English

• 1. INTRODUCTION

  Second-order nonlinear optical (NLO) materials play important roles in the laser and laser-related fields in view of their ability to transfer laser's frequencies to new ones. As to the double-frequency effect demonstrated by the NLO materials, the ones in the ultraviolet, visible, and near-infrared regions have been developed well and can basically satisfy the application's requirement, which have been witnessed by the success of BBO, LBO, KDP, KTP, etc. While for ones in the deep-ultraviolet and middle infrared (MIR) regions, commercial ones like KBBF, AGS, AGSe, and ZGP cannot be widely applied on account of their intrinsic disadvantages. For the MIR NLO materials AGS, AGSe, and ZGP, their low laser-induced damage thresholds (LIDTs) or harmful double phonon absorption restrict their further applications. To meet the modern society's requirement for high-efficient MIR NLO materials, it is necessary to discover new ones with better balanced NLO properties^[1-3]. Compared with those in other transparent regions, the MIR ones are usually chosen from chalcogenides, halides, or iodates, and there are also a few cases discovered in oxides or phosphides. Recently, hundreds of chalcogenides or halides have been studied as MIR NLO materials^[4]. Different from the previous summary work on them, we here try to introduce specifically some very freshly discovered MIR NLO chalcogenides and halides containing multiple anions by us and other groups^[5], and give some meaningful conclusions on the respective subgroup candidates. According to their chemical compositions, they can be classified into chalcohalides, oxychalcogenides, mixed chalcogenides, mixed halides, and so on.

  2. ADVANCE

  2.1 Chalcohalides

  A chalcohalide combines the X^– (X = F, Cl, Br, I) and Q^2– (Q = S, Se, Te) anions to one structure. As chalcogenides usually have wide infrared windows and large NLO coefficients, and halides usually possess large band gaps and high LIDTs, this combination may meanwhile function halide's and chalcogenide's advantages, and contribute the MIR NLO materials with balanced performance. To date, dozens of NLO-active chalcohalides have been studied^{[6, 7]}. According to their chemical compositions and structural characteristics, they can be further classified into adduct-, salt-inclusion-, and normal chalcohalides.

  Adduct-type chalcohalides include SnI₄·(S₈)₂^[8] and AI₃·(S₈)₃ (A = As, Sb)^{[9, 10]}. All of them feature 0D structures comprised of isolated SnI₄/AI₃ and S₈ units, and these two units are linked together by weak Van der Waals interaction (Fig. 1). These adducts exhibit moderate or large NLO effects, and high LIDTs, phase matchability. Especially for the latter two ones, their NLO effects are larger than that for SnI₄·(S₈)₂, ascribing to the contribution of the stereochemical active lone-pairs from A³⁺ cations, and has little relationship with the S₈ unit. Though these adducts behave promising NLO properties, their quantity is really too few. We hope that more NLO-active adducts can be experimentally studied.

  Figure 1

  

  Figure 1.  Structure of SnI₄·(S₈)₂. ©2018 Wiley

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  Recently, salt-inclusion-type chalcohalides have received their new era, following the previous ones containing B₁₂Q₁₂ clusters^[11-13]. Representative ones include [ABa₂Cl][Ga₄S₈] (A = Rb, Cs)^[14], Li[LiCs₂Cl][Ga₃S₆]^[15], and [ABa₃Cl₂]-[Ga₅S₁₀]^[16]. Take the first one for example, their noncentrosymmetric (NCS) structures can be transferred from centrosymmetric (CS) RbGaS₂ via polycation substitution (Fig. 2). This type of cation substitution inducing CS-NCS structural transformation has also been witnessed by our recent work on K₃Ga₃(Ge_7−xM_x)Se₂₀ (M = Si, Sn)^[17] and (A_2xBa_1-x)Ga₂Se₄ (A = Na, K)^{[18, 19]}. [ABa₂Cl][Ga₄S₈] (A = Rb, Cs) demonstrates the NLO effects 10.4~15.3 × KDP@1064 nm and 0.9~1.0 × AGS@1910 nm, and LIDTs 11~12 × AGS. The wonderful NLO performance is ascribed to the orderly packing T2-supertetrahedral Ga₄S₁₀ motifs.

  Figure 2

  

  Figure 2.  Structures of RbGaS₂ and [RbBa₂Cl][Ga₄S₈]. ©2020 American Chemical Society

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  The normal chalcohalides include pseudo-layered NaBa₄Ge₃S₁₀Cl^[20], supertetrahedral [M₄Se₁₀] (T1) and tetrahedral [MSe₄] (T2) built 3D framework Ba₃AGa₅Se₁₀Cl₂ (A = Cs, Rb, K)^[21] and Ba₄MGa₄Se₂₀Cl₂ (M = Zn, Cd, Mn, Cu/Ga, Co, Fe)^[22], etc. These materials usually show large SHG responses, while almost all of them cannot realize phase matchability. They have been well summarized in earlier work^[4].

  2.2 Oxychalcogenides

  Exploration of oxychalcogenides as new MIR NLO materials tries to combine the advantages of oxides and chalcogenides, viz. the large band gaps and high LIDTs from oxides, and large SHG responses and wide MIR windows from chalcogenides. Compared with NLO-active chalcohalides, NLO-active oxychalcogenides are relatively rare, mainly including Sr₅Ga₈O₃S₁₄^[23], SrZn₂S₂O^[24], BaGeOSe₂^[25], Sr₆Cd₂Sb₆O₇S₁₀^[26], and so on. Here, we just take Sr₆Cd₂Sb₆O₇S₁₀ as the example.

  The structure of Sr₆Cd₂Sb₆O₇S₁₀ features {[CdSb₂OS₅]^4–}_n zig-zag layers, which are constructed by the connection between [CdS₄]^6– tetrahedra, [SbS₅]^7– and [SbOS₄]^7– tetragonal pyramids. Besides, the {[SbO_2.5]^2–}_n pseudo-chains locate the interlayers (Fig. 3). It exhibits a SHG response 4 × AGS@2090 nm, and theoretical calculation suggests that the [SbS₅]^7– and [SbOS₄]^7– tetragonal pyramids contribute dominantly to the SHG effect, indicating the importance of stereochemical active lone-pair electrons from Sb³⁺ cations.

  Figure 3

  

  Figure 3.  Structure of Sr₆Cd₂Sb₆O₇S₁₀. ©2019 Wiley

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  2.3 Mixed chalcogenides

  It is well known that for isostructural homologues, sulfides usually have higher LIDTs while smaller SHG responses than those of selenides. Therefore, it is possible to realize a balance between these two important NLO indicators by combining different Q (Q = S, Se, Te) elements into structures, namely, mixed chalcogenides. In view of the synthetic difficulty, the available mixed chalcogenides are not too many^{[27, 28]}, not to mention the NLO-active ones. As far as we know, there are only Ba₆Ag_2.67+4δSn_4.33-δS_16-xSe_x^[29], Na₂In₄SSe₆^[30], and so forth. Here, Na₂In₄SSe₆ will be introduced as an example.

  Na₂In₄SSe₆ shows a 3D anionic framework structure, which is constructed by two types of 1D In(S/Se)₄ tetrahedral chains along the c-axis, which are then connected along the a- and b-axes with a zig-zag style via sharing S/Se atoms. The In(S/Se)₄ tetrahedra are linked together to form two kinds of supertetrahedra (In₄S/Se₁₁)^10– and (In₄S/Se₁₂)^12– (Fig. 4). It exhibits a SHG intensity 7.0 × AGS and LIDT 5.8 × AGS, however, it cannot realize phase matchability. Theoretical calculation suggests that the superpolyhedra contribute dominantly to the SHG response.

  Figure 4

  

  Figure 4.  Structure of Na₂In₄SSe₆. ©2017 American Chemical Society

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  2.4 Mixed halides

  Similar with mixed chalcogenides, mixed halides may balance the NLO performance for single-X halides on a certain extent. Based on this consideration, several mixed halides have been studied as IR NLO materials, including Rb₂CdBrI₃^[31], Rb₂CdBr₂I₂^[32], Cs₂HgI₂Cl₂^[33], etc. Here, Rb₂CdBrI₃ will be described.

  The 0D structure of Rb₂CdBrI₃ features isolated [CdBrI₃]^2– tetrahedra, and all the tetrahedra are aligned parallel to the bc plane (Fig. 5). The dipole moments of two adjacent tetrahedra are not fully offset, and produce a coupled dipole moment along the c-axis accounting for the SHG response. Its SHG response can reach to twice that of KDP and its LIDT is measured to be 115 MW/cm², around 46 times that of AGS.

  Figure 5

  

  Figure 5.  Structure of Rb₂CdBrI₃. © 2020 Royal Society of Chemistry

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  2.5 Chalcogenide borates

  Apart from the above chalcogenides and halides containing multiple anions, other types of chalcogenides and halides are much fewer. For a long time, the researchers on IR NLO materials try to combine the merits of borates and chalcogenides to produce new IR NLO materials with good properties^[34], as it is well known that borates and chalcogenides are the main sources for ultraviolet and IR NLO materials, respectively. The former have large band gaps/high LIDTs, and the latter have large NLO coefficients and wide IR windows. If these advantages can be well modulated, new IR NLO materials with balanced properties can be obtained. Guided by this idea, our group focuses on two subgroups for the combination of borates and chalcogenides, viz. chalcogen-borates and chalcogenide borates. They differ whether there are Q^2– (Q = S, Se, Te) anions or not. The former has been witnessed by B₁₂Q₁₂ cluster-based chalcohalides^[11–13], and the latter is developed relatively slower.

  To date, about ten chalcogenide borates with determined structures have been studied, including Eu₉MgS₂B₂₀O₄₁^[35], RE₃S₃BO₃ (RE = Sm, Gd)^{[36, 37]}, RE₆Ta₂MgSB₈O₂₆ (RE = Sm, Eu, Gd; Q = S, Se)^[38], Eu₂SB₅O₉^[39], Eu_4.5(B₅O₉)₂SI^[39], YSeBO₂^[40], and Zn₈Se₂(BO₂)₁₂^[41]. These very limited chalcogenide borates can be ascribed to their synthetic difficulties. As the sizes of borate and Q^2– ions are largely different, their chemical compatibility is not good. Besides, their thermodynamic and kinetic control conditions are significantly different, so chalcogenides are usually obtained at much higher temperature than borates. Among these chalcogenide borates, the latter three ones are NLO-active. All of them demonstrate interesting structures, and Eu_4.5(B₅O₉)₂SI will be introduced below in view of its promising NLO performance.

  Eu_4.5(B₅O₉)₂SI is isostructural with Eu₂SB₅O₉, differing in different oxidation states of Eu ions and inclusion of I^– anion for charge compensation. In the structure of Eu_4.5(B₅O₉)₂SI, three BO₄ and two BO₃ units connect together to build a 3D polyanionic network {[B₅O₉]^3–}_n, with the cavities occupied by two types of Eu atoms to form a tubular accumulation framework [Eu₂(B₅O₉)]_n (Fig. 6). It has the NLO response around 0.5 times that of AGS and its LIDT can reach to 15 times that of AGS for the powder sample. The study of chalcogenide borate is still in the infancy, and its discovery opens a new avenue for IR NLO material exploration.

  Figure 6

  

  Figure 6.  Structures of Eu₂B₅O₉S (a, c) and Eu_4.5(B₅O₉)₂SI (b, d). ©2020 American Chemical Society

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  3. CONCLUSION

  The maturity and development of MIR NLO materials have accumulated many efforts from various research groups, and the source exploration for new potential compounds is especially supported by several groups in China. Hitherto, there are hundreds of MIR NLO-active compounds studied, and a part of them demonstrates nice performances. However, it is still necessary to explore new ones with improved performances to fulfill the current and future application requirements for MIR NLO materials, especially for the systems that are insufficiently explored. We hope this short review can evoke more interest in this topic.

  
  
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  Figure 1  Structure of SnI₄·(S₈)₂. ©2018 Wiley

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  Figure 2  Structures of RbGaS₂ and [RbBa₂Cl][Ga₄S₈]. ©2020 American Chemical Society

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  Figure 3  Structure of Sr₆Cd₂Sb₆O₇S₁₀. ©2019 Wiley

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  Figure 4  Structure of Na₂In₄SSe₆. ©2017 American Chemical Society

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  Figure 5  Structure of Rb₂CdBrI₃. © 2020 Royal Society of Chemistry

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  Figure 6  Structures of Eu₂B₅O₉S (a, c) and Eu_4.5(B₅O₉)₂SI (b, d). ©2020 American Chemical Society

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