedral unit MgSe4 effectively widens the band gap of DCL compounds. The title compound exhibits a wide band gap of 2.96 eV, resulting in a high laser induced damage threshold (LIDT) of ≈3.0 × AgGaS2 (AGS). Furthermore, the compound shows a suitable second harmonic generation (SHG) response (≈0.9 × AGS) with a type‐I phase‐matching (PM) behavior and a wide IR transparent range. The results indicate that DCL‐MgGa2Se4 is a promising mid‐to‐far IR NLO material and give some insights into the design of new CL compound with outstanding IR NLO properties based on the AEM tetrahedra and the structure predication and experiment combined strategy.

The Combination of Structure Prediction and Experiment for the Exploration of Alkali‐Earth Metal‐Contained Chalcopyrite‐Like IR Nonlinear Optical Material

With the development of laser technology, nonlinear optical (NLO) crystals with optimal performance are urgently required. Finding new NLO-active units is necessary to expand the exploration systems for NLO materials. Herein, a π-conjugated planar hydrogensquarate (HC4O4)− with large hyperpolarizabilities and giant polarizability anisotropies is identified as a new NLO-active unit. On the basis of this idea, a new NLO crystal NaHC4O4·H2O is found by using the first-principles high-throughput screening pipeline for nonlinear optical materials (FHSP-NLO). It is the first NLO crystal found in squarates. Subsequently, the crystal with a size of 30 × 6 × 5 mm was grown by a facile aqueous solution method. NaHC4O4·H2O shows a strong powder SHG effect of 2.3 × KDP and a giant birefringence of 0.52 at 1064 nm. Electronic structure calculation and analysis reveal that the π orbitals of the (HC4O4)− groups are the dominant source of the SHG coefficient. This study provides novel NLO-active units and a materials system to design and find NLO crystals. 

Finding the First Squarates Nonlinear Optical Crystal NaHC4O4·H2O with Strong Second Harmonic Generation and Giant Birefringence

To design nonlinear optical (NLO) materials, we focused on combinations of d10 metal cation (Cd2+)-based chloride and morpholine molecules to form organic-inorganic hybrids. The O of morpholine containing lone-pair electrons can be integrated with Cd2+ by a ligand-to-metal charge transfer (LMCT) strategy to build acentric structures benefiting from the second-order Jahn-Teller effect. Introduction of the high-electronegativity chlorine can make polyhedrons of acentric crystals more distorted and conducive to a strong second harmonic generation (SHG) response. Therefore, (Morpholinium)2Cd2Cl6 crystals were constructed and synthesized by a solvent evaporation method. (Morpholinium)2Cd2Cl6 belongs to the orthorhombic P212121 space group and shows a one-dimensional (1D) structure with distorted [CdCl6] and [CdCl4O2] octahedrons. The short cutoff edge of (Morpholinium)2Cd2Cl6 was determined to be about 230 nm. The SHG response of (Morpholinium)2Cd2Cl6 exhibited an intensity of approximately 0.73 × KDP as estimated by the powder second harmonic generation technique. Furthermore, related theoretical calculations were performed to study the relationship of the band structure, refractive anisotropy, electronic state, and nonlinear optical response. Besides, (Morpholinium)2Cd2Cl6 showed relatively good thermal stability. This work can serve as a guide for the design and synthesis of new large NLO hybrid crystals with d10 transition metals.

Organic-Inorganic Hybrid Noncentrosymmetric (Morpholinium)2Cd2Cl6 Single Crystals: Synthesis, Nonlinear Optical Properties, and Stability.

Stereochemically active lone-pair (SCALP) cations are one attractive type of nonlinear optical (NLO)-active units because of their large microcosmic polarizability and anisotropy. Currently, the single and/or dual lone-pair cation-based noncentrosymmetric (NCS) oxides have been extensively investigated and verified to be one class of outstanding NLO materials. From the perspective of function optimization, the integration of three kinds of SCALP cations into one crystal may synergistically improve the NLO properties, which is greatly expected but unexplored to date. Herein, by introducing flexible metal halide bonds to guarantee the stereochemical activity and overcome the energetically favorable antiparallel arrangements of lone-pair cations, the first type of three lone-pair-cation (Pb2+, Bi3+, and Se4+)-coexisting NCS oxides PbBi(SeO3)2F (I) and Pb2Bi(SeO3)2Cl3 (II) was obtained. As expected, both compounds show outstanding NLO properties, such as the strong second-harmonic-generation signal (10.5× and 13.5 × KDP), large birefringence (0.103 and 0.186), relatively wide energy band gaps (3.75 and 3.45 eV), and good physicochemical stability. Theoretical calculations demonstrated the effect of three lone-pair-cation-based polyhedra and the halide anion on NLO properties.

PbBi(SeO3)2F and Pb2Bi(SeO3)2Cl3: Coexistence of Three Kinds of Stereochemically Active Lone-Pair Cations Exhibiting Excellent Nonlinear Optical Properties.

Oxyhalides possessing the merits of oxides and halides have widely received attention for their comprehensive physical performances, especially as potential nonlinear optical (NLO) crystals. Here, based on conventional strategy for obtaining acentric compounds, a Te4+ lone-pair cation was introduced into oxyhalides, and one oxyfluoride, HgTeO2F(OH), was obtained via a hydrothermal reaction. Crystallized in the polar space group Pca21, the layered structure of HgTeO2F(OH) is composed of V-shaped [HgO2]2- groups and [TeO2F(OH)]2- pyramids, in which the [TeO2F(OH)]2- pyramid first served as the NLO functional motif. Its powder sample exhibits a phase-matchable SHG response of 1.1 × KH2PO4 at 1064 nm, and its birefringence (0.09@1064 nm) is sufficient for phase-matchable behavior, which manifests its comprehensive capacity as a promising NLO candidate. Theoretical calculations about electronic structure and optical properties are also carried out, revealing that the Te4+ lone-pair cation makes the predominant contribution to the SHG effect and synergizes with the [HgO2]2- groups. 

HgTeO2F(OH): A Nonlinear Optical Oxyfluoride Constructed of Active [TeO2F(OH)]2– Pyramids and V-Shaped [HgO2]2– Groups

Dimensionality reduction made high-performance mid-infrared nonlinear halide crystal

A new alkali metal-rare earth metal anhydrous sulfate, KTb(SO4)2 (KTSO), was synthesized by a hydrothermal method. It crystallized in the non-centrosymmetric (NCS) orthorhombic space group P212121 with cell parameters a = 5.43950(10), b = 8.8579(3), c = 13.3962(4), Z = 2. Its structure is composed of [KO11], [TbO8], and [SO4] groups, which are interconnected to constitute a one-dimensional (1D) chain, and then further linked to construct a three-dimensional (3D) network structure through [SO4] units. The results of the powder frequency doubling test show that KTb(SO4)2 has a phase-matching second-order nonlinear optical effect of 0.3 times that of KH2PO4 (KDP). The measured birefringence of the crystal (0.01) agrees well with the calculated value (0.019). Meanwhile, its infrared and ultraviolet spectrum and thermal properties were studied, respectively. Its optical properties and band gap structure are studied by theoretical calculation. This work provides a NCS structure of anhydrous sulfates and offers guidance for further exploration of new anhydrous sulfates.

Synthesis, Crystal Structure, and Nonlinear Optical Property of an Anhydrous Sulfate KTb(SO4)2.

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Dimensionality reduction made high-performance mid-infrared nonlinear halide crystal

Synthesis, Crystal Structure, and Nonlinear Optical Property of an Anhydrous Sulfate KTb(SO4)2.