Sn2Ga2S5: A Polar Semiconductor with Exceptional Infrared

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Sn2Ga2S5: Polar Semiconductor with Exceptional Infrared Nonlinear Optical Properties Originating from Combined Effect of Mixed Asymmetric Building Motifs Meng-Yue Li, Bingxuan Li, Hua Lin, Zuju Ma, Li-Ming Wu, Xin-Tao Wu, and Qi-Long Zhu Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b02389 • Publication Date (Web): 26 Jul 2019 Downloaded from pubs.acs.org on July 26, 2019

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Chemistry of Materials

Sn2Ga2S5: Polar Semiconductor with Exceptional Infrared Nonlinear Optical Properties Originating from Combined Effect of Mixed Asymmetric Building Motifs Meng-Yue Li, †,‡ Bingxuan Li, § Hua Lin,*, † Zuju Ma, ‖Li-Ming Wu,┴ Xin-Tao Wu, † and Qi-Long Zhu*,† †State

Key Laboratory of Structural Chemistry, Fujian Institute of Research on the

Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China ‡College §Key

of Chemistry, Fuzhou University, Fujian 350002, China

Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute

of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China ‖

School of Materials Science and Engineering, Anhui University of Technology,

Maanshan, 243002, China ┴

Key Laboratory of Theoretical and Computational Photochemistry, Ministry of

Education, College of Chemistry, Beijing Normal University, Beijing, 100875, China

ABSTRACT: Non-centrosymmetric (NCS) metal-chalcogenides have emerged as a kind of candidate for infrared nonlinear optical (IR-NLO) materials, but it remains an enormous challenge to achieve simultaneously large second-harmonic-generation (SHG) coefficient (dij), strong laser-induced damage threshold (LIDT),

wide

phase-matching (PM) range and low melting-point (MP) in a single material. Herein, a novel ternary mixed-metal chalcogenide, Sn2Ga2S5, was prepared via a facile 1

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mid-temperature fluxing method. It adopts a polar space group Pna21 (No. 33) and shows a distinctive 3D NCS network made by

2 2– ∞[𝐺𝑎2𝑆5 ]

layers and

1 8– ∞[𝑆𝑛2𝑆6 ]

chains via the sharing common corners. Significantly, Sn2Ga2S5 exhibits an excellent comprehensive performance for IR-NLO applications that surpasses current benchmark AgGaS2, including strong SHG response dij (2.5  AgGaS2), high LIDT (6.6  AgGaS2), wide PM range (> 725 nm), broad transparent region (0.57–13.8 μm) and low MP (ca. 958 K). Furthermore, the detailed theoretical calculation results elucidate that the strong dij of Sn2Ga2S5 can be ascribed to the combined effect of two asymmetric building motifs (ABMs), i.e., dimeric [Sn2S6], and [Ga2S5] units. Such a systematic work would provide some useful guidance for the prediction and discover of new IR–NLO chalcogenides with mixed ABMs.

INTRODUCTION Nonlinear optical (NLO) crystal is a key component for switching laser sources to new spectral regions through the frequency-conversion technology. Among them, IR laser has been widely used in diverse fields, such as military communication, laser lithography, medical treatment, resource detection and so on.1–5 Usually, a distinguished IR-NLO crystal should possess the following features: wide phase-matching (PM) range, strong second-harmonic-generation (SHG) coefficient (dij), large laser-induced damage threshold (LIDT), broad IR transparent window, stable physicochemical properties, etc.6,7 Currently, as one of the optimal candidates for IR-NLO applications, non-centrosymmetric (NCS) metal-chalcogenides have been 2


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aroused great interest due to their diverse structural chemistry and outstanding optical properties.8–14 To date, only AgGaS215 and AgGaSe216 have be successfully commercialized. Unfortunately, both of them suffer from their own weaknesses such as small LIDTs that limit their high-power applications. Thus, searching of new IR-NLO chalcogenides with high performances, especially to achieve simultaneously large dij and strong LIDT in a single material, is a hot yet challenging research topic. Over the past decade, various strategies were adopted for designing and exploring new NCS metal-chalcogenides with outstanding NLO performances. Among them, a promising design idea is to create multiple asymmetric building motifs (ABMs), mainly including main-group [MQ4] tetrahedra (e.g., M = IIIA and IVA elements; Q = chalcogen),

transition-metal [TMQ4] tetrahedra (e.g., TM = Cu,

Ag, Zn Cd and Hg), and stereochemically active lone-pair (SALP)-containing [XQn] polyhedra (e.g., X = As, Sb, Bi, Pb, etc.) into one crystal structure. For instance, orthorhombic La2CuSbS5 possesses a 3D NCS framework built by [SbS4] and [CuS4] ABMs, featuring a type-I PM behaviour with moderate dij (0.5  AgGaS2) and large LIDT (6.7  AgGaS2) at 150–210 μm sized particles.17 PbGa2GeSe6, a polar quaternary semiconductor constructed by [PbSe4], [GaSe4], and [(Ga/Ge)Se4] ABMs demonstrates exceptional NLO properties including giant dij (about 5  AgGaS2), large LIDT (about 3.7  AgGaS2) and moderate birefringence (n = 0.114).18 Ba2BiInS5, an orthogonal 1D chain structure formed by tetragonal-pyramid [BiS5] and tetrahedral [InS4] ABMs, exhibits strong dij (about 0.8  KTP) at the incident wavelength of 2050 nm.19 3



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Among these above-mentioned NCS metal-chalcogenides, the ABMs containing the SALP electrons always play a vital role in achieving the high NLO performance. Nevertheless,

in

comparison

with

the

widespread

studies

on

complex

multi-component compounds, there are few reports on ternary systems.20–23 Guided by these ideas, our research focuses on the X/Ga/Q systems. When screening for potential candidates from the ICSD database, a NCS material Sn2Ga2S5 captured our attention.24 So far, there are no physicochemical properties reported for it. In this work, Sn2Ga2S5 is successfully obtained at relatively mid-temperature with the help of the SnCl2 flux for the first time. Specifically, Sn2Ga2S5 exhibits impressive IR-NLO comprehensive properties with strong dij (2.5  AgGaS2), high LIDT (6.6  AgGaS2), wide PM range (> 725 nm), broad transparent region (0.57–13.8 μm) and low MP (ca. 958 K). Moreover, detailed theoretical calculations are discussed to unveil an in-depth cognition of the relationship between NCS structure and NLO property.

EXPERIMENTAL SECTION Materials and Instruments All of the chemicals were obtained from commercial sources and used without further purification. Sn shot (5N), Ga ingot (5N), S powder (5N) and SnCl2 (4N) was purchased from Alfa-Aesar. Elemental analysis was carried out using a field emission scanning electron microscope (FESEM, JSM6700F) equipped with an energy dispersive X-ray (EDX) spectrometer (Oxford INCA). Powder X-ray diffraction (PXRD) patterns were collected on a Rigaku Mini-Flex II powder diffractometer by 4


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using Cu-Kα radiation at room temperature. UV-vis-NIR absorption measurement was performed in the region of 200−2500 nm at room temperature using an UV−Vis−NIR spectrometer (Perkin-Elmer Lambda 950). The reflectance spectrum of the BaSO4 powder was collected as the baseline and the diffuse reflectance data were converted to absorbance internally by the instrument by use of the Kubelka-Munk function.25 The thermal stability test was recorded by a NETZSCH STA 449C simultaneous analyser under a constant flow of N2 atmosphere at a heating rate of 10 K/min. The IR transmittance was measured on the PerkinElmer Spectrum One FT-IR Spectrometer in the range of 400−4000 cm-1. The powder SHG and LIDT measurements were carried out with the Kurtz-Perry26 and single pulse measurement method,27 respectively. AgGaS2 was used as a benchmark material, which is offered from Anhui Institute of Optics and Fine Mechanics Chinese Academy of Sciences. For getting more details, please see the previous report.28–30 Synthesis Sn2Ga2S5 was firstly synthesized in 1983 by Mazurier and co-workers by reacting SnS with Ga2S3 at 903 K.24 We have tried to repeat this experiment; however, the large high-quality single crystals in millimeter size cannot be obtained. In addition, the pure phase was failed to be obtained from a stoichiometric mixture of the elements, including adjusting the sintering temperatures or mole ratios of reagent. In this study, the crystals of Sn2Ga2S5 were obtained by a facile mid-temperature fluxing method using SnCl2 as the flux. Typically, a graphite crucible was fitted with Sn (2 mmol), Ga (2 mmol), S (5 mmol), and SnCl2 (2.8 mmol) and then put it inside a silica tube that 5



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was evacuated to 10–3 Pa and sealed. The tube was heated in a computer-controlled furnace to 723K at 5 K/h, the temperature was maintained for 72 h, and cooled to 473k at 2/h at which temperature the furnace was shut off. The products were cleaned with distilled water to remove the excess SnCl2 and then dried by ethanol. Finally, red crystals of Sn2Ga2S5 were obtained (Fig. 1a) and they are stable in air. Semi-quantitative analyses by EDX indicated a Sn/Ga/S molar ratio of approximately 2/2/5 on several crystals (see Fig. 1b). The homogenous distribution of the constituent atoms (Sn, Ga and S) is confirmed by the elemental mapping analysis (Fig. 1c). Then a crystal with good quality was manually selected for single-crystal XRD characterization. Moreover, the pure phase of target product was confirmed by PXRD analysis, in which the experimental peaks are in well agreement with the simulated data (Fig. 1d). Single Crystal Structure Determination X-ray single-crystal data was recorded by a high-optical quality crystal using a Saturn 724 diffractometer (Mo-Kα radiation, λ = 0.71073 Å). The absorption correction was done by the multi-scan method and structures were solved by direct methods31 and refined by full-matrix least-squares fitting on F2 by SHELX–2014 program package.32 All of the non-hydrogen atoms were refined with anisotropic thermal parameters. The refinement data are listed in Table 1 and Tables S1–S2. CCDC number: 1940887. Computational Sections Theoretical calculations were performed by using the VASP software.33–35 The ion-electron interactions were described by the projected-augmented-wave (PAW)36 6


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method. The generalized gradient approximation in the Perdew-Burke-Ernzerhof (PBE) form37 was used. A Γ-centered 7×9×7 Monkhorst-Pack grid for the Brillouin zone sampling38 and a cutoff energy of 500 eV for the plane wave expansion were found to obtain convergent total energies. The so-called length-gauge formalism is adopted to calculate the static χ(2) coefficients (dij) 39, 40 and a scissor operator has been added to correct the conduction band energy.

RESULTS AND DISCUSSION Structural Description and Comparison Since the crystal structure was already well described by Mazurier et al.,24 we are briefly reviewing the key structural features in this section. As displayed in Figure 2a, the complex three-dimensional (3D) framework was constructed from layers and

1 8– ∞[𝑆𝑛2𝑆6 ]

2 2– ∞[𝐺𝑎2𝑆5 ]

chains via the sharing common corners. There exist two such

layers and chains in the unit cell and they are associated by a 21 screw axis running 2

along the c-direction. The remarkable ABM of the anionic∞[𝐺𝑎2𝑆2– 5 ] layer is the dimeric [Ga2S5] polyhedra. Such polyhedral groups via two shared corners to form a 1 2– ∞[𝐺𝑎2𝑆5 ]

chain along the b-axis. Then, these chains strand arrange parallel with

duplicate handedness and connect neighbouring chains to make up the

2 2– ∞[𝐺𝑎2𝑆5 ]

layers by vertex-sharing S atoms (Figure 2b). In addition, from another perspective, the

2 2– ∞[𝐺𝑎2𝑆5 ]

layers can be viewed as the one consisting of the [Ga6S15]6– rings

(Figure 2b, red dotted ring) link to each other along the bc-plane. In the structure, the two crystallographically distinct Ga atoms (namely, Ga1 and Ga2) are coordinated in 7



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a distorted tetrahedral fashion with normal The Ga–S distances. Among them, those involving terminal S atoms (S1 and S4) range between 2.296(4) and 2.312(4) Å, and those involving bridging S atoms (S2, S3 and S5) are in the range from 2.250(4) to 2.312(4) Å. In addition, the angles of S–Ga–S bonds range from 103.2(2) to 118.1(2) for the Ga1 atom and from 100.9(2) to 115.4(2) for the Ga2 atom (see Table S3). An isostructural example is found in Na2Ge2Se5, which exhibits a very large SHG response.41 As shown in Figure 2c, 1D

1 8– ∞[𝑆𝑛2𝑆6 ]

chain was made by corner sharing

of dimericc polar [Sn2S6] units. Each [Sn2S6] unit was constituted with one [Sn1S4]6− and one [Sn2S4]6− pyramid via sharing vertex (see Figure 2c, blue dotted box). However, the reports about such a 1D

1 8– ∞[𝑆𝑛2𝑆6 ]

chain are extremely rare and limited

only to BaSnS2.39 Each Sn atom is coordinated with four S atoms with the Sn–S distance ranging from 2.636(4) to 3.157(6) Å, which is in line with those of 2.639– 3.185 Å in Sn2SiS4,40 and 2.707–3.242 Å in SnGa2GeS6.41 Moreover, taking the coordination geometry of Sn2 atom as an example, the crystal orbital Hamilton populations (COHP) and integrated COHP (ICOHP) curves were performed to discuss the Sn–S bonding.42–44 As revealed in Figure 2d, positive and negative numbers in the –COHP curve represent bonding and antibonding states, respectively. Strong bonding interactions are embodying in the energy range between –8 and –6 eV. While the energies of antibonding interactions are in the range from –2 to 0 eV for the occupied states and range from 2 to 6 eV for the unoccupied states. As displayed in Figure 2e, the –ICOHP values of the short Sn–S bonds (2.644–3.157 Å) are 2.87–0.88 eV/bond, while the values of the long Sn–S bonds (3.446–3.703 Å) 8


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show insignificant levels hereunder 0.29 eV/bond. Both –COHP and –ICOHP values illustrate that the bonding interactions of the short distances (Sn–S < 3.16 Å) are much stronger than those of the long ones (Sn–S > 3.45 Å), which confirms that the geometry environment around Sn atoms is certainly 4-fold coordination as mentioned above. Optical Properties and Thermal Stabilities Optical transparency is an important parameter for IR-NLO materials, single-crystal Sn2Ga2S5 shows wide transparent window from the UV-vis spectral region (0.57 μm) to far-IR range (13.8 μm), as displayed in Figure 3a. This IR transparency is comparable with the top IR-NLO material AgGaS2 (0.48–11.4 μm).46 Besides, the solid-state diffuse-reflectance spectrum of Sn2Ga2S5 was tested at room temperature, and the experimental energy-gap (Eg) was calculated to be 2.14 eV (the insert in Figure 3a), which is accordant with its polycrystalline colours (Figure 1a). Such value is slightly smaller than that of commercial AgGaS2 (Eg = 2.56 eV),47 but still greater than those of other top IR-NLO compounds, for example, AgGaSe2 (Eg = 1.75 eV)16 and ZnGeP2 (Eg = 1.65 eV).48 Moreover, the TG-DSC diagrams are shown in Figure 3b, these results indicate that Sn2Ga2S5 melt congruently and the melting point (MP) is 957 K, which was confirmed by the PXRD analysis for the recycled compound after the DSC measurement (Figure S1). Such data is far below than those of the state-of-the-art IR-NLO chalcogenides, e.g., AgGaSe2 (MP = 1133 K),49 BaGa4Se7 (MP = 1241 K),50 AgGaS2 (MP = 1271 K),51 ZnGeP2 (MP = 1298 K)48 and BaGa4S7 (MP = 1361 K),52 and thus would be favorable for the growth of large crystals by the 9



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Bridgman–Stockbarger method. NLO Properties and Powder LIDTs Owing to Sn2Ga2S5 crystallizes in the NCS space group Pna21, we systematically investigated the NLO properties. On the one hand, the powder SHG test was performed through the Kurtz-Perry method26 with polycrystalline AgGaS2 serving as the references. As shown in Figure 4a, particle-dependent SHG intensities from 30−210 μm were performed using 2050 nm radiation, and the recorded signals were compared to those collected for the benchmark AgGaS2 with similar particle sizes. The experimental results show that Sn2Ga2S5 possesses a typical type-I PM feature and the SHG response is approximately 2.5  AgGaS2 in the 150−210 μm particle size range. Evidently, such a strong SHG efficiency is sufficiently for IR-NLO applications. It is also larger than those of most of the top Sn-based IR-NLO metal-chalcogenides, such as Li2CdSnS4 (0.1  AgGaS2),53 BaNa2SnS4 (0.5  AgGaS2),54 BaLi2SnS4 (0.7  AgGaS2),55 and SnGa4S7 (1.3  AgGaS2).21 On the other hand, as another essential parameter for an IR-NLO material, the powder LIDT properties were also examined via the single pulse test method. As displayed in Figure 4b, the value of powder LIDT for Sn2Ga2S5 was about 9.9 MW/cm2, which is 6.6 times larger than that of AgGaS2 (1.5 MW/cm2) in the same sized particles (150−210 μm). The combination of PM ability, a strong SHG response and a high LIDT, indicates that Sn2Ga2S5 satisfies the vital needs of a favorable IR-NLO candidate.

10


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Theoretical Studies The electronic band structure of Sn2Ga2S5 was calculated and shown in Figure 5a with the highest occupied state set as EF = 0 eV. The valence band maximum (VBM) resides at G point (0, 0, 0) whereas the conduction band minimum (CBM) was found along the Y–S path. The fundamental Eg was therefore found to be an indirect type with a value of 1.66 eV. The calculated Eg is less than the experimental Eg (2.14 eV), which, however, is expected because of the well-known fact that the DFT underestimates the Eg of bulk solids. 56–58 As plotted in Figure 5b, the partial density of states (PDOS) of Sn, Ga, and S in the energy range of –10 eV to 10 eV were divide into five sub-sections (VB-I, VB-II, VB-III, CB-I and CB-II) according to different orbital features. VB-I is a large contribution from the valence electrons of the Sn element (5s and 5p states) that mix with the S-3p states. Interestingly, the existence of band dispersion in this section is mainly caused by the Sn lone pair, namely, Sn-5s character. VB-II is dominated by the p orbitals of Sn (5p), Ga (4p) and S (3p), indicating that Sn, Ga and S atoms have strong covalent bonding. VB-III is related to the bonding states of all the atoms (Sn-5s and Ga-4s states with strong mixing from the S-3p states). The bottom of CB, which contains CB-I and CB-II sections, is primarily derived from Sn-5s and Ga-4s states especially in the lower energy part (1.7–4.0 eV) with strong mixing from the S-3p states. Since the optical absorption of a material can be mostly ascribed to the change transitions between the states of VB and CB near the Eg, i.e., VB-I and CB-I, the 11



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[SnS4] and [GaS4] groups should make the absolute contributions to the optical properties. In order to grasp the structure-property relationships and reveal the origin of the NLO response deeply, the DFT study based on length-gauge formalism39,40 was systematically analysed for Sn2Ga2S5. As displayed in Figure 5c, Sn2Ga2S5 has three non-zero independent dij (d15, d24, and d33) under the restriction of Kleinman’s symmetry,59 because of its space group (Pna21) belongs to class mm2. The theoretical dij values are d15 = 33.56 pm/V, d24 = 26.93 pm/V, and d33 = 20.52 pm/V, respectively, which are stronger than that (d36 = 18.2 pm/V) of AgGaS2 at 2050 nm. In addition, another important optical index, birefringence Δn (Δn = nx – ny), was also calculated to be 0.25 at 0.61 eV (i.e., 2050 nm) (Figure S6), which is obviously larger than that of AgGaS2 (Δn = 0.034) under the same condition. This result indicates that Sn2Ga2S5 can easily realize PM in the IR range, which is in well agreement with the experimental observation (see Figure 4a). Furthermore, the calculated PM range based on the dispersion curves of refractive indices as shown in Figure 5d, the minimum PM value of the SHG output for Sn2Ga2S5 is 725 nm. As given in Figure 6a, the variation of static dij (pm/V) vs. cut-off energy (eV) was adopted to evaluate the contribution of the respective electronic orbitals to the overall NLO response. It can be noticed that these dij coefficients display apparent transformations in the energy intervals of VB-II and CB-I. On account of the associated PDOSs (Figure 5b) and the corresponding partial charge density maps (Figure 6b), the dominant contributions in these two intervals are the states of Sn, Ga 12


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and the surrounding S atoms. Consequently, it can be concluded that the strong dij of Sn2Ga2S5 can be down to the combined effect of two ABMs, i.e., dimeric [Sn2S6] and [Ga2S5] units. Finally, the results of the detailed calculated linear and NLO parameters for Sn2Ga2S5 and AgGaS2 are summarized in Table 2 (see Figures S2–6 in SI). Furthermore, we quantified the local dipole moments (LDMs) of the asymmetric distinct units, [SnS4] pyramid and [GaS4] tetrahedron, of Sn2Ga2S5 to study the effect of crystal polarity on SHG effect. The LDMs of these two units were calculated by using the Gaussian 09 Package60 with a large quadruple-zeta Weigend-Ahlrichs basis set61 and listed in the Table S3 and Figure S7, from which some conclusions can be deduced: (i) the μtotal of pyramidal [SnS4] units are much higher than those of tetrahedral [GaS4] units; (ii) the LDMs of the [SnS4] and [GaS4] units are enhanced along the z-director in the unit cell. Therefore, the enhancement of SHG effect of Sn2Ga2S5 mainly originates from the combined effect of two ABMs, i.e., dimeric [Sn2S6], and [Ga2S5] units.

CONCLUSION In summary, by using a facile mid-temperature fluxing method, we successfully obtained a ternary polar semiconductor Sn2Ga2S5, which possesses a unique 3D NCS framework structure constructed by

2 2– ∞[𝐺𝑎2𝑆5 ]

layers and

1 8– ∞[𝑆𝑛2𝑆6 ]

chains.

Significantly, Sn2Ga2S5 is a promising new IR-NLO crystal with several fascinating properties, such as, a strong dij (2.5  AgGaS2), an exceptional LIDT (6.6  AgGaS2), a wide phase-matching (PM) range (>725 nm), together with relatively low 13



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melting-point (MP = 958 K) and air stability. Moreover, theoretical calculations and results show that the NLO response originates from the combined effect of the mixed asymmetric building motifs, namely, dimeric [Sn2S6] units and [Ga2S5] polyhedra. These results may provide a new strategy for the seek and design of IR-NLO chalcogenides. Further striving for the performance optimization and the large-scale crystal growth is in progress.

ASSOCIATED CONTENT Supporting Information CIF data, SHG measurements, LIDT measurements, computational details and additional figures and tables. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]; *E-mail: [email protected]. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This research was supported by the National Natural Science Foundation of China (21771179, 21771182, 21501177, 21571020, and 21301175), the Strategic Priority Research Program of the Chinese Academy of Sciences (No. XDB20000000), the 14


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support from “Chunmiao Projects” of Haixi Institute of Chinese Academy of Sciences, the Natural Science Foundation of Fujian Province (2019J01133) and the One Thousand Young Talents Program under the Recruitment Program of Global Youth Experts. We thank Professor Yong-Fan Zhang at Fuzhou University for helping with the DFT calculations.

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Figures and Tables

Figure 1. Experimental results of Sn2Ga2S5: (a) photographs of title crystals; (b) EDX results; (c) SEM image and corresponding elemental mapping analysis; (d) PXRD patterns.

23



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Figure 2. Crystal structure of Sn2Ga2S5: (a) complex 3D framework was constructed from

2 2– ∞[𝐺𝑎2𝑆5 ]

layers and

schematic of the

2 2– ∞[𝐺𝑎2𝑆5 ]

1 8– ∞[𝑆𝑛2𝑆6 ]

chains via the sharing common corners; (b)

layer along bc-plane. The atom number, unit cell, and

[Ga6S15]6– (red dotted) ring are marked; (c) view of the 1D chain

1 8– ∞[𝑆𝑛2𝑆6 ]

with the

atom number is marked; (c) –COHP and (d) –ICOHP curves for Sn–S lengths (inset: the coordination geometry of Sn2 atom).

24


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Figure 3. (a) Optical transparency from UV-vis to IR spectral region (inset: diffuse-reflectance spectrum); (b) TG and DSC (inset) diagrams.

Figure 4. (a) Plot of SHG intensity vs. particle size for Sn2Ga2S5, inserted is the relative SHG signals of Sn2Ga2S5 and AgGaS2 (reference) in the particle size range of 150–210 μm. (b) Comparison of the relative SHG and LIDT intensities of Sn2Ga2S5 and AgGaS2 in the particle size range of 150−210 μm.

25



Page 26 of 30

Figure 5. Theoretical calculation results of Sn2Ga2S5: (a) band structure; (b) the total and partial DOSs; (c) frequency-dependent SHG coefficients (dij) and AgGaS2 (as a reference); (d) phase-matching (PM) range at 1450 nm. The minimum refractive index at 725 nm (n1) located between the minimum (n2) and maximum refractive index (n3) at 1450 nm indicates the fulfilment of the requirement of the PM condition.

26


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Figure 6. (a) The variation of static coefficient (dij) as a function of the cut-off energy (eV) for Sn2Ga2S5. (b) The associated partial charge density maps in the energy regions of VB-II and CB-I are shown with an isovalue of 0.015 e/Bohr3.

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Table 1. Crystallographic data and refinement details for Sn2Ga2S5.

formula

Sn2Ga2S5

fw

537.12

crystal system

Orthorhombic

crystal color

Red

space group

Pna21 (No.33)

a (Å)

12.428(5)

b (Å)

6.226(3)

c (Å)

10.894(4)

α (deg.)

90

β (deg.)

90

γ (deg.)

90

V (Å3)

842.9(6)

Z

4

Dc (g/cm3)

4.23

μ (mm-1)

13.326

GOOF on F2

1.115

R1, wR2 (I > 2σ(I))a

0.0416, 0.0774

R1, wR2 (all data)

0.0448, 0.0787

largest diff. Peak / hole (e/Å3)

0.969 / –1.039

aR 1

= Σ||Fo|–|Fc||/Σ|Fo|, wR2 = [Σw(Fo2–Fc2)2/Σw(Fo2)2]1/2

28


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Table 2. Property comparison between selected Sn2Ga2S5 and AgGaS2. Sn2Ga2S5

AgGaS2

d15 = 33.56 dij (pm/V) at 2.05 μma

d24 = 26.93

d36 = 18.2

d33 = 20.52 0.57(obs)b

0.60(obs)b

0.41(cal)a

0.46(cal)a

0.57–13.8c

0.48–11.4c

2.14(obs)b

2.56(obs)b

1.66(cal)a

1.07(cal)a

static birefringencea

0.236

0.039

average refractive index at 2.05 μma

2.81

2.49

average static dielectric cons.a

7.64

6.15

absorp. edge (μm) trans. range (μm) Eg (eV)

aCalculated

value. See more details in the SI.

bMeasured

on polycrystalline sample.

cMeasured

on single crystals.

29



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TOC

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Sn2Ga2S5: A Polar Semiconductor with Exceptional Infrared

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