Ternary Mixed-Metal Cd4GeS6: Remarkable Nonlinear-Optical

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Ternary Mixed-Metal Cd4GeS6: Remarkable Nonlinear-Optical Properties Based on a Tetrahedral-Stacking Framework Meng-Yue Li,†,‡ Bing-Xuan Li,† Hua Lin,*,† Yong-Fang Shi,† Zuju Ma,*,§ Li-Ming Wu,*,†,∥ Xin-Tao Wu,† and Qi-Long Zhu*,†

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State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China ‡ College of Chemistry, Fuzhou University, 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 S Supporting Information *

effective design strategies for the discovery of novel quaternary NCS mixed-metal chalcogenides with remarkable NLO properties because of the superposition of polarizations via two kinds of asymmetric building units (ABUs). 9−39 For example, Na2ZnGe2S6, a monoclinic 3D tunnel structure formed by GeS4 and ZnS4 ABUs, exhibits excellent NLO properties including large band gap (Eg = 3.25 eV), moderate dij (0.9AGS), and high LIDT (6.0AGS).20 Trigonal RbCd4In5Se12 adopts a 3D diamond-like framework structure and displays strong powder SHG (ca. 40AGS) at the particle size of 46−74 μm.12 Tetragonal Na2Hg3Ge2S8 achieves the perfect balance between dij (2.2AGS) and LIDT (3.0AGS).19 Compared with the extensive research works on quaternary systems, the studies on ternary chalcogenometallates of group 14 metals with divalent B-subgroup metal cations MIIxMIVyQz are relatively scarce. In this contribution, we successfully obtained a ternary mixed-metal sulfide, Cd4GeS6. Until recently, only a single-crystal structure and a few optical properties of Cd4GeS6 have been reported,40,41 but the systematic experimental and theoretical investigations of its NLO qualities (e.g., dij and LIDT) in the MFIR region have never reported. In what follows, the simple synthesis and crystal-structure assembly of Cd4GeS6 will be described. In addition, the SHG response, LIDT along with computational NLO properties, is systemically illustrated for the first time. Cd4GeS6 belongs to the NCS monoclinic system [space group, Cc (No. 9); Pearson symbol, mC44; idealized Wyckoff sequence, a].11 In a symmetric unit, there are 11 crystallographically unique atoms, including four Cd atoms, one Ge atom, and six S atoms, respectively, and all of them are at Wyckoff sites of 4a. As shown in Figure 1, Cd4GeS6 exhibits a complex tetrahedral-stacking framework structure consisting of two types of [Cd2S7] asymmetric groups (denoted as G1 and G2, hereafter) and dispersed GeS4 tetrahedra. The G1 is formed by Cd1S4 and Cd2S4 tetrahedra adopt corner-sharing with normal Cd−S distances (2.933−2.774 Å) (Figure 1a), and each G1 is further interconnected with other 4 neighboring groups into a

ABSTRACT: Noncentrosymmetric (NCS) mixed-metal chalcogenides have attracted significant attention lately because of their structural multiplicities, strong secondharmonic-generation (SHG) efficiencies (dij) and large laser-induced damage thresholds (LIDTs), which make them promising nonlinear-optical (NLO) materials in mid- and far-IR (MFIR) applications. In this work, a ternary mixed-metal material, Cd4GeS6, has been synthesized by reacting CdS with GeS2 via a solid-state method at 1273 K. It exhibits a unique tetrahedral-stacking NCS framework structure consisting of two types of [Cd2S7] asymmetric groups and dispersed GeS4 tetrahedra. Remarkably, Cd4GeS6 shows type-I phase-matching ability and achieves a desired balance between strong dij (about 1.1AgGaS2) and large LIDT (about 3.6AgGaS2), demonstrating that this material satisfies the essential requirements as a promising MFIR NLO candidate. Moreover, theoretical calculations were performed for a better understanding of the structure−property relationships.

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econd-order nonlinear-optical (NLO) crystals are crucial to laser technology and science, and the search for new NLO materials, especially mid- and far-IR (MFIR, 2−20 μm) applications, has attracted widespread concern.1,2 Noncentrosymmetric (NCS) metal chalcogenides with wide MFIR transmittance, structural multiplicities, and strong secondharmonic-generation (SHG) efficiencies (dij) are one of the most suitable materials for MFIR NLO applications.3−6 Currently, widely used NCS chalcogenides such as AgGaS2 (AGS) and AgGaSe2 exhibit several of the necessary conditions required of MFIR NLO crystals; however, low laser-induced damage thresholds (LIDTs) and low thermal conductivities are inherent drawbacks that are encountered as well.7,8 Therefore, the search for new MFIR NLO materials with a subtle balance between strong dij and large LIDTs is still a great challenge and a hot research topic. In recent years, the combination of B-subgroup metal ions with d10 electronic configurations (Zn2+, Cd2+, Hg2+, etc.) and group 13 or 14 metal anions has proven to be one of the most © XXXX American Chemical Society

Received: June 19, 2018

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DOI: 10.1021/acs.inorgchem.8b01682 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

Figure 1. Depiction of the unique tetrahedral-stacking framework of Cd4GeS6: (a) two types of Cd2S7 groups (denoted as G1 and G2) formed by different Cd atoms; (b) 2D single [Cd2S5]∞ layer parallel to the c axis; (c) arrangement of a 1D [Cd2S6]∞ chain in the ab plane; (d) 2D [Cd2S5]∞ layers stacking along the c direction in ABAB mode, in which A has a glide of a/2 to B along the a axis; (e) 3D Cd−S network consisting of 1D [Cd2S6]∞ chains with the unit cell marked; (f) complex 3D Cd−Ge−S framework.

eV).9 In addition, Cd4GeS6 is thermally stable up to 973 K and resolved after this temperature, which is confirmed by characterization of the temperature-dependent PXRD pattern (Figure S1). Because Cd4GeS6 belongs to a NCS space group (Cc), its powder SHG property was systemically investigated. The SHG measurement was carried out using the Kurtz−Perry method44 and AGS was used as the reference. As shown in Figure 3a, the

2D single [Cd2S5]∞ layer parallel to the ab plane (Figure 1b). Eventually, these layers are stacking in a sequence in ABAB mode along the ab-plane, in which A has a glide of a/2 to B along a-axis as shown in Figure 1d. The G2 consists of Cd3S4 and Cd4S4 tetrahedra, and these G2 groups are connected to form a 1D [Cd2S6]∞ chain in the ab-plane via sharing vertexes (Figure 1c). Subsequently, these chains are interconnected together to further create the 3D Cd−S network along the a-axis (Figure 1e). Finally, the 3D complex Cd−Ge−S framework is constructed from the dispersed GeS4 tetrahedra and 3D Cd−S network (Figure 1f). The homogeneous polycrystalline target product of Cd4GeS6 was confirmed by the powder X-ray diffraction (PXRD) patterns, as illustrated in Figure 2a. Figure 2b shows the diffuse-reflectance spectrum of Cd4GeS6, and the experimental Eg is 2.52 eV (corresponding to 492 nm) based on the Kubelka− Munk function.42 Such data is essentially in agreement with the benchmark AGS (Eg = 2.56 eV) but still larger than those of wellknown MFIR NLO crystals, such as mixed-metal semiconductors ZnGeP2 (Eg = 1.65 eV)43 and AgGaSe2 (Eg = 1.75

Figure 3. (a) Phase-matching curves for Cd4GeS6 and AGS (reference). (b) Relative values of the measured SHG and LIDT for Cd4GeS6 and AGS (reference) at the same particle size (150−210 μm).

SHG responses of Cd4GeS6 increase with the particle size, increasing in the range of 30−210 μm, which indicates a type-I phase-matching behavior at the 2050 nm laser. Cd4GeS6 shows a strong SHG effect that is approximately 1.1 times that of benchmark AGS in the same particle size (150−210 μm), as shown in Figure 3b. As is known, the single-pulse measurement on polycrystalline samples reported by the Guo group is a feasible and effective way to estimate the LIDTs of NLO materials in recent years,45 especially for a new MFIR NLO compound, of which its worthiness to grow large-sized single crystals is not currently

Figure 2. Experimental results of Cd4GeS6: (a) PXRD patterns; (b) diffuse-reflectance spectra. Inset: Picture of the polycrystalline products. B

DOI: 10.1021/acs.inorgchem.8b01682 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

theoretical values of three main dynamic frequency-dependent din (d11, d13, and d15) are as follows: d11 = 11.82 pm V−1, d13 = 11.25 pm V−1, and d15 = 12.04 pm V−1. In addition, the cutoff energy as a function of the largest d15 has been studied to reveal the source of the SHG constituent of Cd4GeS6. Figure 4d exhibits that in the sections of VB-1 (dominated by the S 3p and Cd 4d states) and CB-2 (dominated by the S 3p, Cd 5p, and Ge 4s/4p states), the d15 value has dramatic changes in these two regions, which mainly influence the NLO coefficient. Thus, the NLO activity of Cd4GeS6 originates from the synergism of CdS4 and GeS4 ABUs that build the 3D tetrahedral-stacking structure. Also notice that the SHG coefficient d15 of Cd4GeS6 is close to the AGS (d36 = 18 pm V−1),12 which is well consistent with the experimental observation at the wavelength of 2050 nm (Figure 3a). In summary, an outstanding ternary mixed-metal MFIR NLO material, Cd4GeS6, with a unique NCS tetrahedral-stacking structure was systematically investigated for the first time. Remarkably, the powdered experimental results demonstrate that Cd4GeS6 shows a perfect balance of a strong dij (1.1AGS) and a large LIDT (3.6AGS), as well as good type-I phasematchable ability. Moreover, theoretical calculations reveal that the NLO effects of Cd4GeS6 originate from the synergism of CdS4 and GeS4 ABUs that build the 3D-ordered stacking dense framework. Such an interesting structure−property relationship would shed some useful light on the design or prediction of new MFIR NLO chalcogenides in the related systems.

clear. In this study, the powdered LIDT of Cd4GeS6 was systemically compared with a crushed AGS single crystal (reference) using a 1064 nm pulse laser with a pulse width τp of 8 ns, as summarized in Figure 3b. The measured value of Cd4GeS6 (5.18 MW cm−2) is 3.6 times that of AGS (1.44 MW cm−2) under the same granularity, that is, 150−210 μm. Such data are comparable to some outstanding MFIR NLO chalcogenides, for instance, single-crystal BaGa 4 S 7 (3.0AGS),46 powdered BaNa2SnS4 (1.0AGS),47 BaLi2GeSe4 (1.0AGS),48 and AgGa2PS6 (5.1AGS).49 For a more thorough analysis and understanding of the crystal structure−NLO property relationships in Cd4GeS6, a firstprinciple calculation based on density functional theory was carried out. The result of the band structure suggests a semiconducting state with a direct Eg of 1.33 eV for Cd4GeS6 at the same G point between the valence-band maximum (VBM) and conduction-band minimum (CBM), as shown in Figure 4a. The theoretical value is less than the experimental value (Eg = 2.55 eV), attributable to the underestimation of the GGA calculation.50

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01682. Synthesis, property characterization, and computational

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Figure 4. Theoretical calculations for Cd4GeS6: (a) band structure, the Fermi level (EF) is set at 0.0 eV; (b) PDOSs; (c) dynamic frequencydependent SHG coefficients din; (d) cutoff energy versus d15.

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AUTHOR INFORMATION

Corresponding Authors

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

The projected density of states (PDOSs) of Cd4GeS6 calculated by the Perdew−Burke−Ernzerhof (PBE) approach are shown in Figure 4b. Around the Fermi level (EF), S 3p and Cd 4d electrons contribute mostly in the VB-1 region, whereas the S 3p and Cd 5s electrons make a major contribution in the CB-1 region. Therefore, the Eg absorption is mainly determined by the charge transitions in [CdS4]6−tetrahedral units along with smaller contributions from [GeS4]4− tetrahedral units. Note that S atoms have two different coordination environments in the title compound, four-coordinated with Cd atoms and threecoordinated with Cd and Ge atoms. As illustrated in Figure S2, we can see that the three-coordinated S atoms contribute more to the upper part of VB and the bottom of CB than that of fourcoordinated S atoms. Such an effect of nonbonding electrons can be more clearly seen from the electron density distributions of VBM and CBM, as shown in Figure S3. A similar feature has recently been reported in the Hg2GeSe4 compound.51 Moreover, dij for Cd4GeS6 was also calculated. Cd4GeS6 adopts class m and has 10 nonzero dij values, of which 6 are independent (d11, d12, d13, d15, d24, and d33) under the restriction of Kleinman’s symmetry.52 As revealed in Figure 4c, the

ORCID

Hua Lin: 0000-0002-7241-9623 Qi-Long Zhu: 0000-0001-9956-8517 Author Contributions

M.-Y.L. and B.-X.L. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (Grants 21771179, 21771182, 21501177, 21571020, and 21301175), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant XDB20010200), the “Chunmiao Projects” of Haixi Institute of Chinese Academy of Sciences, and the One Thousand Young Talents Program under the Recruitment Program of Global Youth Experts. C

DOI: 10.1021/acs.inorgchem.8b01682 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.8b01682 Inorg. Chem. XXXX, XXX, XXX−XXX