Na2Hg3M2S8 (M = Si, Ge, and Sn): New Infrared Nonlinear Optical

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Na2Hg3M2S8 (M = Si, Ge, and Sn): new infrared nonlinear optical materials with strong second harmonic generation effects and high laser-damage thresholds Kui Wu, Zhihua Yang, and Shilie Pan Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b00683 • Publication Date (Web): 05 Apr 2016 Downloaded from http://pubs.acs.org on April 11, 2016

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Chemistry of Materials

Na2Hg3M2S8 (M = Si, Ge, and Sn): new infrared nonlinear optical materials with strong second harmonic generation effects and high laser-damage thresholds Kui Wu, Zhihua Yang, Shilie Pan* Key Laboratory of Functional Materials and Devices for Special Environments of CAS; Xinjiang Key Laboratory of Electronic Information Materials and Devices; Xinjiang Technical Institute of Physics & Chemistry of CAS, 40-1 South Beijing Road, Urumqi 830011, China Corresponding author: Shilie Pan, E-mail: [email protected] Phone: (+86)991-3674558; Fax: (+86)991-3838957.

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Abstract: A new family of non-centrosymmetric (NCS) isostructural compounds, Na2Hg3M2S8 (M = Si, Ge, and Sn), were successfully synthesized. They crystallize in the tetragonal space group P 4 c2 with Z = 2. Their major structures are composed of infinite cross-connected ∞(HgS3)n chains and isolated [MS4] ligands, and show the interesting tunnel features. Interestingly, compared with the structures of A2Hg3M2S8 (A = alkali metal, Na−Cs), it can be found that the structural symmetries show a gradually rising tendency from Cs to Na analogues as a result of cation size effect, which rarely exists in quaternary alkali metal chalcogenides. Property measurements show that title compounds exhibit strong second harmonic generation (SHG) effects with a phase-matching behavior at 2.09 µm, wide transparency range in the IR region, and

large

laser-damage

thresholds (LDTs).

Remarkably,

Na2Hg3Si2S8 and

Na2Hg3Ge2S8 achieve the suitable balance between large SHG effects (1.3 and 2.2 × benchmark AgGaS2) and high LDTs (4.5 and 3 × AgGaS2), respectively, and can be expected to be potential nonlinear optical (NLO) candidates in the IR region. Moreover, band structures and NLO properties of title compounds are also theoretically studied and the calculated NLO coefficients are consistent with the experimental observations.

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Introduction Developing new infrared (IR) light sources has become increasingly important in satisfying various practical applications.1,2 An efficient approach to realize the output of IR laser is frequency-conversion technology. IR nonlinear optical (NLO) materials, as critical devices, are indispensable for the above technology.3−7 Unfortunately, commercial IR NLO materials still exist a series of self-defects to hinder their wide application.8−10 For that reason, to explore the new IR NLO materials with excellent properties (broad IR transparency range, large second harmonic generation (SHG) response, and high damage resistance) is extremely urgent.11−37 Recently, increasing attentions have been focused on Hg (d10 element)-containing metal chalcogenides, since the Hg cation has variable coordination numbers (2 to 4) with S atoms and HgS is usually amenable to participate in the reaction while introducing alkali-metals into reaction system, which produce an efficient approach to design interesting crystal structures.38−45 Several Hg-containing metal chalcogenides with excellent properties as promising IR NLO materials were discovered as yet. For instance, HgGa2S4 crystallizes in tetragonal I 4 space group with a “defect” chalcopyrite structure, and has shown good application in high-power ultrafast laser system with large NLO coefficient (d36 = 34 pm/V) and high damage resistance.46,47 BaHgS2 belongs to NCS space group (Pmc21) with two different types of Hg atoms (HgS4 tetrahedron and linear−like HgS2 unit) in its structure, and appears large SHG response (d33= −59.7 pm/V) about 5 times that of benchmark AgGaS2.48 α− and β− K2Hg3M2S8 (M = Ge, Sn) crystallize in different space groups (α, Aba2; β, C2), and β−phase exhibits good 3



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NLO coefficient (deff = 20 pm/V) and large laser-damage threshold (LDT).49 In addition, tetrahedral MIVS4 (MIV = Si, Ge, and Sn) with heavy metals or lively alkali metals have an opportunity to enlarge the odds of forming NCS structures.50−56 More importantly, introducing alkali metals into crystal structure is also helpful to improve the band gap of material and further increase the LDT.57−59 Based on the above strategies, we have focused the investigation on Na−Hg−M−S (M = Si, Ge, and Sn) systems that combine the MS4 tetrahedra with heavy element Hg and alkali metal Na to obtain new phases. Three new isostructural NCS compounds, Na2Hg3M2S8 (M = Si, Ge, and Sn), were successfully synthesized. Remarkably, Na2Hg3Si2S8 and Na2Hg3Ge2S8 exhibits the outstanding properties as potential IR NLO candidates, such as strong SHG responses (1.3 and 2.2 × AgGaS2), wide IR transparency ranges (~ 20 µm), and high LDTs (4.5 and 3 × AgGaS2), respectively, which achieve the rational balance of two important parameters (large NLO coefficient and high LDT) for efficient IR material. Experimental Synthesis High-purity raw materials were commercially purchased without further refinement. Ar−filled glovebox was used to complete the preparation processes since the Na metal is easily oxidized in air. Na2Hg3Si2S8. A mixture of Na (2 mmol, 0.046 g), HgS (3 mmol, 0.678 g), Si (2 mmol, 0.056 g), and excess sulfur (8 mmol, 0.256 g) were loaded into a graphite crucible that avoids the reaction between alkali metal (Na) and silica tube, then put it into a tidy 4


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silica tube that was washed with deionized water and dried in high-temperature to remove the inside impurities, and the tube was flame-sealed under 10–3 Pa. The tube was heated to 700 °C in 30 h and kept at this temperature about 60 h, then slowly down to 300 °C by 4 days, finally quickly cooled to ambient temperature. The product was carefully washed with N, N−dimethylformamide (DMF) to remove the excess S and other byproducts, then dried in 100 °C. Many pale-yellow crystals (Na2Hg3Si2S8, 80% yield) and a small amount of unreacted HgS were discovered. In addition, Na2Hg3Si2S8 is still stable in air after several months, which shows good chemical stability (non-hygroscopic). Na2Hg3Ge2S8 and Na2Hg3Sn2S8. Their preparation processes including starting composition and heating profile are similar to that of Na2Hg3Si2S8. But their highest reaction temperature was set to be about 600 °C, which is different with that of Na2Hg3Si2S8 (700 °C). Similarly, yellow crystals were also obtained by washing with DMF and stable in air. Structure Determination High−quality single−crystals were picked for data collections under a Bruker SMART APEX II 4K CCD diffractometer using Mo Kα radiation (λ = 0.71073 Å) at 296 K. Direct method and SHELXTL program package were used to solve and refine the crystal structures.60 Multi−scan method was chosen for absorption correction.61 Finally, rational anisotropic thermal parameters for all atoms were obtained by the anisotropic refinement and extinction correction. Energy dispersive spectrometer (EDS) elemental analysis method was used to check the actually existed elements in 5



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the crystal structure, and no other elements are found. Final structures were also checked with PLATON program and no other symmetries were found. Crystallographic data for title compounds are reported in Table 1, and selected bond distances and angles are shown in Table S1 in the Supporting Information. Powder XRD Measurement The obtained single-crystals by spontaneous crystallization were firstly ground to micro-crystal powders, and then the micro-crystal powders were used for powder XRD measurement by an automated Bruker D2 X-ray diffractometer at room temperature. In comparison with calculated and experiment results (Figures 1 and Figure S1 in the Supporting Information), it can be found that the experimental powder XRD patterns are basically consistent with calculated values except for a few of extra peaks corresponding to small amounts of unreacted HgS with Na2Hg3Si2S8 (Figure S1a in the Supporting Information). UV–Vis–NIR Diffuse-Reflectance Spectra The ground micro-crystal powders of title compounds were placed in a pallet for measurement. With Shimadzu SolidSpec-3700DUV spectrophotometer, optical diffuse reflectance spectra were measured in the wavelength range from 190 to 2600 nm. Raman Spectra Hand-picked single-crystals were put on an object slide, and then a LABRAM HR Evolution spectrometer equipped with a CCD detector by a 532 nm laser was used to record the Raman spectra. The integration time was set to be 5 s. 6


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Infrared Spectra IR spectra were measured on the ground micro-crystal powders of title compounds mixed with dried KBr powder. A Shimadzu IRAffinity-1 Fourier transform infrared spectrometer was recorded the measurement data in the range from 400 to 4000 cm−1 with a resolution of 2 cm−1. Second-Harmonic Generation Measurements Powder SHG responses of title compounds were investigated by a Q-switch laser (2.09 µm, 3 Hz, 50 ns) on ground micro-crystal powders with different particle sizes, including 38−55, 55−88, 88−105, 105−150, and 150−200 µm. The AgGaS2 crystal was ground and sieved into the same size ranges as the reference. LDT Measurements We have estimated the LDTs of powdered title compounds with powdered AgGaS2 sample as the reference by a pulse laser (1.06 µm, 10 Hz, and 10 ns) in the same condition. The detail test procedure is as follow: the LDTs of title compounds were evaluated on micro-crystal powders (150−200 µm) with a pulsed YAG laser (1.06 µm, 10 ns, 10 Hz). Similar size of AgGaS2 is chosen as the reference. The judgment criterion is as follows: with increasing laser energy, the color change of the powder sample is constantly observed by optical microscope to determine the damage threshold. To adjust different laser beams, an optical concave lens is added into the laser path. The size of damage spot is measured on the scale of optical microscope. Computational Description In order to further investigate the relationship of structure–property, the electronic 7



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structures of title compounds were studied by density functional theory (DFT) based on ab initio calculations.62 The exchange-correlation potential was calculated using Perdew–Burke–Ernzerhof

(PBE) functional within

the

generalized

gradient

approximation (GGA) with the scheme.63 The following orbital electrons were treated as valence electrons, Na: 2p6 3s1, Hg: 5d10 6s2, Si: 3s2 3p2, Ge: 4s2 4p2, Sn: 5s2 5p2, S: 3s2 3p4. To achieve energy convergence, a plane-wave basis set energy cutoff was 900 eV within normal-conserving pseudo-potential (NCP),64,65 and the Monkhorst–Pack scheme was 4 × 4 × 4 in the Brillouin Zone (BZ) of the primitive cell are chosen. As important parameters for NLO crystals, SHG coefficients were also calculated. Owing to the discontinuity of exchange correlation energy, the experimental value is usually larger than that of calculated band gap. Thus, scissors operators are used to make the conduction bands agree with the experimental values.66 Results and Discussion Description of Structures Title compounds crystallize in same NCS space group P 4 c2 and Na2Hg3Ge2S8 is chosen as the representative to illuminate its crystal structure. As for Na2Hg3Ge2S8, its whole structure is composed of infinite ∞(HgS3)n chains and isolated [GeS4] tetrahedra units to form a 3D tunnel structure with charge balanced Na cations (Figure 2b). There should be noted that the HgS4 units have two different types of geometries: one is highly distorted “seesaw-shaped” tetrahedra Hg2S4 with two short lengths (2.396 Å) and two long lengths (2.831 Å) of Hg−S, and the angle between the two longest Hg-S contacts is 101.9(61)° (Figure 2d); another is regular “seesaw” Hg1S4 with four 8


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identical d(Hg−S) = 2.553 Å and the angle between the two legs of Hg1S4 “seesaw” is 122.4(39)° (Figure 2c). In addition, the Hg1S4 units are surrounded by four Hg2S4 ligands to make up similar tetrahedral shapes, but the Hg2S4 units only connect with two Hg1S4 ligands to form dumbbell-shapes (Figure 2a). Moreover, the Hg1S4 and Hg2S4 tetrahedra are alternately linked with each other by sharing corner to form infinite ∞(HgS3)n chains along the a or b axis, then infinite ∞(HgS3)n chains are cross connected with each other to form layer structure in the ab plane (Figure 2a). Remarkably, multiple types of tunnel structures that formed with the interconnection of the NaS6 octahedra and the GeS4 tetrahedra are also found in their structures (Figure 2b), such as completely closed (tunnel A), wide-open (tunnel B), and semi-closed states (tunnel C), which may benefit many promising applications, such as hydrogen-storage, ionic adsorption and photo-catalysis. While compared with the structures of other alkali metal analogues, it can be interestingly found that they crystallize in different space groups and their structural symmetries show a gradually rising tendency from Cs to Na analogues (Cs, P1 ; Rb, P21/c; α−K, Aba2; and Na, P 4 c2),49,50 and this phenomenon rarely exists in the quaternary alkali metal chalcogenides. Main structural differences are as follow: (i) although all above mentioned compounds have the two unique Hg atoms in their asymmetric units, the coordination numbers of HgSn units are obviously different, such as pseudotrigonal HgS3 and dumbbell-shaped HgS2 for Cs analogue (n = 3 and 2), seesaw−like HgS4 and linear−like HgS2 for Rb and α−K analogues (n = 4 and 2), and only the seesaw−like HgS4 units for title compounds (n = 4 and 4); (ii) the coordination 9



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numbers of alkali metals in each compound are also different (Cs, 10; Rb, 7; α−K, 8; and Na, 6); (iii) while a two-dimensional layer structure with the large Cs cation is found, the smaller alkali metals (Rb, K, and Na) prefer a 3D framework structure. Thus, it indicates that crystal structures and dimensionalities tend to be changed with different cation sizes, and this phenomenon can be also found in other relative systems, such as BaGa4S7 (Pmn21)15 and PbGa4S7 (Pc)67, Ba2GeSe4 (P21/m)68 and Mg2GeSe4 (Pnma)69, Na4MgSi2Se6 (C2)16 and Na8Pb2(Si2Se6)2 (C2/m)70, Li2CdSnS4 (Pmn21)58 and Na2CdSnS4 (C2)71. Optical Properties Measured Raman spectra of title compounds (Figure 3 and Figure S2 in the Supporting Information) show a shift to lower absorption energies from Na2Hg3Si2S8 to Na2Hg3Sn2S8, but an overall view from the spectra shows the similar patterns among title compounds. The absorption peaks above approximately 300 cm–1, including Na2Hg3Si2S8 (470, 434 cm–1), Na2Hg3Ge2S8 (410, 395, 362 cm–1) and Na2Hg3Sn2S8 (360, 334 cm–1), can be assigned to the characteristic absorptions of Si–S, Ge–S and Sn–S modes, respectively. Note that their peak positions are severely affected by the tetravalent (IV) metals and this result is similar to these of other alkali metal analogues.49,50 Moreover, several peaks located between 200 and 300 cm–1, such as Na2Hg3Si2S8 (245, 216 cm–1), Na2Hg3Ge2S8 (297, 272 cm–1) and Na2Hg3Sn2S8 (292, 254 cm–1) are attributed to Hg–S bonding interactions. In addition, the absorptions below 200 cm−1 are primarily corresponding to Na–S vibrations for the title compounds. IR transmission range as a critical parameter is also evaluated on 10


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micro-crystal powders and the results indicate that they have broad transmission regions from 4000 to 500 cm–1, namely, 2.5 – 20 µm, which are comparable to the IR absorption edges of other IR NLO powdered compounds, such as LiCdGeS4 (~ 22 µm),13 AgGaS2 (~ 23 µm),8 SnGa4S7 (~ 25 µm),18 BaGa4Se7 (~ 18 µm),14 and CsCd4Ga5S12 (~ 25 µm).19 Based on the comparable transmission range between the title compounds and powdered AgGaS2, it can be concluded that IR absorption edges on single crystal for title compounds may be comparable to that of single-crystals AgGaS2 (~ 13 µm) and Rb2Hg3Ge2S8 (~ 12 µm) that covers the two important atmospheric transparent windows (3−5 and 8−12 µm). These Optical band gaps of title compounds were also measured based on their polycrystalline samples (Table 2). Among them, Na2Hg3Si2S8 and Na2Hg3Ge2S8 have larger band gaps (2.86 and 2.68 eV), respectively, than that of commercial AgGaS2 (2.64 eV).72 Na2Hg3Sn2S8 has a relatively narrow band gap (2.45 eV) corresponding to the crystal color (deep yellow), but it is still much larger than those of other typical crystals, such as ZnGeP2 (1.65 eV)10 and AgGaSe2 (1.75 eV).72 Nowadays, low LIDT has become a serious issue for one IR NLO material. In general, large optical band gap commonly contributes to improve the LDT for an IR NLO material and eliminate the effect of two-photon absorption at convenient pump laser.73 To estimate the LDTs of new NLO compounds on powder samples has become a feasible and semi-quantitative method according to the research reported by Guo23 and Cheng18 et al. Thus, in this work, the LDTs on micro-crystal powders of title compounds were systemically evaluated with powdered AgGaS2 sample as the reference under a pulse laser (1.06 µm, 10 Hz, and 10 ns) and 11



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the result is shown in Table 2. Among them, the LDT of Na2Hg3Sn2S8 is comparable to that of AgGaS2 owing to its slightly smaller band gap. However, Na2Hg3Si2S8 and Na2Hg3Ge2S8 show good laser damage resistances and the measured LDTs (54 and 36 MW/cm2) are about 4.5 and 3 times that of powdered AgGaS2 (12 MW/cm2), respectively, which are comparable to those of powdered SnGa4Se7 (4.6 × AgGaS2),18 powdered Na2In2MS6 (4.0 × AgGaS2),24 and single crystal BaGa4Se7 (3.7 × AgGaS2).18 Thus, Na2Hg3Si2S8 and Na2Hg3Ge2S8 can be applied in high-energy laser system as potential candidates. We have also investigated their SHG responses since the title compounds belong to a NCS space group (P 4 c2). With a Q-switched pulse laser (2.09 µm, 3 Hz, and 50 ns), the powder NLO properties of title compounds were systemically studied in different particle sizes. Figure 4 shows the curves of the particle size versus the SHG intensity, which indicate that title compounds exhibit good SHG conversion efficiencies18 of about ηsample/ηAGS = 1.3 for Na2Hg3Si2S8, 2.2 for Na2Hg3Ge2S8, and 2.8 for Na2Hg3Sn2S8 times that of the benchmark AgGaS2 at 150−200 µm particle size with a phase-matching behavior. As known to all, for an outstanding IR NLO material, it must have the following desired characters, such as broad spectral transmission region, good chemical stability, a large NLO coefficient, and high LDT. Based on our overall experimental results, it shows that Na2Hg3Si2S8 and Na2Hg3Ge2S8 basically satisfy the above conditions as promising IR NLO materials (Table 3), such as strong SHG responses (1.3 and 2.2 × AgGaS2), wide IR transparency range (~20 µm), high LDTs (4.5 and 3 × AgGaS2), and good chemical stability (non–deliquescent), which achieve 12


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the balance of two important parameters (large NLO coefficient and high LDT) and should have good prospect to be applied in the IR field, and may give us a good way to design new promising IR NLO materials by introducing the alkali metal and d10 elements. Furthermore, large-size crystals of title compounds will be grown in the future and their application will be further evaluated in the IR region. Theoretical Studies To investigate deeply the electronic structures of title compounds, DFT method was chosen to complete the relative calculation. From the calculated results (Figure 5 and Figure S3 in the Supporting Information), it can be seen that all of them are direct band-gap compounds with the calculated values of 1.73, 1.65, and 1.58 eV from Na2Hg3Si2S8 to Na2Hg3Sn2S8, respectively, smaller than the experimental observations, which is a common problem that underestimated with the test values for GGA calculation.63 Three compounds have the similar density of states (DOS) with the similar crystal structures (Figure 5), thus, Na2Hg3Ge2S8 is discussed as a representative here. From –25 to –20 eV, only Na–3s orbital appears at this region and illustrates no contribution to the Na–S bond. The valence states between –15 and –10 eV show the obvious hybridization of Ge 4s, 4p and S 3p orbitals, corresponding to the Ge–S chemical bond. And the bands in region (−7.5 to −5.0 eV) are mainly from Hg 5d, Ge 4s,4p and S 3s,3p states, which generate some effects to the Ge−S and Hg−S bonds. Moreover, for the upper part of valence bond (−5 eV to FL), there exists a few of hybridizations between Ge 4s, S 3p and Hg 5s,5p orbitals. In addition, the bottom of the conduction 13



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bond is mainly composed of S 3p, Ge 4s,4p, and Hg 6s states. Thus, the HgS4 and GeS4 units determine the optical band gap of Na2Hg3Ge2S8. Based on the similar band-structures of title compounds, their optical absorptions can be viewed as the effect of both the HgS4 and MS4 (M= Si, Ge, Sn) units, respectively. Moreover, with the scissors-corrected method, the NLO coefficients of title compounds are also calculated. Since title compounds belong to the point group 4 2m, there only have one independent SHG tensor (d36) for them under the restriction of Kleinman symmetry. The calculated SHG coefficient (d36) is 15.4 for Na2Hg3Si2S8, 29.2 for Na2Hg3Ge2S8, and 33.2 pm/V for Na2Hg3Sn2S8, which match well with the experimental observations. Conclusion In summary, a new family of NCS quaternary metal sulfides, Na2Hg3M2S8 (M = Si, Ge, and Sn), were synthesized by solid-state method. All of them are isostructural and crystallize in space group P 4 c2 of tetragonal system. Their structures are composed of infinite

∞(HgS3)n

chains and isolated GeS4 tetrahedra units, and form a

three-dimensional (3D) tunnel structure with charge balanced Na cations. Compared with the structures of alkali-metal analogues, it can be interestingly found that their structural symmetries show a gradually rising tendency from Cs to Na analogues as a result of cation size effect, which rarely exists in quaternary alkali metal chalcogenides. Title compounds exhibits strong SHG responses at a laser radiation wavelength of 2.09 µm with a phase-matching behavior. Optical properties (Raman, IR and diffuse reflection spectra) were also studied and indicate that they have wide 14


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transparent region (~20 µm) that covers the two critical atmospheric transparent windows (3−5 and 8−14 µm). In addition, Na2Hg3Si2S8 and Na2Hg3Ge2S8 have high powder LDTs of 4.5 and 3 times that of benchmark AgGaS2, respectively. Accordingly, Na2Hg3Si2S8 and Na2Hg3Ge2S8 satisfy the key requirements as potential IR NLO crystals, including wide transparent region (~ 20 µm), good chemical stability (non–deliquescent), high LDTs (4.5 and 3 × AgGaS2), and strong SHG responses (1.3 and 2.2 × AgGaS2), which may give us a good way to design new promising IR NLO materials by introducing the alkali metal and d10 elements.

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Supporting Information. CIF files, selected bond distances and angles, powder XRD, diffuse reflection, IR, and Raman spectra, electronic structure, TDOS and PDOS.

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ACKNOWLEDGMENTS This work was supported by the Xinjiang Program of Cultivation of Young Innovative Technical Talents (Grant No. 2014731029), the Western Light Foundation of CAS (Grant No. XBBS201318), the National Natural Science Foundation of China (Grant Nos. 51402352 and 51425206).

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REFERENCES (1) Duarte, F. J. Tunable Laser Applications, 2nd ed.; CRC Press: Boca Raton, FL, 2008; Chapters 2, 9, and 12. (2) Demtröder, W. Laser Spectroscopy, 3rd ed.; Springer, Berlin, Germany, 2009. (3) Nikogosyan, D. N. Nonlinear Optical Crystals: A Complete Survey, 1st ed.; Springer: New York, 2005. (4) Bloembergen, N. Nonlinear Optics, World Scientific, River Edge, NJ, 4th edn, 1996. (5) Dmitriev, V. G.; Gurzadyan, G. G.; Nikogosyan, D. N. Handbook of Nonlinear Optical Crystals. Springer: New York, 1999. (6) Chung, I.; Kanatzidis, M. G. Metal Chalcogenides: A Rich Source of Nonlinear Optical Materials. Chem. Mater. 2014, 26, 849−869. (7) Jiang, X. M.; Guo, S. P.; Zeng, H. Y.; Zhang, M. J.; Guo, G. C. Large Crystal Growth and New Crystal Exploration of Mid-Infrared Second-Order Nonlinear Optical Materials. Struct. Bond 2012, 145, 1−43. (8) Okorogu, A. O.; Mirov, S. B.; Lee, W.; Crouthamel, D. I.; Jenkins, N.; Dergachev, A. Y.; Vodopyanov, K. L.; Badikov, V. V. Tunable Middle Infrared Downconversion in GaSe and AgGaS2. Opt. Commun. 1998, 155, 307−312. (9) Boyd, G. D.; Storz, F. G.; McFee, J. H.; Kasper, H. M. Linear and Nonlinear Optical Properties of Some Ternary Selenides. IEEE J. Quantum Electron. 1972, 8, 900−908.

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Exhibiting

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Laser-Damage

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List of Figures & Tables Figure 1. Powder XRD pattern of Na2Hg3Ge2S8. Table 1. Crystal data and structure refinement for Na2Hg3M2S8 (M = Si, Ge, and Sn). Figure 2. (a) A layer with corner-shared HgS4 groups in the ab plane. Hg1S4: pink; Hg2S4: green. (b) View of the structure of Na2Hg3Ge2S8 along the c-axis. A: completely closed tunnel; B: wide-open tunnel; C: semi-closed tunnel. (c) Coordination environment of regular “seesaw” Hg1S4. (d) Coordination environment of distorted “seesaw” Hg2S4. Figure 3. UV–Vis–NIR diffuse-reflectance, IR, and Raman spectra of Na2Hg3Ge2S8. Figure 4. (a) SHG intensity versus particle size for title compounds and AgGaS2; (b) Comparison on SHG intensities for title compounds and AgGaS2 at the particle size (150–200 µm). Table 2. Experimental optical properties for Na2Hg3M2S8 (M = Si, Ge, and Sn). Figure 5. Band structure (a), and PDOS and TDOS plot (b) of Na2Hg3Ge2S8. Table 3. Property comparison on Na2Hg3Si2S8 and Na2Hg3Ge2S8 with AgGaS2.

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Figure 1. Powder XRD pattern of Na2Hg3Ge2S8.

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Table 1. Crystal data and structure refinement for Na2Hg3M2S8 (M=Si, Ge, and Sn). Empirical formula

Na2Hg3Si2S8

Na2Hg3Ge2S8

Na2Hg3Sn2S8

Formula weight Temperature Crystal system Space group

960.41

1141.61

a = 8.725(5) Å c = 8.972(11) Å 2, 683.0(10) Å3 4.670 g/cm3 0.146 × 0.080 × 0.079

1049.41 296 (2) K Tetragonal P 4 c2 a = 8.848(2) Å c = 9.032(4) Å 2, 707.1(4) Å3 4.929 g/cm3 0.115 × 0.083 × 0.076

a = 9.077(4) Å c = 9.195(7) Å 2, 757.6(7) Å3 5.005 g/cm3 0.145× 0.103 × 0.085

99.8 %

100.0 %

99.8 %

1.002 R1 = 0.0339 wR2 = 0.0558 R1 = 0.0589 wR2 = 0.0630 −0.01(3) 0.0035(2) 1.340 and −1.549 e Å−3

1.000 R1 = 0.0247 wR2 = 0.0474 R1 = 0.0344 wR2 = 0.0513 0.021(19) 0.0054(2) 1.054 and −1.164 e Å−3

1.005 R1 = 0.0228 wR2 = 0.0363 R1 = 0.0327 wR2 = 0.0392 0.028(19) 0.00315(10) 0.990 and −0.920 e Å−3

Unit cell dimensions Z, V Density (calculated) crystal size (mm3) Completeness to theta = 27.28 Goodness-of-fit on F2 Final R indices [Fo2> 2σ(Fo2)][a] R indices (all data) [a] Absolute structure parameter Extinction coefficient Largest diff. peak and hole [a]

R1 = Fo − Fc / Fo and wR2 = [w (Fo2 − Fc2)2 / wFo4]1/2 for Fo2 > 2σ (Fo2)

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Figure 2. (a) A layer with corner-shared HgS4 groups in the ab plane. Hg1S4: pink; Hg2S4: green. (b) View of the structure of Na2Hg3Ge2S8 along the c-axis. A: completely closed tunnel; B: wide-open tunnel; C: semi-closed tunnel. (c) Coordination environment of regular “seesaw” Hg1S4. (d) Coordination environment of distorted “seesaw” Hg2S4.

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Figure 3. UV–Vis–NIR diffuse-reflectance, IR, and Raman spectra of Na2Hg3Ge2S8.

33



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Figure 4. (a) SHG intensity versus particle size for title compounds and AgGaS2; (b) Comparison on SHG intensities for title compounds and AgGaS2 at the particle size (150–200 µm).

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Table 2. Experimental optical properties of Na2Hg3M2S8 (M = Si, Ge, and Sn). Compounds

Trans. region (µm)

Eg (eV)

Relative SHG intensity

LDT

Na2Hg3Si2S8

0.40−20

2.86

1.3 × AGS

4.5× AGS

Na2Hg3Ge2S8

0.45−20

2.68

2.2 × AGS

3× AGS

Na2Hg3Sn2S8

0.50−20

2.45

2.8 × AGS

~AGS

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Figure 5. Band structure (a), and PDOS and TDOS plot (b) of Na2Hg3Ge2S8.

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Table 3. Property comparison on Na2Hg3Si2S8 and Na2Hg3Ge2S8 with AgGaS2. Na2Hg3Si2S8

Na2Hg3Ge2S8

AgGaS2a,19

1.3× AGS

2.2 × AGS

AGS

SHG coeff. (pm/V)

15.4

29.2

12.5

Eg(exp.) (eV)

2.86

2.68

2.56

Eg(cal.) (eV)

1.73

1.65

2.70

Trans. range (µm)

0.40−20

0.45−20

0.47−20

LDTs (MW/cm2)

54 (4.5× AGS)

36 (3 × AGS)

12 (AGS)

Relative SHG intensities

a

AGS = AgGaS2

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Na2Hg3M2S8 (M = Si, Ge, and Sn): New Infrared Nonlinear Optical

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