Rare-earth-based chalcogenides and their derivatives: an encouraging IR nonlinear optical material candidate.

With the continuous development of laser technology and the increasing demand for lasers of different frequencies in the infrared (IR) spectrum, research on infrared nonlinear optical (NLO) crystals has garnered growing attention. Currently, the three main commercially available types of borate materials each have their drawbacks, which limit their applications in various areas. Rare-earth (RE)-based chalcogenide compounds, characterized by the unique f-electron configuration, strong positive charges, and high coordination numbers of RE cations, often exhibit distinctive optical responses. In the field of IR-NLO crystals, they have a research history spanning several decades, with increasing interest. However, there is currently no comprehensive review summarizing and analyzing these promising compounds. In this review, we categorize 85 representative examples out of more than 400 non-centrosymmetric (NCS) compounds into four classes based on the connection of different asymmetric building motifs: (1) RE-based chalcogenides containing tetrahedral motifs; (2) RE-based chalcogenides containing lone-pair-electron motifs; (3) RE-based chalcogenides containing [BS3] and [P2Q6] motifs; and (4) RE-based chalcohalides and oxychalcogenides. We provide detailed discussions on their synthesis methods, structures, optical properties, and structure-performance relationships. Finally, we present several favorable suggestions to further explore RE-based chalcogenide compounds. These suggestions aim to approach these compounds from a new perspective in the field of structural chemistry and potentially uncover hidden treasures within the extensive accumulation of previous research.

Ba10In2Mn11Si3O12S18: First Hexanary Oxychalcogenide Containing an Infrequent Three-Dimensional Noncentrosysmmetric Framework.

Noncentrosymmetric (NCS) oxychalcogenides have attracted great attention in recent years due to their immense potential as candidates for IR nonlinear-optical (NLO) applications. Despite notable advancements in this field, the discovery of oxychalcogenides with three-dimensional (3D) framework structures remains a formidable challenge. In this study, we report the discovery of the first hexanary oxychalcogenide, Ba10In2Mn11Si3O12S18, exhibiting second-order NLO activity, using a high-temperature solid-phase method. This compound showcases a novel structure type, featuring an uncommon NCS 3D [In2Mn11Si3O12S18]20- framework formed by vertex-sharing [(Mn/In)S6] octahedra, [(Mn/In)OS3] tetrahedra, and [SiO4] tetrahedra, with charge-balanced Ba2+ cations occupying the channels. Our study serves as a source of inspiration for researchers to further investigate the synthesis of novel NLO-active oxychalcogenides with 3D frameworks.

Breaking Through the Trade-Off Between Wide Band Gap and Large SHG Coefficient in Mercury-Based Chalcogenides for IR Nonlinear Optical Application.

It is substantially challenging for non-centrosymmetric (NCS) Hg-based chalcogenides for infrared nonlinear optical (IR-NLO) applications to realize wide band gap (Eg > 3.0 eV) and sufficient phase-matching (PM) second-harmonic-generation intensity (deff > 1.0 × benchmark AgGaS2 ) simultaneously due to the inherent incompatibility. To address this issue, this work presents a diagonal synergetic substitution strategy for creating two new NCS quaternary Hg-based chalcogenides, AEHgGeS4 (AE = Sr and Ba), based on the centrosymmetric (CS) AEIn2 S4 . The derived AEHgGeS4 displays excellent NLO properties such as a wide Eg (≈3.04-3.07 eV), large PM deff (≈2.2-3.0 × AgGaS2 ), ultra-high laser-induced damage threshold (≈14.8-15 × AgGaS2 ), and suitable Δn (≈0.19-0.24@2050 nm), making them highly promising candidates for IR-NLO applications. Importantly, such excellent second-order NLO properties are primarily attributed to the synergistic combination of tetrahedral [HgS4 ] and [GeS4 ] functional primitives, as supported by detailed theoretical calculations. This study reports the first two NCS Hg-based materials with well-balanced comprehensive properties (i.e., Eg > 3.0 eV and deff > 1.0 × benchmark AgGaS2 ) and puts forward a new design avenue for the construction of more efficient IR-NLO candidates.

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 (Eg ) and strong second harmonic generation (SHG) response (deff ), 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 Eg (>3.5 eV) and large deff (>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 Eg and strong deff are pursued.

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.

Oxychalcogenides as Promising Ultraviolet Nonlinear Optical Candidates: Experimental and Theoretical Studies of AEGeOS2 (AE = Sr and Ba).

Oxychalcogenides have gained widespread attention as promising infrared nonlinear optical (IR-NLO) candidates. However, high-performance oxychalcogenides have rarely been reported in the ultraviolet (UV) region owing to the low energy gaps (Eg < 4.0 eV). Herein, two non-centrosymmetric (NCS) oxychalcogenides with one-dimensional (1D) chain structures and wide Eg (>4.3 eV), namely, AEGeOS2 (AE = Sr and Ba), have been discovered by combined experiments and theory calculations as a new source of UV-NLO materials. Significantly, they exhibit excellent comprehensive performance comparable to the commercial UV-NLO material KH2PO4 (KDP), including large phase-matching ranges (>380 nm), sufficient second harmonic generation intensities (0.7-1.1 × KDP), high laser-induced damage thresholds (1.2 × KDP), wide transparent regions (0.26-12.2 µm), and good thermal stability (up to 1100 K). Moreover, systematic structure-activity relationship analysis illustrates that the 1D homochiral helical [GeOS2]2- chains composed of heteroanionic [GeS2O2] units make major contribution to the desirable UV-NLO performance. This work makes the two compounds shine out as new energy in the UV-NLO field and offers a new perspective for the exploration of structure-driven functional oxychalcogenides.

The Rise of Infrared Nonlinear Optical Pnictides: Advances and Outlooks.

Infrared (IR) nonlinear optical (NLO) materials are the core devices to realize IR laser output, which are of vital importance in civilian and military fields. Non-centrosymmetric chalcogenide and pnictide compounds have already been widely accepted as favorable systems for IR-NLO materials. Compared to the extensively investigated IR-NLO chalcogenides during the past few decades, the research of non-centrosymmetric phosphides as IR-NLO materials is relatively scarce. In this frontier article, the recent progress of pnictides as emerging IR-NLO candidates has been highlighted based on the perspective of new crystal exploration. These IR-NLO pnictides recently reported were divided into three groups from binary to quaternary according to their chemical compositions. The synthetic methods, structural chemistry, and structure-activity relationships are analyzed and summarized in detail. Finally, current problems and the future development of this field are also proposed.

Quaternary Chalcohalides CdSnSX2 (X = Cl or Br) with Neutral Layers: Syntheses, Structures, and Photocatalytic Properties.

Inorganic chalcohalides are attracting a tremendous amount of attention because of their remarkable structural variety and desirable physical properties. Although great advances have been made in recent years, functional inorganic chalcohalides with two-dimensional neutral layers are still rare. Herein, two novel chalcohalides CdSnSX2 (X = Cl or Br) with high yields were obtained by reacting CdX2 with SnS using a traditional solid-state method at 823 K. Both of these chalcohalides adopt orthorhombic space group Cmcm (No. 63) with the following structural values: a = 4.014(4)-4.064(2) Å, b = 12.996(2)-13.746(3) Å, c = 9.471(2)-9.621(2) Å, V = 494.1(8)-537.5(2) Å3, and Z = 4. The prominent architectural feature is the unique two-dimensional [CdSnSX2] neutral layer consisting of composite [CdX2] and [SnS] sublattices that are connected alternately through the Cd-S-Sn bonds along the ac plane. The [CdX2] sublattice consists of a single octahedral chain of Cd-centered [CdX4S2] groups sharing cis-X edges, while the [SnS] sublattice consists of a bend-shaped chain of unusual [SnS2X2] units sharing vertices of S atoms. Significantly, each CdSnSX2 form (X = Cl or Br) shows high visible-light-induced photocatalytic activity for rhodamine B degradation, which is ∼7.0 times higher than that of nitrogen-doped TiO2 (TiO2-xNx) under the same experimental conditions. This discovery enriches the categories of inorganic chalcohalides and provides more choices of candidate materials for photocatalytic applications.