Li4 MgGe2 S7 : The First Alkali and Alkaline-Earth Diamond-Like Infrared Nonlinear Optical Material with Exceptional Large Band Gap

Ba3HgGa2S7: A Zero-Dimensional Quaternary Sulfide Featuring a Unique [Hg2Ga4S14]12- String and Exhibiting a High Photocurrent Response

The discovery of new types of metal sulfides is attractive because of their rich structures and diverse physical properties. Here, a novel quaternary sulfide, Ba₃HgGa₂S₇ (BHGS), is obtained by a solid-state reaction at 1123 K. It crystallizes in the monoclinic space group P2₁/c, and its zero-dimensional structure features two seesaw-like HgS₂ units and four GaS₄ tetrahedra, constructing a unique [Hg₂Ga₄S₁₄]^12- string. BHGS has a wide band gap of 3.64 eV and a large birefringence of 0.09 at 2100 nm. Specifically, BHGS exhibits a remarkable photocurrent response. This work may be extended to a new family of AE₃M^IIM^III₂Q₇ (AE = Mg, Ca, Sr, Ba; M^II = Zn, Cd, Hg; M^III = Al, Ga, In; Q = S, Se) chalcogenides.

Eu2 P2 S6 : The First Rare-Earth Chalcogenophosphate Exhibiting Large Second-Harmonic Generation Response and High Laser-Induced Damage Threshold

Metal chalcogenophosphates are receiving increasing interest, specifically as promising infrared nonlinear optical (NLO) candidates. Here, a rare-earth chalcogenophosphate Eu₂ P₂ S₆ crystallizing in the monoclinic noncentrosymmetric space group Pn was synthesized using a high-temperature solid-state method. Its structure features isolated [P₂ S₆ ]^4- dimer, and two types of EuS₈ bicapped triangular prisms. Eu₂ P₂ S₆ exhibits a phase-matchable second-harmonic generation (SHG) response ≈0.9×AgGaS₂ @2.1 μm, and high laser-induced damage threshold of 3.4×AgGaS₂ , representing the first rare-earth NLO chalcogenophosphate. The theoretical calculation result suggests that the SHG response is ascribed to the synergetic contribution of [P₂ S₆ ]^4- dimers and EuS₈ bicapped triangular prisms. This work provides not only a promising high-performance infrared NLO material, but also opens the avenue for exploring rare-earth chalcogenophosphates as potential IR NLO materials.

NH4IO2F2 and (NH4)3(IO2F2)3·H2O: A Series of Ammonium-Containing Fluoroiodates with Wide Band Gaps

A series of ammonium-containing fluoroiodates, NH₄IO₂F₂ and (NH₄)₃(IO₂F₂)₃·H₂O, with isolated [IO₂F₂] units have been fabricated by a fluorine-oxygen substitution strategy from NH₄IO₃. The two compounds crystallize in the orthorhombic system, but in different space groups, noncentrosymmetric Pca2₁ for NH₄IO₂F₂ and centrosymmetric Pnma for (NH₄)₃(IO₂F₂)₃·H₂O, and show wide band gaps of 4.53 eV for (NH₄IO₂F₂) and 4.55 eV for ((NH₄)₃(IO₂F₂)₃·H₂O). In addition, NH₄IO₂F₂ exhibits a 1.2 × KDP second harmonic generation response, a short ultraviolet cutoff edge in iodates, and a good crystal growth habit. The crystal of NH₄IO₂F₂ with a size of 11 × 5 × 2 mm³ was obtained by the aqueous solution method. The results enrich the structural diversity of iodate and supply a greater understanding of the design of new functional materials based on the fluoroiodates.

AgGaGeSe4: An Infrared Nonlinear Quaternary Selenide with Good Performance

The symmetry of crystals is an extremely important property of crystals. Crystals can be divided into centrosymmetric and non-centrosymmetric crystals. In this paper, an infrared (IR) nonlinear optical (NLO) material AgGaGeSe4 was synthesized. The related performance analysis, nonlinear optical properties, and first-principle calculation of AgGaGeSe4 were also introduced in detail. In the AgGaGeSe4 structure, Ge4+ was replaced with Ga3+ and produced the same number of vacancies at the Ag+ position. The low content of Ge doping kept the original chalcopyrite structure and improved its optical properties such as the band gap. The UV-Vis diffuse reflection spectrum shows that the experimental energy band gap of AgGaGeSe4 is 2.27 eV, which is 0.48 eV larger than that of AgGaSe2 (1.79 eV). From the perspective of charge-transfer engineering strategy, the introduction of Group IV Ge elements into the crystal structure of AgGaSe2 effectively improves its band gap. The second harmonic generation (SHG) effect of AgGaGeSe4 is similar to that of AgGaSe2, and at 1064 nm wavelength, the birefringence of AgGaGeSe4 is 0.03, which is greater than that of AgGaSe2 (∆n = 0.02). The results show that AgGaGeSe4 possessed better optical properties than AgGaSe2, and can been broadly applied as a good infrared NLO material.

Design of Infrared Nonlinear Optical Compounds with Diamond-like Structures and Balanced Optical Performance

Infrared (IR) nonlinear optical (NLO) crystals are the major materials to widen the output range of solid-state lasers to mid-infrared regions, but they are still inadequate for application due to the difficulties in balancing the large band gaps and strong NLO response. The diamond-like structure is a potential structural template to explore IR NLO materials. Herein, a computational workflow is proposed for exploring compounds with diamond-like structures, a series of LiMgGaSe₃ structures were predicted successfully through this workflow, and LiMgGaSe₃-I-III exhibited good optical performances in a large band gap (2.75-2.92 eV), strong SHG response (1.2-1.3 × AGS), and suitable birefringence (0.0470-0.0783 at 1064 nm). The in-depth mechanism explorations strongly demonstrate that the synergistic effect of alkaline earth metal tetrahedral [MgSe₄] and [GaSe₄] units is the main origin of large SHG response. The foregoing results suggest that our workflow can accelerate the discovery of new mid-IR NLO materials with diamond-like structures.

Strong Nonlinearity Induced by Coaxial Alignment of Polar Chain and Dense [BO3 ] Units in CaZn2 (BO3 )2

Discovery of new efficient nonlinear optical (NLO) materials with large second-order nonlinearity for the short-wave ultraviolet spectral region (λ_PM ≤266 nm, PM=phase-matching) is still very challenging. Herein, a new beryllium-free borate CaZn₂ (BO₃ )₂ with Sr₂ Be₂ B₂ O₇ (SBBO) double-layered like configuration was rationally designed, which not only preserves the structural merits but also eliminates the limitations of the SBBO crystal. CaZn₂ (BO₃ )₂ shows a large PM second harmonic generation (SHG) reponse of 3.8×KDP, which is 38 times higher than that of its barium analogue. This enhancement mainly originates from the ¹ [Zn₂ O₆ ]_∞ polar chains with a large net dipole moment and [BO₃ ] units with a high NLO active density. Our findings show the great significance of the [ZnO₄ ] tetrahedra introduced strategy to design beryllium-free SBBO-type NLO crystals and also verify the feasibility of using simple non-isomorphic substitution to induce giant second-order nonlinearity enhancement.

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

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.

Second-Harmonic Generation-Positive Na2Ga2SiS6 with a Broad Band Gap and a High Laser Damage Threshold

The development of high-power solid-state lasers is in urgent need of new infrared nonlinear optical (IR NLO) materials with a wide band gap and a high laser-induced damage threshold. A new infrared nonlinear optical material Na₂Ga₂SiS₆ has been synthesized for the first time, crystallizing in the Fdd2 (no. 43) noncentrosymmetric space group. Its three-dimensional tunnel framework consists of two typical NLO active motifs [GaS₄] and [SiS₄], with Na⁺ cations located inside the tunnels. Na₂Ga₂SiS₆ exhibits comprehensive optical properties, namely, a wide transmission range, a high laser-induced damage threshold (10 × AgGaS₂), a type-I phase-matching second-harmonic generation response (0.2 × AgGaS₂), and especially a wide band gap (3.93 eV), which is the largest in the A₂M^III₂M^IVQ₆ (A = alkali metals; M^III = IIIA elements; M^IV = IVA elements; Q = S and Se) family. Therefore, Na₂Ga₂SiS₆ does not produce two-photon absorption under a 1064 nm laser pump and could be used in high-energy laser systems, which makes Na₂Ga₂SiS₆ a promising candidate for high-energy IR NLO applications.

Uncovering a Vital Band Gap Mechanism of Pnictides

Pnictides are superior infrared (IR) nonlinear optical (NLO) material candidates, but the exploration of NLO pnictides is still tardy due to lack of rational material design strategies. An in-depth understanding structure-performance relationship is urgent for designing novel and eminent pnictide NLO materials. Herein, this work unravels a vital band gap mechanism of pnictides, namely P atom with low coordination numbers (2 CN) will cause the decrease of band gap due to the delocalization of non-bonding electron pairs. Accordingly, a general design paradigm for NLO pnictides, ionicity-covalency-metallicity regulation is proposed for designing wide-band gap NLO pnictides with maintained SHG effect. Driven by this idea, millimeter-level crystals of MgSiP2 are synthesized with a wide band gap (2.34 eV), a strong NLO performance (3.5 x AgGaS2 ), and a wide IR transparency range (0.53-10.3 µm). This work provides an essential guidance for the future design and synthesis of NLO pnictides, and also opens a new perspective at Zintl chemistry important for other material fields.

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

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-MgGa2 Se4 , 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-MgGa2 Se4 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 Growth of High-Quality Hexagonal GaTe Nanosheets Induced by ZnO Nanocrystals

The monoclinic and hexagonal gallium tellurides (m-GaTe and h-GaTe) show different applications in optoelectronic devices. Compared to the m-GaTe, the h-GaTe is a metastable phase, which generally exists in ultrathin samples and is difficult to obtain by direct chemical reaction. Herein, a hexagonal ZnO-induced crystal growth strategy was used for the design and fabrication of h-GaTe. The high-quality h-GaTe nanosheets were successfully grown on the (001) surface of hexagonal ZnO by the chemical vapor deposition method under ambient pressure. The SEM, XPS, XRD, and HRTEM characterizations uncovered a flower-like nanosheet morphology and a hexagonal crystal structure for the obtained GaTe samples. Meanwhile, the conductive atomic force microscope measurement indicates that the obtained h-GaTe nanosheet is a p-type semiconductor. Based on the electron localization function simulation, the lattice-induced crystal growth of h-GaTe was demonstrated. The results give an insight into the synthesis of metastable phase crystal and open an avenue for fabricating new two-dimensional devices by p-type h-GaTe.

NaGa3Se5: An Infrared Nonlinear Optical Material with Balanced Performance Contributed by Complex {[Ga3Se5]-}∞ Anionic Network

Second-order nonlinear optical (NLO) materials are extensively applied in laser-related techniques. For developing IR NLO materials, chalcogenides are the main candidates. Here, NaGa₃Se₅ was explored as inspired by its unique anionic structure. It crystallizes with the orthorhombic chiral P2₁2₁2₁ structure, featuring 12 types of GaSe₄ tetrahedra built into a three-dimensional {[Ga₃Se₅]^-}_∞ anionic network, representing a new NLO-functional motif. NaGa₃Se₅ exhibits large and phase-matchable NLO response 1.37 × AgGaS₂. It has the largest band gap among the noncentrosymmetric A-M^III-Se (A = alkali metal; M = Ga, In) compounds. The NLO properties' origin is explored via theoretical analysis. The success of NaGa₃Se₅ contributes a practical case for exploring new NLO materials.

The synthesis and structure-property relation analysis of metal chalcohalide crystals Cs2InPS4X2 (X = Cl, Br) with mixed anions

Inorganic metal chalcohalides are significant semiconductive materials for photovoltaics, photodetetion and infrared optics. Thus it is considerably rewarding to develop a new synthetic strategy to provide more degrees of freedom for atomic coordination to tune the optical and electronic properties of metal chalcohalides. In this work, the mixed-anion strategy is performed to synthesize two new metal chalcohalides Cs₂InPS₄X₂ (X = Cl, Br) with mixed-anion structure by the reaction of InPS₄ and CsX. Single-crystal X-ray diffraction analysis shows that they are isostructural and crystallize in the centrosymmetric space group P2₁/n, consisting of zero-dimensional structure [In₂P₂S₈X₄]^4- (X = Cl, Br) built from tetrahedral [PS₄]^3- and octahedral [InS₄X₂]^7- (X = Cl, Br) through edge-sharing, with Cs cations filling in intervening voids. The UV-vis-NIR diffuse reflectance spectroscopy measurement reveals that Cs₂InPS₄Cl₂ and Cs₂InPS₄Br₂ exhibit large optical bandgaps of 3.21 eV and 3.12 eV, respectively. The electronic structure calculations show that the bandgap mainly originates from the [InS₄X₂]^7- (X = Cl, Br) mixed-anion groups. First-principles calculations indicate that the birefringence of Cs₂InPS₄Cl₂ and Cs₂InPS₄Br₂ is ∼0.08 and ∼0.05 at 2090 nm, respectively. Furthermore, thermal analysis reveals that the Cs₂InPS₄X₂ (X = Cl, Br) are thermostable up to 400 °C. This discovery enriches the structural diversity of inorganic chalcohalides and provides an insight for the exploration of new semiconductive materials.

Anion-Centered Polyhedron Strategy for Strengthening Photon Emission Induced by Electron-Phonon Coupling

Electron-phonon coupling emerges as a growing frontier in the heart of condensed matter from physical symmetry to the electronic quantum state, but its quantitative strength dependence on the chemical structure has not been assessed. Here, we originally proposed the anion-centered polyhedron (ACP) strategy for elaborating the electron-phonon coupling interaction in rare-earth (RE) materials comprising three chemical factors, RE-O bond length, the effective charge of the coordinated atom, and structural dimensionality. Using Gd³⁺ cation with 4f⁷ configuration as a fluorescence probe, we found that the "free-O"-centered polyhedron is the most crucial motif in strengthening the phonon-assisted energy transfer and photon emission. The temperature-dependent Huang-Rhys S factors were calculated to identify the electron-phonon coupling intensity based on the fluorescence spectrum quantitatively. Finally, beyond conventional wisdom, a series of structural criteria were presented, serving as useful guidelines for discovering strongly coupled rare-earth optical materials. Our study breaks the long-time "blind"-searching diagram and provides reliable principles for many functional materials associated with electron-phonon coupling, such as superconductors, multiferroics, and phosphors.

Optimizing the Nonlinear Optical Performance of an A-N-M-Q (A: Alkali Metal; N: d10 Metal; M: Main Group Metal; Q: Chalcogen) System

Exploring new infrared nonlinear optical (IR NLO) materials with superior overall properties is scientifically and technically important. However, large second-order harmonic generation (SHG) efficiencies and high laser-induced damage thresholds (LIDT) are incompatible, which makes realizing this goal a challenge. The IR NLO performance of an A-N^IIB-M^IIIA-Q (Q: chalcogen) system was optimized by simultaneously modulating A/(M + N) and M/N ratios (A: alkali metal; N, M: tetra-coordinated metals), and SHG-LIDT balance was achieved. Three new sulfides, KCd₃Ga₅S₁₁ (1), RbCd₄Ga₃S₉ (2), and Cs₂Cd₂Ga₈S₁₅ (3), containing the same CdS₄ and GaS₄ but with different A/(Ga + Cd) and Ga/Cd ratios were obtained. Among these compounds, compound 3 exhibits both the largest SHG efficiency (0.5 × AgGaS₂) and LIDT (35 × AgGaS₂), which can be ascribed to the Ga/Cd modulation for enhancing the NLO functional motif distortions and SHG efficiency as well as the A/(Ga + Cd) modulation for enlarging the band gap and LIDT. Remarkably, compound 3 is the first phase-matchable IR NLO material in the A-N^IIB-M^IIIA-Q family. This article proposes a novel avenue to explore infrared nonlinear materials with superior comprehensive properties by modulating the A/(M + N) and M/N ratios.

AAg3Ga8Se14 (A = Rb, Cs): Second-Harmonic Generation Responses Realized through the Parallel Arrangement of AgSe4 and GaSe4 Tetrahedrons

Two new quaternary selenides AAg3Ga8Se14, (A = Rb, 1; Cs, 2) were synthesised via sold-state reaction in sealed silica tubes. Compounds 1 and 2 crystallised in the monoclinic space group...

APb2(C7H3NO4)2I (A = K, Rb, Cs): rare stable nonlinear optical crystals with second-harmonic generation response and highly distorted lead core coordination polyhedra

APb2(C7H3NO4)2I (A = K, Rb, Cs) features a 3D NCS cubic framework consisting of highly distorted [PbNO5] and [PbNO4I] coordination polyhedra, a moderate SHG response, a wide transparent window and a high thermal stability above 300 °C.

A2P2S6 (A=Ba, Pb): A Good Platform to Study Polymorph Effect and Lone Pairs Effect to Form Acentric Structure

Three ternary thiophosphates α-Ba2P2S6, β-Ba2P2S6, and Pb2P2S6, were synthesized via a high temperature salt flux method or an I2 transport reaction. β-Ba2P2S6 and Pb2P2S6 were previously structurally characterized without investigating...

Na4Ga8S14: A Ga-riched wide band gap ternary alkali-metal sulfide with unique [Ga12S42] 12-menbered rings

A Ga-riched ternary alkali-metal sulfide Na4Ga8S14 has been synthesized by high temperature solid-state reaction. It crystallizes in the centrosymmetric Pbca (no. 61) space group with cell parameters a = 13.5260(4)...

Two New Tellurite Halides with Cationic Layers: Syntheses, Structures, and Characterizations of CdPb2Te3O8Cl2 and Cd13Pb8Te14O42Cl14

Two new tellurite halides, CdPb2Te3O8Cl2 and Cd13Pb8Te14O42Cl14 with mixed cationic layered structures, have been synthesized by high-temperature solution method. CdPb2Te3O8Cl2 crystallizes in the noncentrosymmetric Aba2 space group, built by [CdPb2Te3O8]...

An excellent lead oxyiodide with a strong second-harmonic generation response and a large birefringence induced by the oriented arrangement of highly distorted [PbO4I2] polyhedra

K2I[PbI(OOCCH2COO)] featuring the strongest second-harmonic generation response among malonates, a large birefringence, a wide transparent window and good stability is induced by the oriented arrangement of highly distorted bifunctional [PbO4I2] polyhedra.

RbMo2P3O14 with large birefringence mainly induced by highly distorted [MoO6] in uncommon [Mo2P3O14]∞ layers

The introduction of d⁰ transition metal Mo⁶⁺ cations into a phosphate generates a new acentric molybdophosphate, RbMo₂P₃O₁₄. It shows uncommon [Mo₂P₃O₁₄]_∞ layers composed of isolated [MoO₆] octahedra and [P₃O₁₀] groups. To the best of our acknowledge, it exhibits the largest birefringence (a calculated value of 0.166 at 546 nm) among reported molybdophosphates. In addition, it also possesses a shorter UV cut-off edge (about 250 nm) than other molybdates and molybdophosphates, indicating that it can be used as a birefringent crystal in the UV optical region. First-principles electronic structure analysis suggests that the large birefringence mainly originates from highly distorted [MoO₆].

Balanced infrared nonlinear optical performance achieved by modulating the covalency and ionicity distributions in the electron localization function map

The nonlinear optical (NLO) efficiency (d_ij) and laser-induced damage threshold (LIDT) of a material are mainly determined by their covalency and ionicity, respectively, the incompatibility between which makes balancing the d_ij and LIDT challenging in an IR NLO material. The topological feature (fractal dimension) of the electron localization function (ELF) map (distribution of covalency and ionicity) was evaluated for a series of NLO materials, and, phenomenologically, the fine mixing of covalency and ionicity will benefit a balanced d_ij and LIDT. Chemical bonds with different interaction strengths were introduced simultaneously to mix the covalency and iconicity finely, and three new IR NLO sulfides, A₂Ba₃Li₆Ga₂₈S₄₉ (A = K, 1; Rb, 2; Cs, 3), were obtained. They exhibit a strong NLO efficiency (1.9-2.1 × AgGaS₂ at 1064 nm and 0.5-0.6 × AgGaS₂ at 1910 nm) and high LIDTs (16.7-18.0 × AgGaS₂), which fulfill the criteria of being promising IR NLO candidates. This study provides a new method for designing high-performance IR NLO materials based on the topological features of the ELF.

Designing A New Infrared Nonlinear Optical Material, β-BaGa2 Se4 Inspired by the Phase Transition of the BaB2 O4 (BBO) Crystal

Infrared nonlinear optical (IR NLO) crystals play a significant role in the development of IR laser technology. But rationally designing a new IR NLO crystal remains a huge challenge because of the unpredictability of inorganic structures. Herein, inspired by phase transformation of the famous BaB₂ O₄ (BBO) crystal, a temperature-induced centrosymmetric (CS) to noncentrosymmetric (NCS) transformation approach is employed in CS BaGa₂ Se₄ to uncover a new NCS ternary chalcogenide, β-BaGa₂ Se₄ which can exhibit the well-balanced NLO properties, including moderate phase-matching SHG response (≈0.6×AgGaS₂ ), high LDT (10×AgGaS₂ ), and large birefringence (Δn=0.18 in the visible region). Therefore, β-BaGa₂ Se₄ will be a promising IR NLO crystal. The analysis for the structure-property relationship shows that the excellent NLO properties of β-BaGa₂ Se₄ mainly originate from edge-sharing GaSe₄ tetrahedral chains, which are different from B₃ O₆ groups in β-BBO and respect a new direction for the design of IR NLO crystals. So, we believe that the current research provides not only a new IR NLO crystal but also some insights for the design of new IR NLO crystals.

A new broad-band infrared window material CdPbOCl2 with excellent comprehensive properties

Herein, a new IR window material CdPbOCl₂ is rationally designed and fabricated by a heavy-metal oxide and halide combined strategy. The millimeter-scale CdPbOCl₂ single crystal exhibits a wide IR transparent region (1.4-18.0 μm) and excellent comprehensive properties. The results provide an insight into the exploration of broad-band IR window materials.

Ba4GeSb2Se11: An Infrared Nonlinear Optical Crystal with a V-Shaped Se32- Group Possessing a Large Contribution to the SHG Response

The quaternary selenide Ba₄GeSb₂Se₁₁ was prepared by a high-temperature solid state reaction method. Ba₄GeSb₂Se₁₁ crystallizes in an acentric orthorhombic space group Cmc2₁ with the lattice constants a = 9.370(11) Å, b = 25.850(0) Å, and c = 8.798(10) Å. The compound is composed of a [SbSe₃]^3- trigonal pyramid, [GeSbSe₅]^3- dimers, V-shaped Se₃^2-, and the adjacent Ba²⁺ ions. It has indirect band gap of 1.35 eV and exhibits a second harmonic generation intensity of about 0.2 times that of the benchmark compound AgGaS₂ at the same particle size. Interestingly, theoretical analyses show that the central Se atom of Se₃^2- has the largest contribution (8.1%) to d₃₁ compared to that of other Se atoms, which may be due to its easy swing in the a-axis direction.

Syntheses, Structures and Properties of Alkali and Alkaline Earth Metal Diamond-Like Compounds Li2MgMSe4 (M = Ge, Sn)

Two new diamond-like (DL) chalcogenides, Li2MgGeSe4 and Li2MgSnSe4, have been successfully synthesized using a conventional high-temperature solid-state method. The two compounds crystallize in the non-centrosymmetric space group Pmn21 with a = 8.402 (14) Å, b = 7.181 (12) Å, c = 6.728 (11) Å, Z = 2 for Li2MgSnSe4, and a = 8.2961 (7) Å, b = 7.0069 (5) Å, c = 6.6116 (6) Å, Z = 2 for Li2MgGeSe4. The calculated results show that the second harmonic generation (SHG) coefficients of Li2MgSnSe4 (d33 = 12.19 pm/v) and Li2MgGeSe4 (d33 = -14.77 pm/v), mainly deriving from the [MSe4] (M = Ge, Sn) tetrahedral units, are close to the one in the benchmark AgGaS2 (d14 = 13.7 pm/V). The calculated band gaps for Li2MgSnSe4 and Li2MgGeSe4 are 2.42 and 2.44 eV, respectively. Moreover, the two compounds are the first series of alkali and alkaline-earth metal DL compounds in the I2-II-IV-VI4 family, enriching the structural diversity of DL compounds.