PbGa2GeS6: An Infrared Nonlinear Optical Material Synthesized by

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PbGa2GeS6: An Infrared Nonlinear Optical Material Synthesized by an Intermediate-Temperature Self-Fluxing Method Yi-Zhi Huang,† Hao Zhang,† Chen-Sheng Lin,† Wen-Dan Cheng,† Zhengxiao Guo,‡ and Guo-Liang Chai*,† †

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China ‡ Department of Chemistry, University College London, London WC1H 0AJ, United Kingdom S Supporting Information *

ABSTRACT: A new non-centrosymmetric (NCS) polar sulfide PbGa2GeS6 has been synthesized by a self-fluxing method at a relatively low temperature of 550 °C with desirable properties for nonlinear optical (NLO) applications. Its structure is built by three types of infinite chains intersecting in three dimensions, where the NCS building units are tetragonal pyramids of PbS4 and tetrahedra of MS4 (M = Ga, Ga/Ge). Powder of PbGa2GeS6 exhibits phasematchable (PM) second-order NLO activity with strong second harmonic generation (SHG) intensity of 0.5× AgGaS2 at the particle size of 150−210 μm, high laser-induced damage threshold (LIDT) of 5× AgGaS2, and a wide transmission range in infrared (IR) region (0.47−23 μm). First-principles calculations suggest that the macroscopic SHG response is originated from the cooperation of the lone pairs on Pb2+ and MS4 (M = Ga, Ga/Ge) tetrahedra. Considering its strong PM SHG response, high LIDT, wide IR transmission range, and relatively low synthesis temperature, PbGa2GeS6 should be a promising candidate for high-power IR NLO applications.

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SHG response.4 However, only a few NCS Pb-containing chalcogenides have been reported so far, partly due to the difficulty in the synthesis of such structures. For example, Pb4Ga4GeQ12 (Q = S, Se) and PbGa2MSe6 (M = Si, Ge) are synthesized by solid-state reactions of elements at high temperatures (800−980 °C);5a,6a PbGa4S7 has been obtained from Bi2S3 flux at a high temperature up to 1000 °C;7 Na0.5Pb1.75GeS4 and Li2PbGeS4 are formed in molten A2S (A = Na, Li).8,9 Although utilizing alkali metal sulfide fluxes greatly lowers the synthesis temperatures to 530−600 °C, the preparation of those fluxes needs not only an atmosphere of argon but also involves dangerous liquid ammonia. In our current work, PbCl2, which is readily available and air-stable and often used as a flux for the syntheses of oxides, was tried for chalcogenides, and a NCS sulfide PbGa2GeS6 was obtained at a temperature as low as 550 °C. In this study, we present the overall experimental information for PbGa2GeS6 as a potential IR NLO material, with encouraging PM behavior, strong SHG response, high LIDT, and wide transmission range in the IR region. The lone-pair effects on Pb2+ and the origination of SHG are also discussed based on first-principles calculations.

INTRODUCTION Metal chalcogenides with non-centrosymmetric (NCS) structures have been considered as a rich source of infrared (IR) second-order nonlinear optical (NLO) materials due to their second harmonic generation (SHG) activity and improved transmission in the IR region, compared with metal oxides.1−3 SHG by an NLO crystal is crucial for producing coherent light at frequencies where lasers perform poorly or are unavailable, which can find many applications in civil and military technologies, such as high-capacity communication networks and optical storage. A good NLO material should be phasematchable (PM) and possess a large SHG coefficient, a high laser-induced damage threshold (LIDT), and a wide transmission range. However, materials for commercially available wavelength ranges with reasonable efficiency are still limited. AgGaS2 and AgGaSe2 are the most well-known examples, characterized by high NLO coefficients and wide IR transparency, which have been commercially manufactured and used worldwide.1 However, these have long been criticized for their relatively low LIDT. Therefore, considerable efforts have been dedicated to discover new NCS chalcogenides in recent years.2,3 To design asymmetric crystal structures, Pb2+ with stereochemically active lone pairs especially attracts our attention. On the one hand, its polyhedra are generally known as effective NCS building units due to second-order John−Teller distortion.3 On the other hand, a recent study on the local structural contributions to the macroscopic SHG of Pb2B5O9I reveals that the distorted part of the lone pairs may actively cooperate with other units to enhance the macroscopic © XXXX American Chemical Society

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EXPERIMENTAL SECTION

Synthesis. The following chemicals were used as received: PbCl2 (Sinopharm Chemical Reagent Co., Ltd., 99%), Ga2S3, and GeS2 (CNBM (Chengdu) Optoelectronic Materials Co., Ltd., 99.99%). The Received: November 13, 2017 Published: December 28, 2017 A

DOI: 10.1021/acs.cgd.7b01586 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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the reflectance, α is the absorption coefficient, and S is the scattering coefficient.11 The mid- and far-IR spectra were recorded on a Thermo Scientific Nicolet 6700 FT-IR spectrophotometer in the range of 4000−400 and 600−100 cm−1 with dry KBr and KI as the dilute reagent, respectively. The powder SHG activities were examined by the Kurtz−Perry method with the incident light of 2.05 μm.12 The polycrystalline samples of PbGa2GeS6 were carefully ground and sieved into five distinct particle size ranges (30−46, 46−74, 74−106, 106−150, and 150−210 μm). A piece of AgGaS2 crystal was also ground and sieved into the same size ranges as the reference. The powder LIDT measurements were performed on the polycrystalline samples (150−210 μm) with a focused high-power 1.064 μm laser radiation. The laser pulse duration was set at 10 ns, and the frequency was fixed at 1 Hz. The focal distance of the used lens was 20 cm. First-Principles Calculations. Band structure and density of states (DOS) of the experimental structure were calculated using the density functional theory (DFT) method in CASTEP code.13,14 Local density approximation (LDA) functional was employed. The cutoff energy for plane wave basis set was set at 820 eV, and the Monkhorst−Pack kpoint grid size for Brillouin zone was 2 × 2 × 2.

binary starting materials were mixed in a molar ratio of 1:1:1 and sealed in an evacuated silica tube. The reactants were heated to 550 °C over 10 h, then kept at that temperature for 100 h, cooled to 250 °C over 60 h, and finally cooled to room temperature by shutting off the furnace. Light yellow crystals were found at the bottom and inner wall of the tube, which were handpicked and used for all the following measurements. Single-Crystal Structure Determination. The single-crystal diffraction data were collected on a Rigaku Mercury70 diffractometer equipped with a graphite-monochromated Mo Kα (λ = 0.71073 Å) radiation at 293 K. The experimental absorption corrections were performed. The structure was solved by direct methods and refined by the full-matrix least-squares fitting on F2 by SHELX-97.10 The initial model produced a formula of “PbGa3S6” or “PbGe3S6” since it is difficult for X-ray to differentiate Ga from Ge. Considering the balance of total charge, PbGa2GeS6 was supposed, and then the disorder of Ga and Ge was selectively treated at four crystallographically unique positions (two at Wyckoff position 16b and two at 8a) because of the observed diversity in the related interatomic distances. The result was similar to the case of the selenide isomer PbGa2GeSe66a and supported by the calculated bond-valence sums (VBS);5,22 i.e., the two general positions with a little shorter metal−S distances at 16b were assigned to the disorder occupancy of Ga/Ge = 1:1 (VBS = 3.53 and 3.58), while the other two special positions with a little longer metal−S distances at 8a were assigned to only Ga atoms (VBS = 2.96 and 2.79). It was affirmed by a field emission scanning electron microscope (FESEM, JSM6700F) equipped with an energy-dispersive X-ray spectroscope (EDX, Oxford INCA). No signal about Cl was detected on the crystals, and the molar ratio of Pb/Ga/Ge/S was very close to 1:2:1:6. The additional crystallographic details are given in Table 1. The atomic coordinates and thermal parameters, and selected interatomic distances and angles are provided elsewhere (Tables S1 and S2).

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RESULTS AND DISCUSSION Synthesis Design. Quaternary sulfide PbGa2GeS6 was obtained from a mild solid-state reaction at 550 °C with PbCl2 as the fluxing agent, which also acted as the Pb metal source. The stoichiometric reactions starting from only binary sulfides, i.e., PbS/Ga2S3/GeS2 = 1:1:1, were also assessed. No crystals but only powder-like samples resulted below 700 °C, and the produced crystals were often found to be PbGa2S4 at relatively high temperatures. In another attempt, e.g., the involvement of both PbCl2 and PbS, the results did not seem to improve, but PbGa2S4 crystals appeared at a lower temperature (about 650 °C). Therefore, the possible reaction may be speculated as follows:

Table 1. Crystallographic Data and Structure Refinement Details for PbGa2GeS6

a

formula

PbGa2GeS6

crystal system space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z formula weight density (cal.) (g/cm3) absorption coefficient (mm−1) F(000) θ range (deg) Flack parameter R1, wR2 (obs.)a R1, wR2 (all)a GOF on F2

orthorhombic Fdd2 (No. 43) 45.181(3) 7.2815(5) 11.5928(8) 90 3813.9(4) 16 611.58 4.260 27.551 4352 3.33−27.49 −0.009(14) 0.0256, 0.0485 0.0298, 0.0495 0.850

6PbCl 2 + 6Ga 2S3 + 6GeS2 → 5PbGa 2GeS6 + 2GaCl3 + GeCl4 + PbCl 2

(I)

What is more, a similar reaction also worked for a previously reported selenide analogue PbGa2GeSe6 as follows: 6PbCl 2 + 6Ga 2S3 + 6GeS2 → 5PbGa 2GeS6 + 2GaCl3 + GeCl4 + PbCl 2

(II)

The crystals could be obtained still at 550 °C, which was notably lower than the previously reported 800 °C via only binary selenides.6a In addition to alkali metal sulfides mentioned above, alkali metal chlorides have recently been similarly employed as selffluxing agents for a series of multifunctional chalcogenides AXII4XIII5Q12 (A = K−Cs; XII = Mn, Zn, Cd; XIII = Ga, In; Q = S, Se, Te), where the sulfide and selenide compounds were obtained at a relatively high temperature up to 1000 °C.15−17 The synthesis temperature in this work thus seems surprisingly low. This may mainly benefit from the relatively low melting temperature of lead chloride (497 °C), compared with those of the three alkali metal chlorides (640−770 °C).18 Compared with binary sulfides and chlorides of alkali metal as a flux for chalcogenides, PbCl2, possesses such advantages as air stable, readily accessible and much more effective in lowering the synthesis temperature. It is favorable for the growth of bulk crystal needed for practical applications, and so important for high performance NLO materials.

R1 = ∑||Fo| − |Fc||/∑|Fo|, wR2 = ∑[w(Fo2 − Fc2)2/∑w(Fo2)2]1/2.

Powder X-ray Diffraction. The powder X-ray diffraction (PXRD) patterns were recorded on a Rigaku MiniFlex II benchtop X-ray diffractometer. The observed pattern agrees well with the simulated one, which confirms the purity of the handpicked crystals (Figure S1). Optical Characterizations. The optical diffuse reflectance spectra of powdered sample were measured in the range of 200−2500 nm using a PerkinElmer Lambda 950 UV−Vis−NIR spectrophotometer equipped with an integrating sphere attachment, and BaSO4 was used as a reference. The absorption spectra were calculated via the Kubelka−Munk function: F(R) = (1 − R)2/2R = α/S, in which R is B

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Figure 1. (a) Crystal structure of PbGa2GeS6 viewed down the b axis with Pb−S bonds omitted for clarity; (b) an isolated complex chain of

∞

1 [Pb2Ga 2(Ga/Ge)2 S10] along the b axis; (c) and (d) the simple chains of (Ga4/Ge4)S4 tetrahedra along [0 1 1] and [0 1̅ 1], respectively. (Black, Pb; pink tetrahedron, GaS4; green tetrahedron, (Ga3/Ge3)S4; blue tetrahedron, (Ga4/Ge4)S4).

It is interesting to find a very closed patent (just opened) during the reviewing of this manuscript, where stoichiometric reactions starting from elementals or binary sulfides (without PbCl2) are reported to also produce a quaternary PbGa2GeS6 with a NCS space group Cc.6b Note that the product in the patent is different from our Fdd2 discussed in the below (more detailed discussion about the differences between such two space groups can be found elsewhere).6a And some other synthesis details in the patent, such as high up to 700−800 °C for powder samples and slowly cooling from 900 °C for crystals, highlight once again the high efficiency of using PbCl2 as a self-fluxing agent in the crystal synthesis and growth. Moreover, it is a pity that there is no concrete NLO measurement data such as SHG intensity to be found in the patent although such rout is declared to achieve crystals in the size of centimeter-level with IR SHG activity. Crystal Structure. PbGa2GeS6 crystallizes in the NCS polar space group Fdd2. The three-dimensional (3D) network is knitted by three types of infinite chains as shown in Figure 1.

parallel with the complex chains (i.e., along the b axis) appear in the final 3D framework. The distances of Pb−S, Ga−S, Ga/ Ge−S are 2.848(2)−3.110(2) Å, 2.282(2)−2.310(2) Å, 2.232(2)−2.252(2) Å, respectively, which are comparable to those reported in the related compounds such as Pb4Ga4GeS12, SnGa2GeS6, and BaGa2GeS6.5a,19,20 Recently, some quaternary MIIGa2MIVQ6 chalcogenides including BaGa2MIVQ6 (MIV = Si, Ge, Sn; Q = S, Se), SnGa2GeS6, and PbGa2MIVSe6 (MIV = Si, Ge) have been reported, which are found to be in three different space groups, i.e., R3, Fdd2, and Cc.20,21,19,6a More detailed discussion about the structural diversity among those due to MII can be found elsewhere.19,21 However, there are still another two MIcontaining sulfides crystallizing in Fdd2; i.e., Li2Ga2GeS6 and its isomer AgGaGeS4, need to be mentioned here.22,23 In a 3D stacking mode of ABAC along the longest axis, B and C in them are also formed by two space-crossing chains of (Ga/ Ge)S4 tetrahedra (the a−c plane in Li2Ga2GeS6 and AgGaGeS4 is equivalent to the b−c plane in PbGa2GeS6), respectively. In contrast, A is not formed by isolated complex chains as the case of PbGa2GeS6. It is a compact layer condensed by the infinite zigzag chains of MIS4 tetrahedra (vs isolated PbS4 tetragonal pyramids in PbGa2GeS6) and isolated (Ga/Ge)S4 tetrahedra (vs the zigzag chains of GaS4 and (Ga/Ge)S4 tetrahedra in PbGa2GeS6). As a result, the longest axis shrinks strikingly for Li2Ga2GeS6 and AgGaGeS4 (about half of PbGa2GeS6). Such interesting connection diversity in the same space group may be mainly due to the relatively small ionic radius of Li+ and Ag+ compared with Pb2+ and their different coordination preferences. Optical Properties. The optical transmission range of powder PbGa2GeS6 was measured via UV−vis diffusion reflectance spectra and mid- and far-IR spectra. UV absorption edge was observed at about 0.47 μm, and then the bandgap was estimated to be 2.64 eV (Figure 2a). In the far-IR spectra (the inset of Figure 2b), some wide absorption bands were found at about 440−260 cm−1, which could be related to the overlapped

∞

One is the complex chain of 1 [Pb2Ga 2(Ga/Ge)2 S10] along the b axis, where the primary NCS building units are tetrahedra of Ga1S4, Ga2S4 and (Ga3/Ge3)S4 and tetragonal pyramids of Pb1S4 (Figure 1b). The GaS4 tetrahedra connect with each other, sharing one vertex, to form the central chain, which further joins with the isolated (Ga3/Ge3)S4 tetrahedra via vertex sharing to define a zigzag motif. The PbS4 pyramids just pack into the cavities of such zigzag chain via shared edges and vertexes with the tetrahedra so as to complete the complex ∞

chain of 1 [Pb2Ga 2(Ga/Ge)2 S10]. The other two are simple chains formed by only (Ga4/Ge4)S4 tetrahedra via vertex sharing, and the directions are [0 1 1] and [01̅ 1], respectively (Figure 1c,d). (Note: the complex chains as well as both types of “blue chains” are parallel with the b−c plane, and they three alternately distributed along a axis to form the ABAC··· stacking.) Such three types of chains cross in 3D space via shared vertexes of (Ga/Ge)S4 tetrahedra and so the channels in C

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Figure 3. Powder SHG activities of PbGa2GeS6 induced by a laser at 2.05 μm with AgGaS2 as a reference: (a) SHG intensity dependence on particle size (the curve is drawn to guide the eye); (b) oscilloscope races of SHG signal at a particle size of 150−210 μm.

Figure 2. Optical transmission characterization of powder PbGa2GeS6: (a) UV−vis diffused reflectance spectra, and (b) mid- and far-IR (inset) spectra.

Table 2. Powder LIDT of PbGa2GeS6 at the Particle Size of 150−210 μm with AgGaS2 as the Reference

asymmetric and symmetric stretching of Ga−S−Ga and Ge− S−Ge modes according to the cases of Li2Ga2GeS6 and AgGaGeS4 vs AgGaS2.22 There were no additional absorption bands detected in the mid-IR region (Figure 2b). Therefore, powder PbGa2GeS6 shows a wide transparent region of 0.47− 23 μm. The powder SHG response of PbGa2GeS6 was measured by a laser at a wavelength of 2.05 μm. The SHG intensity increased with particle size growing as shown in Figure 3a. It suggests that PbGa2GeS6 is type-I phase-matchable in the IR region. The signal at the particle size of 150−210 μm was observed to be half of AgGaS2 (Figure 3b), which is almost doubled, compared with the Sn-containing isomer SnGa2GeS6 (about 1/4 × AgGaS2).19 It may be explained as the result of moderate offset among the NCS building blocks of PbGa2GeS6 compared with that of SnGa2GeS6 since they all have cross arrangement as well as parallel mode, as shown in Figure 1. In addition, the powder of PbGa2GeS6 shows a high LIDT of 7.65 MW/cm2 or 5 × AgGaS2 (Table 2), which may benefit from its wide bandgap of 2.64 eV. Although there is no reasonable mechanism for laser-induced damage as yet, it has been widely accepted that a wide bandgap supports a strong resistance to high-power laser.1−3 Theoretical Analyses. The calculated electronic band structure and DOS for PbGa2GeS6 are shown in Figure 4. As noted, near the Fermi level (EF), the highest valence band (VB) exhibits peaks and valleys, and the maximum located at Γ point.

compound

damage energy (mJ)

spot diameter (cm)

damage threshold (MW/cm2)

PbGa2GeS6 AgGaS2

65 13

1.04 1.04

7.65 1.53

On the other hand, the lowest conduction band (CB) seems rather flat, and the minimum locates between Γ and Z points (Figure 4a). Therefore, PbGa2GeS6 is an indirect bandgap semiconductor with a calculated bandgap of 1.68 eV, which is much smaller than the corresponding experimental value of 2.64 eV, estimated from the UV−vis diffuse reflectance spectra. Underestimation of the bandgap of a crystal is well-known for the DFT method. However, this would not affect the qualitative analysis of the origination of the SHG response. As shown in the DOS in Figure 4b, the metal ns/np and S 3p states are all found at both sides of EF, but the contributions from them are very different at the top of VB (above −0.8 eV), i.e., more S 3p, less Pb 6s, a little Pb 6p and negligible Ga/Ge 4s/4p. Here, (Pb 6s−S 3p)−Pb 6p antibonding−antibonding interaction is assumed. It is such a high energetic region dominated by S 3p that is responsible for the asymmetric character of the lone pairs on Pb2+. In contrast, the symmetric inner core of the lone pairs is found at a low energetic region (below −5 eV), which is characterized by the major portion of Pb 6s mixing with less S 3p in a bonding interaction. That is, the stereochemically D

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CONCLUSIONS In summary, a new NCS sulfide PbGa2GeS6 has been synthesized as a NLO material by a self-fluxing method, with a significantly reduced synthesis temperature. Its single crystal structure and optical properties including transmission range, SHG, and LIDT are characterized. The results show that PbGa2GeS6 possesses comprehensive features as a potential IR NLO material for future applications, with advantages of low synthesis temperature, wide transmission range in the IR region, good type-I PM SHG response, and very high LIDT. Moreover, the macroscopic SHG response is explained, from first-principles simulations, to originate from the cooperation of the lone pairs on Pb2+ and MS4 (M = Ga, Ga/Ge) tetrahedra.

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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.cgd.7b01586. Further crystallographic data and powder XRD pattern (PDF) Accession Codes

CCDC 1562862 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 data_ [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 Author

*E-mail: [email protected]. ORCID

Zhengxiao Guo: 0000-0001-5404-3215 Guo-Liang Chai: 0000-0003-3792-2204 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (Grant No. 2014CB845605), the National Natural Science Foundation of China (Grant Nos. 91222204 and 21473203), and EPSRC (Grant No. EP/L018330/1).

Figure 4. (a) Band structure and (b) density of states for PbGa2GeS6.

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2+

activity of lone pairs on Pb does not arise from purely metalbased 6s-6p hybrid, but results from Pb 6s mixing with both S 3p and Pb 6p, which is comparable to the model of the metal 6s and 6p mixing with O 2p supported by DFT calculations and experimental measurements in the case of α-PbO and αBi2O3.24 As a result, the electronic transitions near EF may be explained to originate mainly from the distorted part of the lone pairs on Pb2+ to the unoccupied Ga/Ge−S antibonding states as well as Pb 6p states. It may also explain the microscopic cooperation mechanism between the lone pairs on Pb2+ and the tetrahedra of MS4 (M = Ga, Ga/Ge) contributing to the macroscopic SHG response of PbGa2GeS6 since the optical response of material mainly originates from the electronic transitions close to the bandgap.25

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