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.

Flexible Anionic Groups-Activated Structure Dissymmetry for Strong Nonlinearity in Ln2 Ae3 MIV 3 S12 Family

Structural dissymmetry and strong second-harmonic generation (SHG) responses are key conditions for nonlinear optical (NLO) crystals, and targeted combinatorial screening of suitable anionic groups has become extremely effective. Herein, optimal combination of flexible SnSn (n = 5, 6) groups and highly electropositive cations (lanthanides (Ln3+ ) and alkaline earth (Ae2+ : Sr, Ca) metals) affords the successful synthesis of 12 NLO thiostannates including Ln2 Sr3 Sn3 S12 (Pmc21 ) and Ln2 Ca3 Sn3 S12 (P-62m); whereas 17 rigid GeS4 or SiS4 tetrahedra-constructed Ln2 Ae3 Ge3 S12 and Ln2 Ae3 Si3 S12 crystallize in the centrosymmetric (CS) Pnma. This unprecedented CS to noncentrosymmetric (NCS) structural transformation (Pnma to P-62m to Pmc21 ) in the Ln2 Ae3 MIV 3 S12 family indicates that chemical substitution of the tetrahedral GeS4 /SiS4 units with SnSn breaks the original symmetry to form the requisite NCS structures. Remarkably, strong polarization anisotropy and hyperpolarizability of the Sn(4+) S5 unit afford huge performance improvement from the nonphase-matching (NPM) SHG response (1.4 × AgGaS2 and Δn = 0.008) of La2 Ca3 Sn3 S12 to the strong phase-matching (PM) SHG effect (3.0 × AgGaS2 and Δn = 0.086) of La2 Sr3 Sn3 S12 . Therefore, Sn(4+) S5 is proven to be a promising "NLO-active unit." This study verifies that the coupling of flexible SnSn building blocks into structures opens a feasible path for designing targeted NCS crystals with strong nonlinearity and optical anisotropy.

Na2 Ba[Na2 Sn2 S7 ]: Structural Tolerance Factor-Guided NLO Performance Improvement

The strong mutual coupling of and even the opposite change in the key parameters, such as the band gap (E_g ) and second-order harmonic generation (SHG), leads to the extreme scarcity in high-performance IR nonlinear optical (NLO) chalcogenides. Herein, we report 8 new sulfides, Na₂ Ba[(Ag_x Na_1-x )₂ Sn₂ S₇ ] (1, x=0; 1 series, x=0.1-0.6; Na₂ Ba[(Li_0.58 Na_0.42 )₂ Sn₂ S₇ ], 1-0.6Li); Na₂ Sr[Cu₂ Sn₂ S₇ ] (2); and Na₂ Ba[Cu₂ Sn₂ S₇ ] (3). We use the structural tolerance factor ( t I e x p ${{t}_{I}^{exp}}$ ) to connect the chemical composition, crystal structure, and NLO properties. Guided by these correlations, a better balance between E_g and SHG is realized in 1, which exhibits a large E_g of 3.42 eV and excellent NLO properties (SHG: 1.5×AGS; laser-induced damage threshold: 12×AGS), representing the best performance among the known Hg- or As-free sulfides to date.

DFT study of alkali and alkaline earth metal-doped benzocryptand with remarkable NLO properties

Strategies for designing remarkable nonlinear optical materials using excess electron compounds are well recognized in literature to enhance the applications of these compounds in nonlinear optics. In this study, density functional theory simulations are performed to study alkali and alkaline earth metal-doped benzocryptand using the B3LYP/6-31G+(d, p) level of theory. Vertical ionization energies (VIEs), reactivity parameters, interaction energies, and binding energies exposed the thermodynamic stability of these complexes. FMO analysis revealed that HOMO is located on alkali metals having polarized electrons, which are easy to excite. The doping strategy enhanced the charge transfer with low bandgap energy in the range of 0.68–2.23 eV, which is lower than that of the surface BC (5.50 eV). Also, the lower transition energies and higher oscillator strength indicate that these complexes exhibit excellent electronic and optical properties. Non-covalent interaction analysis suggested the presence of van der Waals interactions between dopants and surface. IR analysis provided information about the frequencies of stretching vibrations present in the complexes due to different bonds. UV-vis analysis revealed that all the newly designed excess electron complexes are transparent in the UV region and possessed maximum absorption in the visible and NIR region, ranging from 753.6 to 2150 nm, which is higher than the surface (244 nm). Thus, these complexes have a potential for high-performance NLO materials in the applications of optics. Natural bond orbital analysis (NBO), transition density matrix (TDM), electron density difference map (EDDM), and density of state (DOS) analyses were also performed to study the charge transfer properties. Moreover, these complexes possessed remarkable optoelectronic properties due to a significant increase in the isotropic linear polarizability (α_iso) in the range of 629.59–1423.23 au. Further, these systems demonstrated an extraordinary large total first hyperpolarizability (β_tl) in the range of 3695.55–910 706.43 au. The rationalization of hyperpolarizability by the two-level model reflected a noteworthy increase in β_tl because of low transition energies (ΔE) and high transition dipole moment (Δμ). Thus, our results showed that alkali and alkaline earth metal-doped BC might be a competitor for efficient nonlinear optical properties with practical applications in the area of optoelectronics.

Strategies for designing remarkable nonlinear optical materials using excess electron compounds are well recognized in literature to enhance the applications of these compounds in nonlinear optics.

Ba6(CuxZy)Sn4S16 (Z = Mg, Mn, Zn, Cd, In, Bi, Sn): High Chemical Flexibility Resulting in Good Nonlinear-Optical Properties

Seven acentric sulfides Ba₆(Cu_xZ_y)Sn₄S₁₆ (Z = Mg, Mn, Zn, Cd, In, Bi, Sn) were grown by a high-temperature salt flux method. The crystal structures of the Ba₆(Cu_xZ_y)Sn₄S₁₆ (Z = Mg, Mn, Zn, Cd, In, Bi, Sn) compounds were determined by single-crystal X-ray diffraction with the aid of solid-state NMR spectroscopy. The Ba₆(Cu_xZ_y)Sn₄S₁₆ (Z = Mg, Mn, Zn, Cd, In, Bi) compounds are isostructural and crystallize in the Ba₆Ag₄Sn₄S₁₆ structure type. The Sn-containing compound exhibits high structural similarity to Ba₆(Cu_xZ_y)Sn₄S₁₆ (Z = Mg, Mn, Zn, Cd, In, Bi) with the presence of an interstitial atomic position partially occupied by Sn atoms. The chemical bonding characteristics of Ba₆(Cu_2.9Sn_0.4)Sn₄S₁₆ were understood with electron localization function calculations coupled with crystal orbital Hamilton population calculations. The Ba-S and Cu-S interactions are dominantly ionic, but the Sn-S interactions consist of strong covalent bonding characteristics in Ba₆(Cu_2.9Sn_0.4)Sn₄S₁₆. The monovalent Cu atoms, mixed with certain metals with various oxidation states, significantly shift the optical properties of the Ba₆(Cu_xZ_y)Sn₄S₁₆ (Z = Mg, Mn, Zn, Cd, In, Bi) compounds. This results in a good balance between the second-harmonic-generation (SHG) response and laser damage threshold (LDT). Ba₆(Cu_1.9Zn_1.1)Sn₄S₁₆ possesses a high SHG response and a high LDT of 2.8 × AGS and 3 × AGS, respectively. A density functional theory calculation revealed that CuS₄ and SnS₄ tetrahedra significantly contribute to the SHG response in Ba₆(Cu₂Mg)Sn₄S₁₆, which also confirmed that CuS₄ tetrahedra are crucial for the stability and optical properties of the Ba₆(Cu_xZ_y)Sn₄S₁₆ (Z = Mg, Mn, Zn, Cd, In, Bi, Sn) compounds revealed by electronic structure analysis.

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...

AAg2PS4 (A = K, Na/K): the first-type of noncentrosymmetric alkali metal Ag-based thiophosphates exhibiting excellent second-order nonlinear optical performances

The first-type of NCS alkali metal Ag-based thiophosphates KAg2PS4 (1) and (Na0.30K0.70)Ag2PS4 (2) exhibit large SHG responses, originating from the ordered arrangement of AgS4 and PS4 tetrahedra, and high LIDTs contributed by alkali metals.

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.

Structural, Optical, and Electronic Properties of Two Quaternary Chalcogenide Semiconductors: Ag2SrSiS4 and Ag2SrGeS4

Quaternary chalcogenide materials have long been a source of semiconductors for optoelectronic applications. Recent studies on I₂-II-IV-X₄ (I = Ag, Cu, Li; II = Ba, Sr, Eu, Pb; IV = Si, Ge, Sn; X = S, Se) materials have shown particular versatility and promise among these compounds. These semiconductors take advantage of a diverse bonding scheme and chemical differences among cations to target a degree of antisite defect resistance. Within this set of compounds, the materials containing both Ag and Sr have not been experimentally studied and leave a gap in the full understanding of the family. Here, we have synthesized powders and single crystals of two Ag- and Sr-containing compounds, Ag₂SrSiS₄ and Ag₂SrGeS₄, each found to form in the tetragonal I4̅2m structure of Ag₂BaGeS₄. During the synthesis targeting the title compounds, two additional materials, Ag₂Sr₃Si₂S₈ and Ag₂Sr₃Ge₂S₈, have also been identified. These cubic compounds represent impurity phases during the synthesis of Ag₂SrSiS₄ and Ag₂SrGeS₄. We show through hybrid density functional theory calculations that Ag₂SrSiS₄ and Ag₂SrGeS₄ have highly dispersive band-edge states and indirect band gaps, experimentally measured as 2.08(1) and 1.73(2) eV, respectively. Second-harmonic generation measurements on Ag₂SrSiS₄ and Ag₂SrGeS₄ powders show frequency-doubling capabilities in the near-infrared range.

Triclinic Layered A2ZnSi3S8 (A = Rb and Cs) with Large Optical Anisotropy and Systematic Research on the Inherent Structure-Performance Relationship in the A2MIIBMIV3Q8 Family

Two triclinic A₂ZnSi₃S₈ (A = Rb and Cs) with layered structures were successfully synthesized, and their physicochemical performances including optical bandgap, thermal behavior, and optical anisotropy were investigated. A₂ZnSi₃S₈ could be viewed as the first discovered Si-based examples in the known A₂M^IIM^IV₃Q₈ family (2-1-3-8 system; A = monovalent alkali metal; M^II = divalent transition metal; M^IV = group 14 metal; Q = chalcogen). The A₂M^IIM^IV₃Q₈ family members crystallize in five different space groups (P1̅, P2₁, P2₁/n, P2₁2₁2₁, and Pa3̅), and their structural transformation and optical performances (bandgap, NLO coefficient, and birefringence) were systematically studied based on the first-principles calculation among 13 A₂M^IIBM^IV₃Q₈ (M^IIB = Zn, Cd, and Hg) compounds without cubic β-K₂ZnSn₃S₈. Research result shows that the above 13 compounds exhibit the layered structures, but diverse wavelike layers and their optical anisotropy (Δn) undergo an increasing trend range from the triclinic to orthorhombic systems. Moreover, P2₁2₁2₁ compounds have very weak NLO effects compared with those of the P2₁ compounds since the polarization directions of anionic groups (M^IIBQ₄ and M^IVQ₄) in P2₁2₁2₁ compounds are directing oppositely and almost completely canceled out by the dipole moment calculation, which further indicates that P2₁ compounds exhibiting the relatively strong NLO effect and large optical anisotropy could be expected as potential IR NLO candidates.

Synthesis, crystal structure, and electronic structure of Li2PbSiS4: a quaternary thiosilicate with a compressed chalcopyrite-like structure

The new quaternary thiosilicate, Li₂PbSiS₄ (dilithium lead silicon tetrasulfide), was prepared in an evacuated fused-silica tube via high-temperature, solid-state synthesis at 800 °C, followed by slow cooling. The crystal structure was solved and refined using single-crystal X-ray diffraction data. By strict definition, the title compound crystallizes in the stannite structure type; however, this type of structure can also be described as a compressed chalcopyrite-like structure. The Li⁺ cation lies on a crystallographic fourfold rotoinversion axis, while the Pb²⁺ and Si⁴⁺ cations reside at the intersection of the fourfold rotoinversion axis with a twofold axis and a mirror plane. The Li⁺ and Si⁴⁺ cations in this structure are tetrahedrally coordinated, while the larger Pb²⁺ cation adopts a distorted eight-coordinate dodecahedral coordination. These units join together via corner- and edge-sharing to create a dense, three-dimensional structure. Powder X-ray diffraction indicates that the title compound is the major phase of the reaction product. Electronic structure calculations, performed using the full potential linearized augmented plane wave method within density functional theory (DFT), indicate that Li₂PbSiS₄ is a semiconductor with an indirect bandgap of 2.22 eV, which compares well with the measured optical bandgap of 2.51 eV. The noncentrosymmetric crystal structure and relatively wide bandgap designate this compound to be of interest for IR nonlinear optics.

New nonlinear optical-active AAgGa6S10 (A = K, Rb, Cs) featuring {[AgGa6S10]−}∞ framework and high laser damage threshold

New structure-type AAgGa₆S₁₀ (A = K, Rb, Cs) featuring unprecedented {[AgGa₆S₁₀]⁻}_∞ anionic framework exhibit the largest band gaps among chalcogenides containing independent Ag site, moderate SHG responses and high LIDTs.

Effect of Pb Substitution in Sr2-xPbxGeSe4 on Crystal Structures and Nonlinear Optical Properties Predicted by DFT Calculations

We investigated the Sr_2-xPb_xGeSe₄ series from 0 ≤ x ≤ 2 to study the impact of Pb on structure and properties. While the noncentrosymmetric (NCS) compounds γ-Sr₂GeSe₄ and α-Pb₂GeSe₄ have already been reported previously, the substitution variants Sr_1.31Pb_0.69GeSe₄ (space group Ama2, a = 10.31220(1) Å, b = 10.39320(1) Å, c = 7.42140(1) Å) and Sr_0.19Pb_1.81GeSe₄ (I4̅3d, a = 14.6177(3) Å) are introduced here for the first time. The experimentally determined optical band gaps decrease as predicted with increasing Pb content from γ-Sr₂GeSe₄ to Sr_1.31Pb_0.69GeSe₄, Sr_0.25Pb_1.75GeSe₄, and α-Pb₂GeSe₄ from 2.00, to 1.65, 1.45 and 1.42 eV, respectively. The nonlinear optical (NLO) properties of the orthorhombic compounds γ-Sr₂GeSe₄ and Sr_1.3Pb_0.7GeSe₄ (approximated with the supercell "Sr₃PbGe₂Se₈") were studied both theoretically, using first-principle calculations, and experimentally. The calculations found the effective nonlinear susceptibility, d_eff, of γ-Sr₂GeSe₄ and "Sr₃PbGe₂Se₈" at the static limit to be 10.8 and 8.8 pm V^-1, respectively. The experimental d_eff values of γ-Sr₂GeSe₄, Sr_1.31Pb_0.69GeSe₄, Sr_0.25Pb_1.75GeSe₄, and α-Pb₂GeSe₄ were 2.6, 2.3, 0.68, and 0.79 pm V^-1, respectively.

Coordinated regulation on critical physiochemical performances activated from mixed tetrahedral anionic ligands in new series of Sr6A4M4S16 (A = Ag, Cu; M = Ge, Sn) nonlinear optical materials

Coordinated regulation on physiochemical performances activated from mixed anionic ligands in a new series of IR NLO materials was systematically investigated.

A review of the AI2BIICIVDVI4 family as infrared nonlinear optical materials: the effect of each site on the structure and optical properties

The AI2B^IIC^IVDVI4 family as promising infrared NLO materials is summarized. The influence of each site substitutions on the structures and properties is systematically analyzed.

Verification of an integral figure of merit for mid-IR nonlinear crystals

The examination and verification of a simple integral figure of merit for nonlinear crystals, taking into account different crystal properties, was studied. The examination was carried out on the basis of experimental data on the broadband sum frequency generation of CO laser radiation in ZnGeP₂, BaGa₂GeSe₆, AgGaSe₂, GaSe, and PbIn₆Te₁₀ mid-IR nonlinear crystals. Taking into account spectral and angular phase-matching bandwidths, the figure of merit provides the best agreement with the experiment and can be applied for qualitative, and even quantitative, comparison of the nonlinear crystals.

A2SrMIVS4 (A = Li, Na; MIV = Ge, Sn) concurrently exhibiting wide bandgaps and good nonlinear optical responses as new potential infrared nonlinear optical materials

A series of A₂SrM^IVS₄ compounds concurrently exhibiting wide bandgaps and good NLO responses were proven as promising IR NLO materials.

Exploration of new nonlinear optical (NLO) materials is of importance for infrared (IR) applications. However, it is an extremely tough challenge to design and synthesize excellent IR NLO materials with optimal performances (e.g., concurrently a large NLO response and wide bandgap). Herein, four new mixed alkali/alkaline earth metal sulfides, A₂SrM^IVS₄ (A = Li, Na; M^IV = Ge, Sn), were successfully synthesized by a motif-optimization approach using the classical AgGaS₂ as a template. Note that all of them concurrently exhibit wide bandgaps (3.1–3.8 eV) and good NLO responses (0.5–0.8 × AgGaS₂) with phase-matching behavior, which satisfy the balance conditions (E_g ≥ 3.0 eV and d_ij ≥ 0.5 × benchmark AgGaS₂) of optical performances and hence are outstanding IR NLO materials. Remarkably, both of Na₂SrM^IVS₄ have the same structure without the structural transformation (Ge to Sn) in the reported related analogues and an interesting cation-dependent structural change is also found in Na₂M^IISnS₄ (M^II: Sr, R3c vs. Ba, I4[combining macron]2d). These results verify that the above design strategy of motif-optimization provides a feasible guide for the discovery of new IR NLO candidates and the A–AE–M–S (A = alkali metal; AE = alkaline-earth metal; M = Ga, In, Ge, Sn) system was identified as the preferred system for IR NLO materials.