Sn2Ga2S5: A Type of IR Nonlinear-Optical Material | Inorganic

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Sn2Ga2S5: A Type of IR Nonlinear-Optical Material Zhi-Hui Shi, Yang Chi, Zong-Dong Sun, Wenlong Liu,* and Sheng-Ping Guo* School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou, Jiangsu 225002, P. R. China

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between 1 and Ga2Q3 (Q = S and Se) continuously stimulates our interest in knowing its NLO activity and potential. Fortunately, both theoretical and experimental results indicate that 1 is NLO-active. Here, its structural chemistry and NLO behaviors are investigated, together with the theoretical calculations on its electronic structure and NLO properties. (Note that during our submission we noticed another paper addressing the NLO properties of 1 that just came online by Chemistry of Materials.32) In this work, red block single crystals of 1 were synthesized using a facile solid-state reaction between stoichiometric SnS, Ga, and S at 950 °C. A pure powder crystalline sample (Figure S1) was picked out for further study. Because the structure of 1 is known, the structure description focuses on the relationship between it and the IR NLO material Ga2S3 (Figure 1),30 which may be helpful to understanding its NLO performance. There are two Sn, two Ga, and five S atoms in the independent unit of 1. Its 3D structure is built by two types of units, the SnS3 triangle pyramid and GaS4 tetrahedron. The Sn−S distances in 1 are in the range of 2.631(1)−2.743(1) Å, which is very common for normal Sn−S bond lengths. Because the chemical formula of 1 can be written as “Ga2S3·2SnS”, we considered whether the structure was relevant with Ga2S3 and SnS. Then, the structures of all known Ga2S3 and SnS polymorphs were checked, and it was found that monoclinic Ga2S3 (Cc)- and orthorhombic SnS (Pnma)-like slabs can be isolated in the structure of 1.30,33 As shown in Figure 1, the 3D structure of 1 can be cut into two parts, marked A and B, respectively. The basic unit in A is a 12-membered ring comprised of six Ga and six S atoms, and each ring links with six such neighboring rings by sharing two Ga and one S atoms. Therefore, part A is a wrinkled layer parallel to the bc plane, which is very similar to the “[Ga2S3]n layer” in monoclinic Ga2S3. Part B is a distorted cis,trans-[SnS]n chain along the b axis, which is almost the same as that in orthorhombic SnS. Therefore, the structure of 1 can be built by the A:2B intergrowth, namely, the combination of monoclinic Ga2S3 and orthorhombic SnS with a ratio of 1:2. This structural relevance is interesting because monoclinic Ga2S3 is a very promising NLO material, although the structure of SnS is centrosymmetric (CS). Segregation of NCS units by CS ones here definitely has an influence on the NLO behavior of 1, different from that of monoclinic Ga2S3, which will be discussed later. To date, there are in total less than 10 MII2MIII2Q5 compounds that can be found from ICSD (Table S4), and only Pb2B2S5 (P41212) and 1 crystallize in the NCS structure.34 The NCS structure and structural relevance to Ga2S3 of 1 allow

ABSTRACT: The deficiency of nonlinear-optical (NLO) materials in the IR region inspires strong research interest in this field. Here, Sn2Ga2S5 (1), crystallizing in the orthorhombic Pna21 space group, demonstrates obvious NLO activity, around a maximum of 1.6 times that of AgGaS2 and a strong laser-induced damage threshold of 9.7 times that of AGS. 1 represents the first NLO-active compound in the MII2MIII2Q5 (MII = divalent Ca, Sr, Ba, Pb, Sn, and Eu; MIII = B, Al, Ga, and In; Q = S and Se) family. The NLO performances of 1 are systematically studied experimentally and theoretically.

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econd-order nonlinear-optical (NLO) crystals applied in the middle-IR (MIR) region are important for a laser’s application covering the important atmosphere windows. However, the already commercial ones cannot fulfill the market’s requirement in view of their intrinsic disadvantages. Therefore, exploring novel MIR NLO crystals is important for lasers’ applications.1−6 When surveying the recent achievements on this topic, it can be noticed that the combination of a strong electropositive alkali-earth (AE) metal and a MQ4 (M = Ga, In, Si, Ge, and Sn; Q = S and Se) tetrahedral functional motif in one structure takes up a large proportion for MIR NLO crystals.7,8 The relatively more studied systems include BaxSnyQz9,10 II−II′−IV−VI4,11,12 II−III2−IV−VI6,13,14 II3− II′−IV2−VI8,15,16 II4−III4−IV−VI12,17 and II−III4−VI718,19 (II = AE metal; II′ = Zn, Cd, and Hg; III = Ga and In; IV = Si, Ge, and Sn; VI = S and Se). Because divalent Sn and Pb have coordination preferences and ionic radii similar to those of AE metals, in most cases, the lattice sites of AE in AE chalcogenides can be occupied by Sn and Pb atoms. This character has been verified by the discovery of many Pb2+/ Sn2+-MQ4 type MIR NLO materials.20−27 Therefore, Pb2+/ Sn2+ chalcogenides also have high opportunities as new MIR NLO materials. When potential chalcogenides containing AE2+/Pb2+/Sn2+ ions and MQ4 tetrahedra are screened, a family of compounds with the formula MII2MIII2Q5 (MII = divalent metal Ca, Sr, Ba, Pb, Sn, and Eu; MIII = B, Al, Ga, and In; Q = S and Se) attract our attention28 because their NLO properties have never been studied, although a few of them crystallize in the noncentrosymmetric (NCS) structures. Convinced by the success of other such chalcogenides and the NLO research gap of II2− III2−VI5 compounds, Sn2Ga2S5 (1) was selected as the pioneer for this family.29 On the other hand, binary Ga2S3 and Ga2Se3 exhibit promising NLO properties,30,31 and the structure of 1 can be considered to be the 1:2 intergrowth of monoclinic Ga2S3 and orthorhombic SnS. This structural relationship © XXXX American Chemical Society

Received: July 8, 2019

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DOI: 10.1021/acs.inorgchem.9b02021 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 2. (a) Size-dependent SHG responses of 1 and AGS under a 2.1 μm incident laser. (b) Comparison of the SHG signals at 25−45 μm particle sizes.

same particle sizes (75−100 μm) were irradiated by a 1064 nm laser with a pulse width τp of 10 ns in a 1 Hz repetition, and they were damaged when the laser energy reached 5.11 and 0.53 mJ, suggesting LIDT values of 20.1 and 2.08 MW cm−2 for 1 and AGS, respectively. Therefore, the LIDT of 1 is around 9.7 times that of AGS, although the band gap of 1 is smaller than that of AGS. This phenomenon is usually observed.9,27,31,37−39 The IR spectrum measured between 4000 and 400 cm−1 (2.5−25 μm) exhibits that there is no IR absorption peak, demonstrating that 1 is transparent in 2.5−25 μm (Figure S2). The ultraviolet−visible-near-IR (UV−vis−NIR) diffuse-reflectance spectrum (Figure S3) suggests an optical band gap of 2.02 eV, which is consistent with its red color. The combination of these two spectra indicates that 1 is transparent from 0.61 to 25 μm. To better understand the NLO behaviors of 1, calculations for the band structure (BS), density of states (DOS), and optical properties were carried out by using the CASTEP code in Material Studio.40 The BS (Figure 3a) indicates a theoretical optical band gap of 1.705 eV for 1, close to the experimental value of 2.02 eV. The bottom of the conduction band (CB) and top of the valence band (VB) locate at the R and G points,

Figure 1. Structural relationship between 1, monoclinic Ga2S3, and orthorhombic SnS.

it has an opportunity to be NLO-active. Therefore, its NLO activities are studied in this work, and it is expected that this work can stimulate more investigations on this MII2MIII2Q5 family. The NLO data of 1 was measured using the modified Kurtz−Perry method under 2.1 μm laser radiation,35 and benchmark AGS with the same particle sizes served as the standard. It can be observed that the second-harmonicgeneration (SHG) intensity increases first, while gradually it decreases when the particle sizes are larger than 45 μm, indicating a typical non-phase-matchable (NPM) behavior (Figure 2a). When the particle sizes are in the range of 25−45 μm, 1 exhibits a maximum SHG response, around 1.6 times that of AGS (Figure 2b). However, the SHG intensity ratio between 1 and AGS becomes smaller and smaller when the particle size increases. Its laser-induced damage threshold (LIDT) was measured using a commonly accepted powder technique.30,36 The powder samples of 1 and AGS with the B

DOI: 10.1021/acs.inorgchem.9b02021 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. BS (a) and DOS (b) for 1. The Fermi level is set at 0 eV.

Figure 4. (a) Calculated frequency-dependent SHG coefficients of 1. (b) Frequency-dependent birefringence Δn of 1.

respectively, indicating an indirect band gap for 1. The total and partial DOSs of 1 are drawn in Figure 3b. Obviously, the top of the VB is mainly contributed by the S 3p orbital, together with minor Sn 5s and Sn 5p orbitals. The bottom of the CB is primarily comprised of the Sn 5p orbital and a little S 3p orbital. These analyses demonstrate that the optical absorption between CB and VB is mainly ascribed to the charge transition from the S 3p to Sn 5p states and has almost no relationship with the Ga element. The NLO coefficients of 1 are obtained by calculating the optical dielectric constants first, which can be expressed by a complex number equation containing real (ε1Re; Figure S4a) and imaginary (ε2Im; Figure S4b) parts along the threecoordinate axis directions. The average ε2Im value suggests that the strongest absorption of 1 locates at 4.66 eV (Figure S4b), primarily assigned to the electronic transition from S 3p to Sn 5p orbitals, which represent the interband electronic transitions between the VB and CB because of the semiconductor-like electronic structure. Compound 1 has five (χ15, χ24, χ31, χ32, and χ33) independent nonzero second-order NLO coefficients (Figure 4a) because of its NCS point group of mm2. They are computed to be 24.9, 29.83, 25.51, 29.5, and 30.46 pm V−1, respectively. The theoretical frequency-dependent birefringence Δn was also calculated according to its structure model, and the value is 0.12 under 2.1 μm (Figure 4b). This Δn value is out of the range of ∼0.03−0.10, the moderate one beneficial to obtaining NLO phase-matchability. Therefore, it is consistent with the fact that 1 is NPM. Some strategies can be used to tune Δn, possibly conducive to realizing PM. Introducing structural units with less delocalized electronic density can decrease Δn. Besides, the exclusion of stereochemically active lone pair cations can decrease Δn.

Therefore, it might be effective in reducing Δn via partial substitution of Sn2+ by alkali-earth metal cations. The NLO data of 1, monoclinic Ga2S3, and AGS are included in Table S5. It is well-known that AGS is hitherto the most successful IR NLO material, and low LIDT is the leading disadvantage restricting its application for high-energy lasers. Therefore, the current research toward IR NLO materials focuses on balancing the NLO response and LIDT.41 This work presents the new NLO material 1, whose LIDT (9.7 × AGS) is much enhanced compared with that of AGS, even though its NLO effect is not encouraging. Moreover, it is transparent in 0.61−25 μm. These results suggest that, although 1 is not competitive with AGS (low SHG intensity and NPM), its high LIDT and not very small SHG response still give us some expectations. On the other hand, the NLO comparison between 1 and Ga2S3 is interesting considering their structural relevance. This case indicates that the insertion of CS SnS into NCS Ga2S3 decreases both the SHG response and LIDT. To better understand the contribution of the SnSlike slab to the NLO performance of 1, we can get some information to a certain extent from the dipole moment change for one unit cell from Cc Ga2S3 to 1 and the respective dipole moments for the SnS3 and GaS4 units in one unit cell. The calculation results indicate that the dipole moment of the GaS4 units in one unit cell is much smaller than that of Cc Ga2S3, and SnS3 units contribute a larger dipole moment than GaS4 units in 1 (Figure S5 and Table S6). Therefore, it may be concluded that the insertion of a SnS-like slab into Cc Ga2S3 will distort GaS4 units in Cc Ga2S3 a lot, causing the GaS4 units in 1 to have much smaller total dipole moments. C

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Inorganic Chemistry In conclusion, 1, as the first MII2MIII2Q5 family member being NLO-active, is extensively studied here. Its 1.6 × AGS NLO effect, 9.7 × AGS LIDT, transparent window of 0.61−25 μm, and NPM behavior suggest that it is promising via some suitable modifications. This study opens a new region for IR NLO material exploration. There are many subjects in the future that should be addressed to make clear this family’s potential as IR NLO materials, including the syntheses of more NCS members and crystal engineering to tune the chemical compositions and crystal structures, with the goal of obtaining materials with desired balanced NLO performances superior to the benchmark ones.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b02021. Experimental details, crystallographic data and some other tables, powder X-ray diffraction pattern, IR and UV spectra, and optical dielectric function figures (PDF) Accession Codes

CCDC 1916921 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Sheng-Ping Guo: 0000-0002-9703-1537 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant 21771159), the Priority Academic Program Development of Jiangsu Higher Education Institutions, and the Qing Lan Project from Yangzhou University.

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DOI: 10.1021/acs.inorgchem.9b02021 Inorg. Chem. XXXX, XXX, XXX−XXX