Design of Pentanary Mixed-Chalcogenides Ag2In2SiS6-xSex (x = 1, 2) Based on the Bucket Effect: Local Structural Difference and High-Performance Nonlinear-Optical Properties Realized by Partial Congener Substitution

Noncentrosymmetric chalcogenides are promising candidates for infrared nonlinear-optical (NLO) crystals, and exploring high-performance ones is a hot topic and challengeable. Herein, the combination of AgQ4, InQ4, and SiQ4 (Q = S, Se) units with different S/Se ratios resulted in the discovery of the tetrahedral chalcogenides Ag2In2SiS4Se2 (1) and Ag2In2SiS5Se (2). They both crystallize in the monoclinic Cc space group with different local structures. Co-occupied S/Se sites only exist in 2, and the arrangement of [In2SiQ3] six-membered rings builds different helical chains and 3D [(In2SiQ6)2-]n polyanionic frameworks in 1 and 2. They show balanced NLO performances, including phase-matchable moderate NLO responses (0.7 and 0.5 × AGS) and enhanced laser-induced damage thresholds (4.5 and 5.1 × AGS). Theoretical calculations reveal that their NLO responses are predominantly contributed by the AgQ4 and InQ4 units.

Rational design via dual-site aliovalent substitution leads to an outstanding IR nonlinear optical material with well-balanced comprehensive properties

The acquisition of a non-centrosymmetric (NCS) structure and achieving a nice trade-off between a large energy gap (E g > 3.5 eV) and a strong second-harmonic generation (SHG) response (d eff > 1.0 × benchmark AgGaS2) are two formidable challenges in the design and development of infrared nonlinear optical (IR-NLO) candidates. In this work, a new quaternary NCS sulfide, SrCdSiS4, has been rationally designed using the centrosymmetric SrGa2S4 as the template via a dual-site aliovalent substitution strategy. SrCdSiS4 crystallizes in the orthorhombic space group Ama2 (no. 40) and features a unique two-dimensional [CdSiS4]2- layer constructed from corner- and edge-sharing [CdS4] and [SiS4] basic building units (BBUs). Remarkably, SrCdSiS4 displays superior IR-NLO comprehensive performances, and this is the first report on an alkaline-earth metal-based IR-NLO material that breaks through the incompatibility between a large E g (>3.5 eV) and a strong phase-matching d eff (>1.0 × AgGaS2). In-depth mechanism explorations strongly demonstrate that the synergistic effect of distorted tetrahedral [CdS4] and [SiS4] BBUs is the main origin of the strong SHG effect and large birefringence. This work not only provides a high-performance IR-NLO candidate, but also offers a feasible chemical design strategy for constructing NCS structures.

Broad transparency and wide band gap achieved in a magnetic infrared nonlinear optical chalcogenide by suppressing d-d transitions

Magnetic infrared (IR) nonlinear optical (NLO) materials, particularly those containing d-block metals, have attracted considerable attention due to the contributions of d-orbitals to large NLO efficiency. However, the d-d transitions from the d-block metals lead to strong optical absorption and narrow band gap, seriously hindering their practical applications. The structural flexibility of salt-inclusion systems provides a good opportunity for modulating the crystal field of magnetic ions to suppress the d-d transitions but allowing the NLO-active d-s and d-p transitions. These ideas afford a new salt-inclusion sulfide [K₃Cl][Mn₂Ga₆S₁₂], which features a rare nanoporous [MnGa₃S₆]^- framework with tunnels of inner diameter of 9.0 Å and possesses a broad transparency (0.39-25.0 μm) and the widest band gap (3.17 eV) among all magnetic IR NLO chalcogenides. Remarkably, it exhibits a strong phase-matchable second-harmonic generation intensity (0.8 × AgGaS₂ at 1910 nm and 3.1 × AgGaS₂ at 1064 nm) and a high laser-induced damage threshold (12.5 × AgGaS₂ at 1064 nm), achieving the important criteria of an advanced IR NLO material.

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.

Two Nonlinear Optical Thiophosphates Cu5Hg0.5P2S8 and AgHg3PS6 Activated by Their Tetrahedra-Stacking Architecture

Infrared (IR) lasers are very critical in military and civil affairs, but it is also challenging and difficult to develop new infrared nonlinear optical (NLO) crystals. Herein, two new mixed-metal thiophosphates, Cu₅Hg_0.5P₂S₈ and AgHg₃PS₆ were discovered with the noncentrosymmetric (NCS) space group Pmn2₁ (No. 31) and Cc (No. 9). Cu₅Hg_0.5P₂S₈ displays a three-dimensional (3D) defective diamond-like structure stacked by _∞²[Cu_2.5Hg_0.25PS₈]^8- layers. AgHg₃PS₆ is characterized by a 3D framework consisting of distorted tetrahedrons. Moreover, the optical spectra show the band gaps of Cu₅Hg_0.5P₂S₈ and AgHg₃PS₆ are 2.12 and 1.85 eV, respectively, with a broad transparent range of 0.7-16.0 μm. In these two compounds, the dipole moments of nonlinear active units are strengthened due to the high-valence element P and the Hg-S bonds with large susceptibility. Therefore, AgHg₃PS₆ exhibits a moderate second harmonic generation (SHG) response that is half that of AgGaS₂ (AGS) at 30-45 μm, while Cu₅Hg_0.5P₂S₈ performs a phase-matching (PM) behavior with a good SHG signal of 0.8 × AGS at 150-200 μm. The origin of NLO performance and electronic structures were revealed by the calculated dipole moments of distorted tetrahedra and theoretical calculations on the basis of density functional theory.

Five coordinated Mn in Ba4Mn2Si2Te9: synthesis, crystal structure, physical properties, and electronic structure

A new structure type Ba4Mn2Si2Te9 containing unique MnTe5 units is synthesized. The structure comprises two independent Mn atoms, each with 50% occupancy. It is a narrow bandgap semiconductor (Eg = 0.6(1) eV) consistent with the DFT studies.

Cd7SiAs6, a Nonchalcopyrite Arsenide with a Strong Nonlinear-Optical Response

A new arsenide, Cd₇SiAs₆, has been successfully synthesized and characterized. It is the first arsenide that adopts a nonchalcopyrite structure and possesses a strong nonlinear-optical (NLO) response. In the structure, the CdAs₃ trigonal planar unit, a kind of π-conjugated planar NLO-active group, was identified for the first time. Furthermore, theoretical calculations reveal that the CdAs₃ planar unit contributes more to the NLO effect than the CdAs₄ tetrahedron does. The result may provide valuable insights for the future exploration of IR NLO materials, especially for application above 10 μm.

Cu4MnGe2S7 and Cu2MnGeS4: two polar thiogermanates exhibiting second harmonic generation in the infrared and structures derived from hexagonal diamond

The new, quaternary diamond-like semiconductor (DLS) Cu₄MnGe₂S₇ was prepared at high-temperature from a stoichiometric reaction of the elements under vacuum. Single crystal X-ray diffraction data were used to solve and refine the structure in the polar space group Cc. Cu₄MnGe₂S₇ features [Ge₂S₇]^6- units and adopts the Cu₅Si₂S₇ structure type that can be considered a derivative of the hexagonal diamond structure. The DLS Cu₂MnGeS₄ with the wurtz-stannite structure was similarly prepared at a lower temperature. The achievement of relatively phase-pure samples, confirmed by X-ray powder diffraction data, was nontrival as differential thermal analysis shows an incongruent melting behaviour for both compounds at relatively high temperature. The dark red Cu₂MnGeS₄ and Cu₄MnGe₂S₇ compounds exhibit direct optical bandgaps of 2.21 and 1.98 eV, respectively. The infrared (IR) spectra indicate potentially wide windows of optical transparency up to 25 μm for both materials. Using the Kurtz-Perry powder method, the second-order nonlinear optical susceptibility, χ⁽²⁾, values for Cu₂MnGeS₄ and Cu₄MnGe₂S₇ were estimated to be 16.9 ± 2.0 pm V^-1 and 2.33 ± 0.86 pm V^-1, respectively, by comparing with an optical-quality standard reference material, AgGaSe₂ (AGSe). Cu₂MnGeS₄ was found to be phase matchable at λ = 3100 nm, whereas Cu₄MnGe₂S₇ was determined to be non-phase matchable at λ = 1600 nm. The weak SHG response of Cu₄MnGe₂S₇ precluded phase-matching studies at longer wavelengths. The laser-induced damage threshold (LIDT) for Cu₂MnGeS₄ was estimated to be ∼0.1 GW cm^-2 at λ = 1064 nm (pulse width: τ = 30 ps), while the LIDT for Cu₄MnGe₂S₇ could not be ascertained due to its weak response. The significant variance in NLO properties can be reasoned using the results from electronic structure calculations.

Synergistic Effect of π-Conjugated [C(NH2)3] Cation and Sb(III) Lone Pair Stereoactivity on Structural Transformation and Second Harmonic Generation

The search for nonlinear optical (NLO) crystals with excellent comprehensive properties is a formidable challenge. In this work, two guanidine antimony fluorides, C(NH₂)₃Sb₂F₇ and C(NH₂)₃SbF₄, were obtained by conjunction of [C(NH₂)₃] groups with π-conjugated configuration and stereochemically active Sb³⁺ cations. Due to the different coordination modes of Sb-F bonds and H-F hydrogen bonds, the crystal structure of C(NH₂)₃Sb₂F₇ is centrosymmetric (CS), while C(NH₂)₃SbF₄ is noncentrosymmetric (NCS). Optical measurements show that the UV cutoff wavelengths of the title compounds were both less than 240 nm. Thermal studies indicate that these crystals are stable up to 250 °C. In addition, the second harmonic generation (SHG) response of C(NH₂)₃SbF₄ is 2 times that of KH₂PO₄ (KDP) with the phase-matchable capacity. Theoretical calculations reveal that the large SHG effects of C(NH₂)₃SbF₄ were attributed to the synergy between the planar [C(NH₂)₃] units and the distorted [SbF₄] groups. These results demonstrate that the guanidine antimony fluorides will have potential value as UV NLO materials.

Synergism of multiple functional chromophores significantly enhancing the birefringence in layered non-centrosymmetric chalcohalides

Compared with one or two functional chromophores materials, Hg₃AsQ₄X (Q = S, Se; X = Cl, Br, I) with multiple ones generate extremely large birefringence due to the synergism of the d¹⁰ cation Hg²⁺, lone pair layer of As³⁺ and mixed anions Q²⁻/X⁻.

Bi32Cd3P10O76: a new congruently melting nonlinear optical crystal with a large SHG response and a wide transparent region

Large size single crystal Bi₃₂Cd₃P₁₀O₇₆ with a large SHG response (4 × KDP) and a wide transmission range covering 0.36–4.9 μm.

EuHgGeSe4 and EuHgSnS4: Two Quaternary Eu-Based Infrared Nonlinear Optical Materials with Strong Second-Harmonic-Generation Responses

Metal chalcogenides play a critical role in the infrared (IR) nonlinear optical (NLO) field. However, Eu-based chalcogenide-type IR NLO materials are still scarce up to now. In this paper, two new quaternary Eu-based chalcogenides, EuHgGeSe₄ and EuHgSnS₄, containing the "NLO active groups" [HgQ₄]^6- (Q = S, Se) and [GeSe₄]^4-/[SnS₄]^4- were synthesized through traditional high-temperature solid-state reactions. They possess noncentrosymmetric structures, crystallizing in the Ama2 space group, and exhibit strong phase-matchable second-harmonic-generation (SHG) responses (3.1× and 1.77× that of AgGaS₂ for EuHgGeSe₄ and EuHgSnS₄, respectively). Meanwhile, the optical band gaps of EuHgGeSe₄ (1.97 eV) and EuHgSnS₄ (2.14 eV) were determined from UV-vis-NIR diffuse reflectance spectra. Differential scanning calorimetry (DSC) analyses reveal the congruent-melting behavior of EuHgGeSe₄. Furthermore, structural analysis and theoretical calculations verify the critical driving effects of [HgQ₄]^6- tetrahedra on the strong SHG activity. The overall results demonstrate that EuHgGeSe₄ and EuHgSnS₄ are potential IR NLO materials.

Non-centrosymmetric sulfides A2Ba6MnSn4S16 (A = Li, Ag): syntheses, structures and properties

Li₂Ba₆MnSn₄S₁₆ and Ag₂Ba₆MnSn₄S₁₆ are synthesized for the first time. By modulating disordered sites with Mn²⁺, they not only show strong SHG responses and wide band gaps (5.1, 2.7 × AgGaS₂; 2.88, 2.76 eV), but also possess paramagnetism.