AMnAs3S6 (A = Cs, Rb): Phase-Matchable Infrared Nonlinear Optical Functional Motif [As3S6]3- Obtained via Surfactant-Thermal Method

Homoatomic Polychalcogenide Nonlinear Optical Anionic Groups with Ultra-Large Optical Anisotropy

Functional assembly of nonlinear optical (NLO) motifs with a large optical anisotropy is vital to the development of advanced NLO and birefringent materials. In this work, we highlight that, in addition to heteroatomic NLO motifs, homoatomic anionic clusters formed by aggregated anions (S, Se, Te) exhibit diverse chain-, ring-, and cage-like chemical structures as well as one-, two-, and three-dimensional motif alignments. The rich structural chemistry enables homoatomic polychalcogenides (HAPCs) to exhibit asymmetric structural features and anisotropic optical properties, with great potential for NLO and birefringent performance. Focusing on totally 55 binary HAPCs A2Qn (n = 2, 3, 4, 5; A = Na, K, Rb, Cs; Q = S, Se, Te) and their ternary analogues, we employ the state-of-the-art first-principles approach to systematically investigate the modulation evolution of their NLO and birefringent properties. Remarkably, Rb2Te3 and Na2TeSe2 exhibit rarely colossal birefringence (>1.0@10 μm) and NLO effects (>20 × AgGaS2), much larger than conventional NLO chalcogenides. Na2Te3 presents the largest birefringence to date (∼3.48@1, 2.72@2, 2.34@10 μm), indicating the unique structural superiority of HAPC in terms of ultra-large birefringence. By mining the intrinsic mechanism, the HAPC anionic groups are identified as novel mid-infrared NLO "material genes", furnishing unique NLO and birefringent performance for the design of novel optoelectronic materials.

AgMAsS3 (M = Cd, Hg): Double d10 Cation-Containing Thioarsenites(III) Exhibiting High Photocurrent Response and Moderate Second Harmonic Generation

Thioarsenites(III) are an advanced functional material platform owing to the stereochemically active lone pair cations. In this paper, two novel quaternary thioarsenites(III), AgMAsS3 (M = Cd, Hg), are successfully obtained by introducing double d10 cations. In the compounds, d10 cations show a variety of different coordination modes ([AgS4] and [HgS4] in AgHgAsS3 vs [AgS5] and [CdS6] in AgCdAsS3). As a result, AgHgAsS3 and AgCdAsS3 crystallize in the noncentrosymmetric Cc space group and centrosymmetric C2/c space group, respectively. The band gaps of AgHgAsS3 and AgCdAsS3 are determined experimentally as 1.90 and 2.20 eV, respectively. Meanwhile, title compounds exhibit strong photocurrent responses. Specifically, AgHgAsS3 has a large birefringence of 0.18 at 2100 nm and a moderate second harmonic generation of (0.5 × AgGaS2). Moreover, the origin of linear and nonlinear optical responses is investigated based on first-principles calculations. This study enriches the family of MI-MII-As-Q (M = Ag, Cu; MII = Zn, Cd, Hg; Q = chalcogen) chalcogenides and helps to understand and design other multifunctional optical materials.

Rational Design of a Rare-Earth Oxychalcogenide Nd3 [Ga3 O3 S3 ][Ge2 O7 ] with Superior Infrared Nonlinear Optical Performance

Inorganic chalcogenides have been studied as the most promising infrared (IR) nonlinear optical (NLO) candidates for the past decades. However, it is proven difficult to discover high-performance materials that combine the often-incompatible properties of large energy gap (E_g ) and strong second harmonic generation (SHG) response (d_eff ), especially for rare-earth chalcogenides. Herein, centrosymmetric Cs₃ [Sb₃ O₆ ][Ge₂ O₇ ] is selected as a maternal structure and a new noncentrosymmetric rare-earth oxychalcogenide, namely, Nd₃ [Ga₃ O₃ S₃ ][Ge₂ O₇ ], is successfully designed and obtained by the module substitution strategy for the first time. Especially, Nd₃ [Ga₃ O₃ S₃ ][Ge₂ O₇ ] is the first case of breaking the trade-off relationship between wide E_g (>3.5 eV) and large d_eff (>0.5 × AgGaS₂ ) in rare-earth chalcogenide system, and thus displays an outstanding IR-NLO comprehensive performance. Detailed structure analyses and theoretical studies reveal that the NLO effect originates mainly from the cooperation of heteroanionic [GaO₂ S₂ ] and [NdO₂ S₆ ] asymmetric building blocks. This work not only presents an excellent rare-earth IR-NLO candidate, but also plays a crucial role in the rational structure design of other NLO materials in which both large E_g and strong d_eff are pursued.

Crystalline chalcogenidometalate-based compounds from uncommon reaction media

Chalcogenides are one of the most versatile inorganic materials families, further subdivided into a large variety of specific groups of compounds, ranging from neat binary or multinary solids and nanoparticles of the same formal compositions, both in crystalline or non-crystalline form, to complicated open-framework structures and cluster compounds, also including organ(ometall)ic derivates of the latter. The large variety regarding both the compositions and the structures is associated with an enormous variety of properties, ranging from simple or high-tech pigments through a multitude of opto-electronic devices and electrolytes to materials for ion separation or high-sophisticated catalysts. Naturally, this also goes hand in hand with a corrosponding breadth of synthesis strategies. Traditionally, chalcogenides have been accessed via high-temperature methods, which continuously have been replaced by lower-temperature approaches for economical and ecological reasons. Moreover, more recent methods also showed that new types of chalcogenide materials can be obtained under such milder conditions that are not accessible via traditional routes. To shed light onto one of the numerous families of chalcogenides, this feature article summarizes current achievements in the generation of multinary chalcogenidometallate-based clusters and networks via non-classical routes, using ionic liquids, surfactants, or hydrazine as reaction media at moderately elevated termperature.

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.

Material research from the viewpoint of functional motifs

As early as 2001, the need for the ‘functional motif theory’ was pointed out to assist the rational design of functional materials. The properties of materials are determined by their functional motifs and by how they are arranged in the materials. Uncovering the functional motifs and their arrangements is crucial in understanding the properties of materials and rationally designing new materials of desired properties. The functional motifs of materials are the critical microstructural units (e.g. constituent components and building blocks) that play a decisive role in generating certain material functions, and could not be replaced with other structural units without losing or significantly suppressing the relevant functions. The role of functional motifs and their arrangements in materials with representative examples was presented. These examples could be classified into six types of material microscopic structures on a length scale smaller than ∼10 nm with maximum subatomic resolution, i.e. the crystal, magnetic, aperiodic, defect, local, and electronic structures. The method of functional motif analysis could be employed in the function-oriented design of materials, as elucidated by taking infrared nonlinear optical materials as an example. Machine learning is more efficient in predicting material properties and screening materials with high efficiency than high-throughput experimentation and high-throughput calculations. In extracting the functional motifs and finding their quantitative relationships, developing sufficiently reliable databases for material structures and properties is imperative.

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...

Cs3CuAs4Q8 (Q = S, Se): unique two-dimensional layered inorganic thioarsenates with the lowest Cu-to-As ratio and remarkable photocurrent responses

Inorganic chalcogenides containing cations with lone-pair electrons have attracted considerable attention because of their potential applications in photocatalysis. In this research, two new copper thioarsenates with the lowest Cu-to-As ratio in the quaternary X/Cu/As/Q (X = inorganic cations; Q = chalcogen) system, namely Cs₃CuAs₄Q₈ (Q = S, Se), were obtained by a simple surfactant-thermal method at a low temperature. These two isostructural compounds belong to the monoclinic space group C2/c (no. 15) and are composed charge-balanced Cs⁺ cations and two-dimensional anionic [CuAs₄Q₈]^3- layers. Notably, photo-electrochemical measurements indicate that Cs₃CuAs₄Q₈ possesses a remarkable photocurrent response under simulated solar-light illumination. Further theoretical studies were performed to gain insights into the relationships between electronic structure and optical properties.

A Discrete Ligand-Free T3 Supertetrahedral Cluster of Gallium Sulfide

Large discrete supertetrahedral clusters of metal chalcogenides are rare due to the difficulty of crystallizing solids in which the negative charge of the cluster is balanced by the positive charges of the countercations. Here, we describe a discrete ligand-free T3 supertetrahedral cluster, [Ga10S16(SH)4]6-, which was successfully synthesized in the presence of the superbase 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) using the neutral surfactant polyethyleneglycol (PEG)-400 as the reaction solvent. Protonated DBUH+ cations are incorporated into the crystal structure of the product, which can be formulated as [C9H17N2]6[Ga10S16(SH)4]. This compound, which represents the first example of a discrete ligand-free T3 cluster of gallium sulfide, was fully characterized by single-crystal and powder X-ray diffraction, elemental analysis, infrared spectroscopy, thermogravimetric analysis, and ultraviolet-visible diffuse reflectance. The results presented here indicate that the use of surfactants as solvents offers potential for the preparation of new compounds containing supertetrahedral clusters.

AZn4Ga5Se12 (A = K, Rb, or Cs): Infrared Nonlinear Optical Materials with Simultaneous Large Second Harmonic Generation Responses and High Laser-Induced Damage Thresholds

Despite the fact that nonlinear optical (NLO) crystals such as AgGaS₂ and AgGaSe₂ have been widely used in the infrared (IR) range due to their large second harmonic generation (SHG) coefficients and wide range of IR transparency windows, the small laser-induced damage threshold (LIDT) remains a great issue hindering their high-power applications. Herein, three noncentrosymmetric (NCS) chalcogenides AZn₄Ga₅Se₁₂ (A = K, Rb, or Cs) are successfully obtained through an appropriate flux method after the extensive design and synthesis of the A/Zn/Ga/Q system. Single-crystal X-ray diffraction data demonstrate that they adopt trigonal space group R3 (No. 146) with three-dimensional diamond-like frameworks composed of [M₉Se₂₄] layers (M = Zn or Ga) stacking in the same direction and filled by charge-balancing A⁺ cations. Noticeably, they all exhibit strong powder SHG responses (2.8-3.7 × AgGaS₂) and amazing LIDTs (19.2-23.4 × AgGaS₂). In addition, theoretical calculations are performed to further determine the relationship between NCS structures and NLO properties. This work provides effective solutions for overcoming the trade-off between strong SHG and high LIDT in IR-NLO materials.

Uncovering a Functional Motif of Nonlinear Optical Materials by In Situ Electron Density and Wavefunction Studies Under Laser Irradiation

Exploring nonlinear optical (NLO) functional motifs (FM, the structural origin of NLO efficiency) is vital for the rational design of NLO materials. Normal spectrum techniques applied in studying photon exciting materials are invalid for NLO materials, in which electrons are not excited substantially but only distorted under laser. A general strategy of determining NLO FM is proposed by comparative studies of experimental electron density (ED) without and under the laser. The in situ experimental ED and wavefunction of typical NLO material LiB₃ O₅ (LBO) under dark and 360 and 1064 nm lasers are investigated. Compared with the initial state under dark, the ED of [B₃ O₅ ]^- unit at functional states under laser irradiation exhibits remarkable changes of topological atomic and bond properties, confirming the NLO FM being [B₃ O₅ ]^- . The work extracts for the first time the FM of a NLO material experimentally and highlights the crucial role of in situ ED analysis in studying NLO mechanisms.

A3Mn2Sb3S8 (A = K and Rb): a new type of multifunctional infrared nonlinear optical material based on unique three-dimensional open frameworks

A new type of multifunctional IR-NLO material, A₃Mn₂Sb₃S₈ (A = K and Rb), with unique 3D open frameworks has been developed using a facile surfactant–thermal method.

Na4SnS4 and Na4SnSe4 exhibiting multifunctional physicochemical performances as potential infrared nonlinear optical crystals and sodium ion conductors

Na₄SnS₄ and Na₄SnSe₄ exhibiting excellent physicochemical performances as potential IR NLO crystals and sodium ion conductors were systematically studied.