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 (Eg > 3.5 eV) and a strong second-harmonic generation (SHG) response (deff > 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 Eg (>3.5 eV) and a strong phase-matching deff (>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.


Introduction
The infrared (IR) laser occupies a more and more important position in military weapons, information storage, precision micromanufacturing and other scientic research. [1][2][3][4][5] Generally, the IR nonlinear optical (NLO) crystal is an integral part for IR laser generation, which requires a phase-matching (PM) feature, a broad energy gap (E g ), a strong second-harmonicgeneration (SHG) intensity (d eff ), a large laser-induced damage threshold (LIDT), an appropriate birefringence (Dn), and a favorable physical and chemical stability. 6 Chalcopyrite-type materials, AgGaS 2 , 7 AgGaSe 2 , 8 and ZnGeP 2 , 9 are commercially available and exhibit sufficient d eff and wide transmittance in the IR region. However, they still suffer from several fatal drawbacks, e.g. low LIDT of AgGaS 2 , non-phase-matching (NPM) behavior of AgGaSe 2 and unexpected multi-phonon absorption of ZnGeP 2 , which hinder their further applications in far-IR regions and high-power lasers. Therefore, it is necessary and urgent to explore new IR-NLO candidates with excellent comprehensive properties.
A non-centrosymmetric (NCS) structure is the prerequisite for a NLO crystal, which is also the rst challenge in the design and synthesis of IR-NLO candidates. In order to overcome this problem, various strategies have been developed in the past decades. Among them, chemical substitution is considered to be the simplest and most effective method. 10,11 On one hand, it can greatly improve the IR-NLO properties of chalcogenides, such as, Li 0.6 Ag 0.4 GaS 2 (d eff ¼ 1.1 Â AgGaS 2 ) versus LiGaS 2 (d eff ¼ 0.4 Â AgGaS 2 ), 12 Cu 5 Zn 0.5 P 2 S 8 (d eff ¼ 0.3 Â AgGaS 2 ) versus Cu 3 PS 4 (d eff ¼ 0.03 Â AgGaS 2 ), 13 and Sr 1.3 Pb 0.7 GeSe 4 (d eff ¼ 16 Â SiO 2 ) versus Pb 2 GeSe 4 (d eff ¼ 2 Â SiO 2 ). 14 On the other hand, it can realize centrosymmetric (CS)-to-NCS structure evolution in the chalcogenide system. Some  versus NCS Sn 7 Br 10 S 2 , 21 and CS Rb 4 P 2 S 6 versus NCS RbBiP 2 S 6 . 22 Notably, compared to a large number of structural transformations achieved by single-site substitution mentioned above, examples of dual-site and multi-site substitution are rarely reported. 23,24 Among the essential conditions for a promising IR-NLO candidate, a large E g and a strong d eff are not only the most vital factors but also the most challenging to achieve concurrently due to their incompatibility. Metal chalcogenides have been considered as promising candidates for IR-NLO materials, and nearly a thousand novel NLO-active chalcogenides have been discovered in the past few decades. [25][26][27][28][29][30][31] Unfortunately, there are only 6 PM chalcogenides that can meet the preferred requirement for a useful IR-NLO crystal, that is, a nice trade-off between a large E g (> 3.5 eV) and strong d eff (> 1.0 Â benchmark AgGaS 2 ), see Table S1 in the ESI for details. † As summarized in Table S1, † some useful information can be obtained as follows: (1) all of them are suldes; (2) most signicant structural features are two-dimensional (2D) or three-dimensional (3D) structures that are constructed from tetrahedral [MS 4 ] basic building units (BBUs) (M ¼ metal elements); (3) the lled cations are mainly alkali metals (A) or polycations. Nevertheless, a similar example based on an alkaline-earth metal (AE) as a lled cation is still not reported to date.
Recently, our research focuses on the ternary AE-M III -Q system (M III ¼ group IIIA metal Ga, In), hoping to obtain NCS chalcogenides. The tetrahedral [M III Q 4 ] BBUs are the benecial NLO-active units for achieving a large d eff , while the introduction of AE elements into this system may have the additional advantage of enlarging the E g , which may help to increase the LIDT once an IR-NLO crystal is obtained. [32][33][34] Our systematic exploratory efforts have led to the discovery of a known ternary sulde in this family, namely, SrGa 2 S 4 . 35 It exhibits a unique 2D [Ga 2 S 4 ] 2À layer that is constructed from common NLO-active [GaS 4 ] units and possesses a wide optical E g (3.93 eV) and a large theoretical birefringence (Dn ¼ 0.147@2050 nm). Unfortunately, the CS space group of Fddd (no. 70) makes this sulde NLO inert, that is, it does not display any SHG signal under laser irradiation. Inspired by the aforementioned chemical substitution strategy and detailed structural analysis, we are eager to realize the CS-to-NCS structural evolution via the replacement of two Ga III sites by "M I + M V " or "M II + M IV " in such a 2D layer. We term this the "dual-site aliovalent substitution" strategy.
Guided by a dual-site aliovalent substitution strategy, a novel quaternary NCS sulde SrCdSiS 4 was successfully discovered herein. Remarkably, SrCdSiS 4 exhibits the PM feature and excellent IR-NLO performances, including a strong d eff (1.1 Â AgGaS 2 ), wide E g (3.61 eV), ultra-high LIDT (20.4 Â AgGaS 2 ), broad transmission range (0.33-18.19 mm) and suitable Dn (0.158@2050 nm), which indicates that it is a promising candidate for IR-NLO materials and eliminates the disadvantageous factors of commercial chalcopyrite-type chalcogenides. Moreover, SrCdSiS 4 is also the rst example of an alkaline-earth metal-based IR-NLO material that breaks through the incompatibility between a large E g (>3.5 eV) and a strong PM d eff (>1.0 Â AgGaS 2 ). In this work, a systematic study of the syntheses, structural evolution, NLO and linear optical properties, and the in-depth mechanism is reported as well.

Results and discussion
In this study, light-yellow crystals of SrCdSiS 4 were prepared by a high-temperature solid-state reaction between stoichiometric SrS, CdS, Si, and S at 1123 K using CsI as the ux. The purity of the polycrystalline sample was checked by powder X-ray diffraction (XRD) analysis (Fig. 1a), and energy-dispersive Xray spectroscopy (EDX) provides average atomic ratios of 1.09/ 1.10/1/4.13 for Sr, Cd, Si, and S elements (Fig. S1 †), which are close to theoretical values determined from single-crystal XRD results. As shown in Fig. 1b, SrCdSiS 4 exhibits desirable thermal stability below 1207 K under N 2 condition and decomposes to Sr 2 SiS 4 and CdS at higher temperatures ( Fig. S2 †). The UV-vis and near-IR absorption spectra of SrCdSiS 4 reveal an optical E g of 3.61 eV (see Fig. 1c) based on the Kubelka Munk function, 36 which is not only keeping the advantage of the wide E g of the parent compound SrGa 2 S 4 (3.93 eV, as plotted in Fig. S3 †) but is also considerably wider than those of commercial IR-NLO materials AgGaS 2 (2.56 eV), 37 AgGaSe 2 (1.83 eV) 38 and ZnGeP 2 (2.0 eV). 39 Notably, such an ultra-wide E g of SrCdSiS 4 can effectively avoid two-or multi-photon absorptions under the incident normal laser, which is helpful to obtain a high LIDT. In addition, the transmittance spectrum ( Fig. 1d) recorded from a well-polished single crystal piece indicates that SrCdSiS 4 exhibits a wide transparent window from 0.33 mm (UV-vis region) to 18.19 mm (far-IR region), which can cover two notable atmospheric windows (3-5 mm and 8-12 mm). Remarkably, such a transparent range is wider than those of distinguished IR-NLO materials AgGaS 2 (0.48-11.4 mm), 39 ZnGeP 2 (0.74-12 mm), 39 AgGaSe 2 (0.76-17 mm) 39 and other recently reported IR-NLO candidates. [40][41][42][43][44][45] The structural evolution from CS SrGa 2 S 4 to NCS SrCdSiS 4 based on the dual-site aliovalent substitution strategy is illustrated in Fig. 2. Comparison of their structures shows that they belong to the same orthorhombic system and possess tetrahedral [MS 4 ] BBUs in their 2D layered structures. However, they still have several signicantly different characteristics in their structures: (i) SrCdSiS 4 crystallizes in the space group of Ama2 (no. 40), while SrGa 2 S 4 adopts the space group of Fddd (no. 70), see Table 1 for details; (ii) the asymmetric unit of SrCdSiS 4 has 6 crystallographically independent sites (i.e., 1 Sr, 1 Cd, 1 Si, and 3 S atoms) and the Z value (number of molecules in a unit cell) is 4, which are different from those of SrGa 2 S 4 [9 unique sites, namely, 3 Sr, 2 Ga, and 4 S atoms) and Z ¼ 32], see Tables 1 and  2 Fig. 2c and e); (iv) the [SrS 8 ] polyhedra are more highly distorted in SrCdSiS 4 than those in SrGa 2 S 4 , e.g., the larger difference (Dd) between the Sr-S bonds (Dd (Sr-S) ¼ 0.15Å) in SrCdSiS 4 than that (0.03Å) in SrGa 2 S 4 , and a similar trend also occurred in the tetrahedral [MS 4 ] BBUs, see Fig. S4 and Tables S2 and S3 for details. † In a word, the dual-site aliovalent substitution led to the above-mentioned obvious changes in their crystal structures, thus realizing the CS-to-NCS structural transformation from ternary SrGa 2 S 4 to quaternary SrCdSiS 4 . Moreover, the detailed symmetric operation change shown in Fig. 2g and h clearly displays the evolution of symmetry breaking, that is, the loss of the different glide planes and the inversion centre from CS SrGa 2 S 4 [high symmetry Fddd (no. 70)] to NCS SrCdSiS 4 [low symmetry Ama2 (no. 40)].
Owing to SrCdSiS 4 possessing the NCS polar structure, we exhaustively investigated and analyzed the NLO performance. Size-dependent SHG effect measurements were performed by using the Kurtz-Perry method 46 at ve different particle size ranges. As illustrated in Fig. 3a, the SHG intensity strength increases with the increase of particle size, indicating that SrCdSiS 4 can achieve type-I PM in the IR region. Under the same particle size of 150-210 mm, the d eff is around 1.1 times that of AgGaS 2 under a 2050 nm Q-switched laser. We also measured the SHG signals under a 1064 nm laser due to the shorter UV absorption edge of SrCdSiS 4 (ca. 330 nm), giving it the potential to be applied in the UV-vis range. As indicated in Fig. 3b, SrCdSiS 4 shows a large SHG effect of 4.5 Â KH 2 PO 4 (KDP) with type-I PM nature. Therefore, SrCdSiS 4 is an excellent dual-band NLO candidate that can be used in both the IR and UV-vis regions. Apart from an adequate SHG response, a large LIDT is also vitally important for an IR-NLO material. So, its LIDT was measured by a single-pulse power technology. 47 As shown in Fig. S5, † the experimental LIDT of SrCdSiS 4 of 57.14 MW cm À2 in the particle size range of 150-210 mm is around 20.4 times higher than that of benchmark AgGaS 2 (2.8 MW cm À2 ) under the same condition (1064 nm, 1 Hz, 10 ns). Such a value shows the outstanding laser tolerance of SrCdSiS 4 , indicating its potential in high-power laser applications. As a new member of the XM II M IV Q 4 (X ¼ Eu, Sr, Ba; M II ¼ Mn, Zn, Cd, and Hg; M IV ¼ group-14 elements; and Q ¼ chalcogen) system, 24,48-63 it is necessary to make a detailed comparison with other compounds. A summary of the two key performance parameters (i.e., d eff and E g ) of the XM II M IV Q 4 family is provided in Fig. 4 and details are listed in Table S4. † Remarkably, SrCdSiS 4 displays superior IR-NLO comprehensive performances, and this is the rst 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 Â AgGaS 2 ) in this system. Furthermore, a more comparative study with other state-of-the-art IR-NLO candidates is worthwhile. 32,64-67 As shown in Fig. S6 and Table S1, † there are 7 PM IR-NLO chalcogenides with E g > 3.5 eV and d eff > 1.0 Â AgGaS 2 ( Fig. S6a †), which have been selected on the basis of literature research. From the perspective of structural dimension, they are mainly constructed in 3D framework (43%) and 2D layered (43%) structures, and only K 2 BaP 2 S 6 67 possess a zerodimensional (0D) cluster structure (14%) (Fig. S6b †). In addition, they can be divided into four categories according to the kind of lled cation: polycation-based (43%), alkali-metal-based (29%), mixed-cation-based (14%) and alkaline-earth-metal-based (14%) (Fig. S6c †). Note that the central atoms in most of the BBUs are main group elements [e.g., Ga (20%), P (20%), Si (13%), Li (13%) and Ge (7%)] and transition metal elements [e.g., Zn (20%), and Cd (7%)] (Fig. S6d †). The production of SrCdSiS 4 not only enlarges the proportion of Cd and Si acting as favorable framework cations but also represents the rst report of an alkaline-earth metal-based IR-NLO material that breaks through the wall of E g > 3.5 eV and d eff > 1 Â AgGaS 2 . Theoretical computations were adopted to better understand the structure-activity relationships of the title compound. According to the electronic structures, the valence band minimum (VBM) and the conduction band maximum (CBM) are at different k-points for SrGa 2 S 4 (Fig. 5a) and SrCdSiS 4 (Fig. 5b), which indicates that they are indirect E g semiconductors. Theoretical results exhibit that the calculated E g values are 2.85 eV for SrGa 2 S 4 and 2.77 eV for SrCdSiS 4 . Such values are smaller than the experimental ones (3.93 eV for SrGa 2 S 4 and 3.61 eV for SrCdSiS 4 , respectively), which is mainly due to the discontinuity of the exchange correlation energy of the GGA functional. [68][69][70] In addition, their partial density of states (PDOSs) in the energy eld from À10 to 10 eV are shown in Fig. 5c and d. From the PDOSs, it is found that the contribution in the VBM is mainly from Ga-4s, S-3p orbitals for SrGa 2 S 4 and S-3p, Si-3p orbitals for SrCdSiS 4 , while the CBM consists of Ga-4p, S-3p orbitals for SrGa 2 S 4 and Cd-5s, S-3p   (9) a U eq is dened as one third of the trace of the orthogonalized U ij tensor.    (Fig. 6b). Typically, a signicant anisotropic structure is benecial to produce a large Dn, that is, dual-site aliovalent substitution induces greater structural distortion from SrGa 2 S 4 to SrCdSiS 4 . Meanwhile, these calculated values are larger than those of commercialized NLO materials, such as AgGaS 2 (Dn ¼ 0.039@2050 nm), 72 ZnGeP 2 (Dn ¼ 0.04@2050 nm) 72 and KDP (Dn ¼ 0.034@1064 nm). 20 Besides, the frequency-dependent refractive index diagrams mean that  under the premise of PM determined at 2050 nm, the lower limit of the SHG output wavelength is 500 nm (Fig. 6c). Based on theoretical studies and experimental observations, we compared SrCdSiS 4 with the illustrious IR-NLO crystal AgGaS 2 .
As illustrated in the radar chart (Fig. 6d) Furthermore, the cut-off energy dependences of the largest static d 33 were analyzed based on a length-gauge formalism method 73,74 with the purpose of revealing the intrinsic source of the SHG response. As shown in Fig. 7a, d 33 values are trending upward in the range of VB-1 (dominated by the S-3p and Si-3p states, CB-1 (dominated by the S-3p and Cd-5s states) and CB-3 (dominated by the S-3p and Si-3p states). Distinctly, these three regions have a predominant impact on the overall NLO response. Considering the PDOS (Fig. 5d) and the relevant partial charge density proles (Fig. 7b)

Conclusions
In conclusion, employing the ternary CS SrGa 2 S 4 as the parent structure, a new NCS quaternary SrCdSiS 4 was successfully designed and synthesized via a dual-site aliovalent substitution strategy, whose 2D layered structure consisted of alternately connected [CdS 4 ] and [SiS 4 ] BBUs through corner-and edgesharing S atoms. Detailed performance analyses indicated that SrCdSiS 4 could be a promising candidate for the UV-vis and IR-NLO crystal due to its advantages including a strong SHG intensity (d eff ¼ 4.5 Â KDP at 1064 nm, or 1.1 Â AgGaS 2 at 2050 nm) with PM feature, a suitable birefringence (Dn (cal.) ¼ 0.165 at 1064 nm, or 0.158 at 2050 nm), a wide transmission window (0.33-18.19 mm), a large E g (3.61 eV), and an ultra-high LIDT (20.4 Â AgGaS 2 ). In addition, theoretical calculations reveal that the large Dn and strong d eff are mainly contributed by the tetrahedral [CdS 4 ] and [SiS 4 ] NLO-active motifs that are nicely arranged in a most favorable stacking. Hopefully, such a simple and effective chemical design strategy can accelerate the discovery of novel NCS materials with advanced NLO properties.

Data availability
Supporting data for this article is presented in the ESI. †

Conflicts of interest
There are no conicts to declare.