From CaBaM2F12 to K2BaM2F12 (M = Zr, Hf): Heterovalent Cation-Substitution-Induced Symmetry Break and Nonlinear-Optical Activity.

Designing short-wavelength nonlinear-optical (NLO) crystals is of vital importance for laser applications. Here, the combination of alkaline-earth metals, d0 transition metals, and F atom has generated two new and isostructural fluorides, CaBaZr2F12 (CBZF) and CaBaHf2F12 (CBHF), which adopt centrosymmetric space group I4/mmm. Taking CBZF and CBHF as the parents, two new fluorides, K2BaZr2F12 (KBZF) and K2BaHf2F12 (KBHF), with an Imm2 polar structure were obtained via a heterovalent cation substitution strategy. All four compounds feature ZrF8-dodecahedra-built {[Zr2F12]4-}∞ chains and show short ultraviolet cutoff edges (<200 nm). KBZF and KBHF show phase-matchable behavior with moderate second-harmonic-generation responses [0.6 and 0.35 × KH2PO4 (KDP)] under 1064 nm laser radiation. This work enriches fluorides as promising short-wavelength NLO materials.

Pentanary Oxythiogermanates Ba3MGe3O2S8 (M = Ca, Zn) Featuring [Ge3O2S8]8- Trimers and {[MGe3O2S8]6-}∞ Chains: Structural Chemistry and Physical Properties.

Oxychalcogenides are increasingly attracting wide attention because they contain multiple anions that may combine the advantages of oxides and chalcogenides. In this work, two new pentanary oxythiogermanates, Ba3MGe3O2S8 [M = Ca (1), Zn (2)], were synthesized by a high-temperature solid-state reaction. They crystallize in the orthorhombic space group Pnma, and their structures contain isolated [Ge3O2S8]8- units constructed by one [GeO2S2] and two [GeOS3] tetrahedra that link with M2+ ions to build the {[MGe3O2S8]6-}∞ chain, representing a new type of oxythiogermanate. Notably, a [ZnS5] square pyramid exists in 2. Their structural chemistry and relationship with relevant structures are analyzed. 1 and 2 exhibit wide band gaps of 3.93 and 2.63 eV, birefringences of 0.100 and 0.089 at 2100 nm, respectively, and also obvious photocurrent responses. This work may be extended to a family of AE3MIIMIV3O2Q8 (AE = alkali-earth metal; MII = Ca, Zn, Cd, Hg; MIV = Si, Ge, Sn; Q = S, Se), and further systematic survey on them can be performed to enrich the study of multifunctional oxychalcogenides.

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.

KREP2S6 (RE = Sm, Gd, Tb, Dy): A Series of Polar Rare-Earth Thiophosphates with High Laser-Induced Damage Thresholds and Moderate Nonlinear-Optical Responses Synergistically Contributed by Isolated P2S6 Units and {[RE2S15]24-}∞ Layers.

As one of the potential candidates of nonlinear-optical (NLO) materials, rare-earth chalcophosphates have demonstrated promising properties. Here, KREP2S6 (RE = Sm, Gd, Tb, Dy) were synthesized using the facile RE2O3-B-S solid-state method. They crystallize with a monoclinic chiral P21 structure, and their layer structures are built by isolated ethane-like P2S6 dimers and RES8 bicapped trigonal prisms built {[RE2S15]24-}∞ layers. By comparing the structures with related ones, the change of the alkali metal or RE3+ ions can cause structural transformation. Their band gaps are tunable between 2.58 and 3.79 eV, and their powder samples exhibit good NLO properties. Theoretical calculations suggest that the NLO properties are mainly contributed by P2S6 units and {[RE2S15]24-}∞ layers synergistically, in which {[RE2S15]24-}∞ layers and P2S6 units dominate the contribution to the band gap and second-harmonic-generation response, respectively. This work enriches the application of rare-earth chalcophosphates as NLO materials.

Hg16O12(NO3)6F2(H2O): A Mercury-Rich Nitrate Oxyfluoride with Diverse Triple-Coordinated Mercury-Based Units That Exhibits an Unparalleled [(Hg16O12F2(H2O))6+]∞ Cationic Framework and a Large Birefringence.

Designing new compounds based on anion regulation has been widely favored due to the production of diverse crystal structures and excellent optical properties. Here, a new nitrate oxyfluoride, Hg16O12(NO3)6F2(H2O), has been obtained through a hydrothermal reaction. It crystallizes in the centric Ibca space group and shows a novel three-dimensional [(Hg16O12F2(H2O))6+]∞ cationic framework composed of interconnected HgO2F, HgO3, and HgO2(H2O) units, with isolated NO3- groups as balanced anions to build the whole structure. Notably, the HgO2F and HgO2(H2O) units are first presented here among mercury (Hg)-based compounds. Additionally, Hg16O12(NO3)6F2(H2O) exhibits a large birefringence of 0.17 at 546 nm. This work enriches the multiformity of Hg-based compounds and provides a route for developing promising birefringent materials.

Polysubstitution Induced Centrosymmetric-to-Noncentrosymmetric Structural Transformation and Nonlinear-Optical Behavior: The Case of Na0.45Ag0.55Ga3Se5.

Obtaining compounds with large nonlinear-optical (NLO) coefficients and wide band gaps is challenging due to their competitive requirements for chemical bonds. Herein, the first member with mixed cations on the A site in the A-M3-Q5 or A-Ag-M6-Q10 (A = alkali metal; M = Ga, In; Q = S, Se, Te) family, viz. Na0.45Ag0.55Ga3Se5 (NAGSe), was obtained by a solid-state reaction. Its structure features [GaSe4] tetrahedra built three-dimensional {[Ga3Se5]-}∞ network, with Na and Na/Ag cations located at the octahedral cavities. Noncentrosymmetric (R32) NAGSe can also be transformed from centrosymmetric RbGa3S5 (P21/c) via multiple-site cosubstitution. NAGSe exhibits the highest NLO response (1.9 × AGS) in the A-Ag-M-Q family. Crystal structure analysis and theoretical calculations suggest that the NLO response is mainly contributed by the regularly arranged [GaSe4] units. This work enriches the exploration of the undeveloped A-M3-Q5 or A-Ag-M6-Q10 family as potential infrared NLO materials.

Three-in-One Strategy Constructing the First High-Performance Nonlinear Optical Sulfide Crystallizing with the P43 Space Group.

The balance between large nonlinear optical (NLO) effect and wide bandgap is the key scientific issue for the exploration of infrared NLO materials. Targeting this issue, two new pentanary chalcogenides KGaGe1.37 Sn0.63 S6 (1) and KGaGe1.37 Sn0.63 Se6 (2) are obtained by the three-in-one strategy, viz. three types of fourfold-coordinated metal elements co-occupying the same site. They crystallize in the tetragonal P43 (1) and monoclinic Cc (2) space group. Their structures can be evolved from benchmark AgGaS2 (AGS) by suitable substitution. Remarkably, 1 is the first NLO sulfide crystallizing with the P43 space group, representing a new structure-type NLO material. The structural relationship between 1 and 2 and the evolution from 1, 2 to AGS are also analyzed. Both 1 and 2 show balanced NLO properties. Specifically, 1 exhibits phase-matchable SHG response of 0.6 × AGS, a wide bandgap of 3.50 eV, and a high laser damage threshold of 6.24 × AGS. Theoretical calculation results suggest that the Ga/Ge/Sn element ratios of the co-occupied sites of 1 and 2 are the most appropriate for stabilizing the structures. The strategy adopted here will provide some inspiration for exploring new high-performance NLO materials.

KREP2 Se6 (RE = Sm, Gd, Tb): The First Rare-Earth Selenophosphates with Remarkable Nonlinear Optical Activities Realized by Synergistic Effect of RE- and P-Based Motifs.

Rare-earth (RE) chalcogenides have been extensively studied as infrared nonlinear optical (NLO) materials because of their nice integrated performances; however, very few RE chalcophosphates are involved for this topic. Here, three quaternary RE selenophosphates, KSmP2 Se6 (1), KGdP2 Se6 (2), and KTbP2 Se6 (3), are profoundly studied for their NLO potentials. Their noncentrosymmetric P21 structures feature RESe8-bicapped trigonal prisms and ethane-like [P2 Se6 ]4 - dimers built {[REP2 Se6 ]-}∞ layers. As the first studied NLO-active RE selenophosphates, 1-3 exhibit second harmonic generation (SHG)responses ≈0.34-1.08 × AgGaS2 at 2.10 µm and laser-induced damage thresholds (LIDTs) ≈1.43-4.33 × AgGaS2 , and they all show phase-matchable behaviors, indicating their wonderful balanced NLO properties. Theoretical calculations demonstrate that the synergistic effect between RESe8 and P2 Se6 units makes the major contribution to the SHG responses.

Three-in-One Tetrahedral Functional Units Constructing Diamondlike Ag2In2SiS3.06Se2.94 with High-Performance Nonlinear-Optical Activity.

Exploration of new functional materials with enhanced performance from known ones is always an attractive strategy. A new infrared (IR) nonlinear-optical (NLO) mixed chalcogenide Ag2In2SiS3.06Se2.94 (1), was obtained through partial congener substitution originated from Ag2In2SiS6 (0). 1 crystallizes in the monoclinic space group Cc, and its three-dimensional (3D) polyanionic network is composed of {[In4Si2Se5(S/Se)11]12-}∞ helical chains sharing S/Se(5) corner atoms with cavities embedded with counterion Ag+ ions. It exhibits a much enhanced NLO response compared to that of 0, reaching 1.1 × AgGaS2. Further theoretical analysis results indicate that the large NLO response can be attributed to the synergistic effect of AgQ4 and InQ4 tetrahedral functional motifs. This work not only reports a new high-performance IR NLO material but also enriches the partial ion substitution strategy to obtain new functional materials.

KHg4Ga3S9: A Hg-Based Sulfide with Nonlinear-Optical Activity in the A-MII-MIII-Q (A = Alkali Metal; MII = d10 Metal; MIII = Ga, In; Q = S, Se) System.

The search for new high-performance infrared (IR) nonlinear-optical (NLO) materials is a hot topic in the fields of laser chemistry and inorganic solid-state chemistry. Here, a new Hg-based sulfide KHg4Ga3S9 in the family of A-MII-MIII-Q (A = alkali metal; MII = d10 metal; MIII = Ga, In; Q = S, Se) was synthesized. It crystallizes in the orthogonal system of the C2221 structure, which is rare for IR NLO chalcogenides. Its anionic framework {[Hg4Ga3S9]-}∞ is constructed by two types of interconnected helical chains, viz., the inner layer ({[Hg6Ga2S29/3]4/3-}∞) and the outer layer ({[Hg2Ga4S25/3]2/3-}∞). It exhibits a moderate NLO response and a high laser-induced damage threshold. Theoretical calculations indicate that the HgS4 unit accounts for its much larger NLO response compared to RbCd4Ga3S9. The influence of alkali metals and d10 metals on the initial phase-matching wavelength is also discussed. This work provides inspiration for improving the properties of NLO materials' properties.

NaREP2Se6 (RE = Y, Sm, Gd-Dy): Quaternary Rare-Earth Selenophosphates with Unique 3D {[REP2Se6]-}∞ Framework Built by RESe8 and P2Se6 Motifs and Multiple Properties.

Rare-earth (RE) chalcophosphates have been widely studied because of their abundant structures. Here, five new RE selenophosphates, NaREP2Se6 (RE = Y, Sm, Gd-Dy), were synthesized by a facile RE oxide-boron-selenium solid-state route. They crystallize in the triclinic P1̅ space group, featuring three-dimensional (3D) structures constructed by RESe8 and P2Se6 motifs, different from common 2D RE chalcophosphates A-RE-P2-Q6 (A = alkali metal; Q = S, Se) system. Their structural chemistry and relationship with related phases are analyzed. Both the size of A and the coordination geometry of RE have important influences on the system's structures. Their optical band gaps are tunable from 1.79 to 2.50 eV, and they exhibit diverse magnetic behaviors, including Van-Vleck-type paramagnetism, antiferromagnetism, and ferromagnetism. Their photocurrent responses and thermal stabilities are analyzed as well. Calculation results suggest that the RESe8 and P2Se6 units make a great contribution to the optical properties. This work enriches the chemistry and multifunctional properties of RE chalcophosphates.

Mixed Rare-Earth Chalcogenide Borate Eu9-xRExMgS2B20O41 (RE = Sm, Gd) Featuring a 3D {[B20O41]22-}∞ Framework Connected by [B6O9(O0.5)6]6- and [B7O13(O0.5)3]8- Clusters.

Rare-earth (RE) chalcogenide borates are very rarely discovered in view of the difficulties in synthesis though they have demonstrated attractive physical performances. Here, the first mixed RE chalcogenide borates Eu5.4Sm3.6MgS2B20O41 (1) and Eu3Gd6MgS2B20O41 (2) are synthesized by combining RE, sulfur, and borate ions into one structure. They crystallize in the centrosymmetric hexagonal space group P63/m, and their 3D honeycomb-like {[B20O41]22-}∞ open frameworks are built by [B6O9(O0.5)6]6- and [B7O13(O0.5)3]8- polyanionic clusters and consolidated by Mg2+ ions; both of which are formed by BO4 tetrahedra and BO3 planar triangles. The coordination modes of RE ions are rare REO6S2 bicapped trigonal prisms and REO8S irregular polyhedra, and their band gaps are determined to be 2.25 and 2.22 eV, respectively. They exhibit antiferromagnetic interactions and distinct photocurrent responses. The corresponding theoretical calculations are also performed. The study of 1 and 2 perhaps stimulates interest in exploring new functional RE chalcogenide borates.

Eu2MGe2OS6 (M = Mn, Fe, Co): Three Melilite-Type Rare-Earth Oxythiogermanates Exhibiting Balanced Nonlinear-Optical Behaviors.

Metal oxychalcogenides as candidates for novel mid-infrared nonlinear-optical materials have attracted great interest due to the distinctive advantages of oxides and chalcogenides in this field. Herein, the first melilite-type rare-earth (RE) oxythiogermanates Eu2MGe2OS6 [M = Mn (1), Fe (2), Co (3)] are obtained by combining RE metals with localized f electrons, magnetic transition metals with delocalized d electrons, and the highly distorted mixed anionic group [GeOS3] into one structure. They belong to the tetragonal P4̅21m space group, and highly distorted [EuOS7] bicapped trigonal prisms bridge adjacent {[MGe2OS6]4-}∞ layers to build the three-dimensional network. Their optical band gaps are determined as 2.40, 2.11 and 2.14 eV, and they show moderate second-harmonic-generation (SHG) responses (0.3, 0.3 and 0.5 × AGS) and large laser-induced damage thresholds (2.77-8.31 × AGS). Theoretical calculation results indicate that the synergistic effect of [EuOS7] and [MS4] units acts on the SHG effect. This work enriches the crystal chemistry of melilite-structure materials.

Dual Monomeric Inorganic Units Constructed Bright Emissive Zero-Dimensional Antimony Chlorides with Solvent-Induced Reversible Structural Transition.

A5M2X11 and A3M2X9 families (A = monovalent organic cation; M = trivalent metal; X = halogen) are receiving increasing attention because of their combination of easy solution processability and superior ferroelectricity properties. However, synthesizing highly efficient A5M2X11 and A3M2X9-type fluorophores with multiple monomeric inorganic units and achieving their structural interconversion remains challenging. Here, we report two novel zero-dimensional (0D) antimony halides, (C10H16N)5Sb2Cl11·C2H3N (1) and (C10H16N)3Sb2Cl9 (2), which not only contain two distinct [SbXn]3-n units but also have excellent orange (590 nm) and yellow-green emission (540 nm) with high PLQY of 17.7% and 31.5%, respectively. Interestingly, a reversible structural conversion could be triggered by acetonitrile steam stimulation, accompanied by luminescence switching properties. This work not only enriches the structure of hybrid Sb-based halides but also provides the possibility of well-known A5M2X11 and A3M2X9 families as structural transformation materials.

Multi-Stimuli Responsive Luminescence and Domino Phase Transition of Hybrid Copper Halides with Nonlinear Optical Switching Behavior.

Although phase transition materials (PTMs) under external stimuli are of great research interest duo to their rich potential applications, it is still challenging to explore multi-responsive PTMs. Herein, two different phases of organic-inorganic hybrid copper-based halides, α- and ß-Gua3 Cu2 I5 (Gua+ =CN3 H6 + ), were synthesized by solvent evaporation method, which they crystalize in the noncentrosymmetric space group Fdd2 with zero-dimensional structure and centrosymmetric space group C2/c with one-dimensional metal-halogen framework, respectively. Interestingly, it is firstly demonstrated that Gua3 Cu2 I5 simultaneously possesses reversible PL conversion and NLO switching properties in response to thermal stimulus. Strikingly, apart from heat, its structural phase transition can also be triggered by crystalline-phase-recognition (CPR) and mechanical force. These new findings may pave a path for future exploration of PTMs with multiple physical properties.

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

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.

Second-Harmonic-Generation-Active Oxyhalides: CuSb2O3X (X = Cl, Br).

Metal oxyhalides have attracted broad interest recently because of their diverse structures and versatile properties. Here, two oxyhalides, CuSb2O3Cl (1) and CuSb2O3Br (2), were studied by focusing on their nonlinear-optical properties. They are crystallized in the noncentrosymmetric monoclinic Cc structure, and the layered structures could be derived from a 1:1 combination of CuX- (X = Cl, Br) and Sb2O3-type slabs. Their energy gaps were determined to be 2.76 and 2.64 eV. The second-harmonic-generation (SHG) test suggests that they are nonlinear-optical-active, and the effects are ascribed to the contribution of CuX3O units. Meanwhile, the SbO3 units' arrangement has a small contribution to the SHG effects. This work is the pioneer SHG investigation of the MI-MIII-O-X (MI = Cu, Ag; MIII = trivalent As, Sb, Bi; X = halogen) family.

Three-in-One Strategy Constructing a Series of Hybrid Tetrahedral Motif-Based Selenides with Balanced Second-Order Nonlinear Optical Performance.

Concurrently achieving suitable second harmonic generation (SHG) effect and high laser-induced damage threshold (LIDT) is challenging for infrared nonlinear optical (NLO) materials. Here, a series of pentanary infrared NLO materials CsMIIIMIVSnSe6 (MIII = Ga, In; MIV = Si, Ge) have been obtained by a three-in-one strategy, viz. three kinds of elements (MIII, MIV, and Sn) in one position, which is first adopted to design NLO materials. Their three-dimensional structures are constructed by the MQ4 (M denotes MIII, MIV, and Sn) tetrahedral units. They exhibit promising hybrid NLO properties, witnessed by their moderate/large SHG effects of 0.52, 0.98, 1.05, and 1.12 × AgGaS2, and high powder LIDT values of 6.9, 4.1, 8.1, and 5.4 × AgGaS2, respectively. These NLO properties are well verified by the DFT calculation results. The three-in-one strategy of designing high-performance infrared NLO materials will stimulate more investigations in this field.

Stepwise Li Substitution Induced Structure Evolution and Improved Nonlinear Optical Performance for Diamond-like Sulfides.

One of the key scientific issues for exploration of novel infrared nonlinear optical (NLO) crystals is to obtain ones with good hybrid NLO behaviors. Herein, we report four diamond-like Ag-based sulfides via stepwise Li substitution of Ag in Ag2ZnSnS4, including Ag2ZnSnS4 (1), (Li1.22Ag0.78)ZnSnS4 (2), (Li1.58Ag0.42)ZnSnS4 (3), and Li2ZnSnS4 (4). With the increase of Li content, the sulfide's noncentrosymmetric crystal structure changes from tetragonal I42m for 1, to orthorhombic Pmn21 for 2 and 3, and to monoclinic Pn for 4. Accordingly, their NLO responses are improved along with the increase of Li content, viz. from non-NLO-active for 1, to non-phase-matchable for 2 and 3, and to phase-matchable for 4. Their optical band gaps also increase regularly. The relationship between their chemical compositions, crystal structures, and NLO activities is investigated by means of chemical structural analysis and theoretical calculation. This work offers a new systematic case for designing promising NLO-active compounds via rarely adopted cation's stepwise partial substitution and understanding the chemical composition-structure-NLO property relationship.

Introduction of Li into Ag-Based Noncentrosymmetric Sulfides for High-Performance Infrared Nonlinear Optical Materials.

The understanding of the structure-performance relationship is beneficial to establish the balance between the second-harmonic generation (SHG) response and laser-induced damage threshold (LIDT) in exploring novel nonlinear optical (NLO) materials. In this work, a facile strategy of partial Li co-occupation with Ag is employed to investigate the structures and NLO performances of the four Ag-based noncentrosymmetric chalcogenides Ag2In2SiS6 (1), (Li0.79Ag1.21)In2SiS6 (2), (Li1.12Ag0.88)In2SiS6 (3), and (Li1.44Ag0.56)In2SiS6 (4). Both the SHG intensity and LIDT are improved as the Li element is introduced, among which 4 exhibits the optimal phase-matchable SHG response of ∼0.8 × AgGaS2 (AGS) and a high LIDT of 4 × AGS. These NLO activities are explained by theoretical calculation and the dipole moment analysis.