Syntheses, Structures, and Nonlinear Optical Properties of Two

Feb 4, 2016 - Synopsis. The first two Na-containing sulfides Na2In2SiS6 (1) and Na2In2GeS6 (2), synthesized by conventional high-temperature solid-sta...
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Syntheses, Structures, and Nonlinear Optical Properties of Two Sulfides Na2In2MS6 (M = Si, Ge) Shu-Fang Li, Bin-Wen Liu, Ming-Jian Zhang, Yu-Hang Fan, Hui-Yi Zeng,* and Guo-Cong Guo* 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 S Supporting Information *

ABSTRACT: The first two new Na-containing sulfides Na2In2MS6 (M = Si (1), Ge (2)) in the Na2Q−B2Q3−CQ2 (B = Ga, In; C = Si, Ge, Sn; Q = S, Se) system were prepared for the first time through conventional high-temperature solidstate reaction. They are isostructural with space group Cc (No. 9) in monoclinic phases and feature three-dimensional frameworks built by the 1∞[In2MS6]2− (M = Si, Ge) chains through corner-sharing InS4 tetrahedra and MS4 (M = Si, Ge) tetrahedra, with Na+ cation located in the cavities. They display moderate second harmonic generation (SHG) conversion efficiencies compared with commercial AgGaS2, with phase-matching behavior at 1800 nm and laser-induced damage thresholds 6.9 and 4.0 times higher than that of AgGaS2, respectively. Therefore, the output SHG intensities of 1 and 2 will be ∼4.3 and 4.0 times larger than that of AgGaS2, when the intensity of incident laser increased to close the damage energy of 1 and 2, indicating their potential for high-power nonlinear optical application.

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K, Rb, and Cs),17 Ba4Ga4SnSe12 and Ba6Ga2SnSe11,18 Li2In2MS6 (M = Si, Ge),19 Ba4CuGa5Q12 (Q = S, Se),20 which have two or three kinds of different metal centers and show fine IR nonlinear properties. NCS chalcogenides are especially attractive for their potential as SHG materials in IR region; a series of acentric compounds in the quasi-ternary systems A2Q−B2Q3−CQ2 (A = Li, Cu, Ag; B = Ga, In; C = Si, Ge, Sn; Q = S, Se) have been reported.21 Examples include Ag2In2GeS6 (Ag2S/In2S3/GeS2 = 1:1:1; space group: Cc),21 AgGaGeS4 (Ag2S/Ga2S3/GeS2 = 1:1:2; space group Fdd2),22 AgGaGe3Se8 (Ag2Se/Ga2Se3/GeSe2= 1:1:6; space group Fdd2),23 Li2Ga2GeS6 (Li2S/Ga2S3/GeS2 = 1:1:1; space group Fdd2).24 It is worthy to note that the quasi-ternary systems A2X−B2Q3−CQ2 show improved NLO properties compared to their ternary analogues AgGaS2 and AgGaSe2 in transparency region, birefringence, LIDTs, etc.21 For example, the quaternary compound AgGaGeS4, constituted with AgGaS2 and GeS2 in 1:1 ratio, has higher LIDT than AgGaS2 with the addition of GeS2 and exhibits a promising alternative to the broadly used AgGaS2.24 Although many compounds of the A2Q−B2Q3−CQ2 system have been reported, nothing is known so far about the Na2Q−B2Q3−CQ2 system. Thus, exploring new quaternary acentric sodium compounds in the Na2Q− B2Q3−CQ2 system is necessary. On the basis of these opinions, we perform the syntheses, crystal structure characterizations, and preliminary optical properties of Na2In2SiS6 (1) and

INTRODUCTION Nonlinear optical (NLO) materials are of importance in optical domains, for instance, laser frequency conversion and optical parameter oscillators.1,2 NLO materials can be divided into ultraviolet (UV) NLO materials, visible NLO materials, and infrared (IR) NLO materials based on the different application wavelength ranges. In the past decades, metal oxide NLO crystals in UV and visible ranges have been broadly explored, a series of excellent NLO crystals such as KTiOPO4 (KTP),3 KH2PO4 (KDP),4 LiNbO3 (LNO),5 BaB2O4 (BBO),6 and LiB3O5 (LBO)7 have been developed, which have extensively fulfilled the practical demands in the UV and visible ranges. In IR region, the current NLO materials are chalcopyrite semiconductors, for instance, AgGaS2 (AGS),8 AgGaSe2 (AGSe),9 ZnGeP2 (ZGP);10 on one hand, they seriously suffer from relatively low laser-induced damage thresholds (LIDTs) due to their relatively small band gaps, limiting their high-power applications. On the other hand, in spite of the Li-containing ternary chalcogenides such as LiInS2 and LiInSe2,11,12 which have higher LIDTs than the above chalcogenides due to their wider band gap,13 it is challenging to prepare large crystal with high quality on account of the corrosivity of Li+ ion to the quartz tube. Consequently, exploring new NLO materials with prospective properties in the IR region are quite desired.14 Since the combination with low electronegativity element, for instance, alkali metal or alkaline earth metal, into structure can broaden the band gap of the compound, a higher LIDT can be generated.15 In recent years, a series of alkali or alkaline earth element-containing NCS quaternary chalcogenides have been reported, such as Ba2Ga8MS16 (M = Si, Ge),16 ANb2PSe10 (A = © 2016 American Chemical Society

Received: September 25, 2015 Published: February 4, 2016 1480

DOI: 10.1021/acs.inorgchem.5b02211 Inorg. Chem. 2016, 55, 1480−1485

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Inorganic Chemistry Na2In2GeS6 (2), the first two members of Na-containing sulfides in the Na2Q−B2Q3−CQ2 system.

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Powder X-ray Diffraction. The powder X-ray diffraction data of 1 and 2 were measured on Rigaku MiniFlex600 diffractometer with Cu Kα (λ = 1.540 57 Å) radiation in reflection mode. The range of 2θ is 5−65° in a step size of 0.02° at room temperature. The experimental results agreed with the simulated patterns generated using the Mercury program (Figure S1), indicating that the powder is pure. Elemental Analysis. The measurement were executed on a JSM6700F scanning electron microscope furnished with an energydispersive X-ray spectroscope (EDX, Oxford INCA), which gave the empirical formula of Na1.98In1.96SiS6.17 for 1 and Na2.02In2.13GeS5.96 for 2. No other elements were detected (Figure S2). UV−vis−near-IR Diffuse Reflectance and FT-IR Spectroscopy. PerkinElmer Lambda 900 UV−vis−NIR spectrometer was used to measure the optical diffuse reflectance spectrum of 1 and 2 over 200− 2500 nm at room temperature with pure BaSO4 as a standard for comparison. The absorption data were converted from reflection spectra using the Kubelka−Munk equation.29 PerkinElmer Spectrum One FT-IR spectrophotometer was used to measure the IR data for 1 and 2 over the scope 4000−400 cm−1 with the pure KBr pellets as the baseline. Second Harmonic Generation Measurements. The powder SHG measurements for 1 and 2 were performed using a modified Kurtz−Perry powder technique laser radiation with a wavelength of 1800 nm.30 The powder samples were prepared by sieving into several discrete particle sizes of 30−50, 50−75, 75−100, 100−150, 150−200, and 200−300 μm. AgGaS2 crystals with similar particle sizes served as the standard. The Andor DU420A-BR-DD CCD camera was used to detect the doubled frequency signals (900 nm). Powder Laser-Induced Damage Threshhold Measurements. LIDTs were estimated for 1, 2, and AGS (as a reference) by single pulse measurement method.31 Both compounds and AGS in the same size (50−75 μm) were selected and pressed into disks. The samples were exposed to an incident laser at 1064 nm with a pulse width τp of 10 ns and 1 Hz repetition. The measurements were performed by the laser beam enhanced until the damage spot was noticed. The damage spots and powers of laser beam were measured by vernier caliper and the Nova II sensor with a PE50-DIF-C energy sensor, respectively. Thermal Analyses. Differential scanning calorimetry (DSC) analysis data were measured using a Mettler Toledo thermal analyzer under a nitrogen atmosphere. Approximately 8 mg of 1 and 2 were sealed in silica tubes evacuated to 1 × 10−3 Torr, heated to 850 °C for 1 and 800 °C for 2 at 10 °C/min, and then cooled to 30 °C at a rate of 10 °C/min. Electronic Band Structure Calculations. Energy band structures of 1 and 2 were completed by built calculation models directly from the single-crystal XRD analysis. The band structures and densities of state (DOSs) were calculated by the CASTEP code based on the density functional theory (DFT) provided by the Materials Studio package.32,33 Na: 2p63s1, In: 4d105s25p1, Si: 3s23p2 (Ge: 4s24p2), and S: 3s23p4 were regarded as the valence electrons. Energy cutoff for 1 and 2 were determined to be 370 eV. And a 1 × 2 × 1 Monkhorst−Pack kpoint in the numerical integration of Brillouin zone was adopted. The Fermi level Ef = 0 eV was selected as the reference.

EXPERIMENTAL SECTION

Syntheses. The starting materials used in reactions, such as Na2S (90%), In (99.99%), Si (99.99%), Ge (99.99%) and S (99.99%), were put in a dry glovebox filled with Ar. The title compounds were synthesized by solid-state reaction of Na2S, In, Si (Ge), S mixed in the mole ratio of 3:1:1:5.25 The mixtures in total 300 mg were finely ground, introduced into a quartz tube, and then flame-sealed under vacuum (∼1 × 10−4 Torr). These tubes were put in a temperaturecontrolled muffle furnace, held at 450 °C for 5 h, raised to 820 °C in 10 h, retained there for 36 h, and then cooled to 400 °C in 100 h and powered off. Crystals of 1 (yellow) and 2 (orange) were gained with the yields based on crystals selected being ∼20% for 1 and 2, respectively. The crystals were stable in moisture conditions and air. The pure powder phases were prepared for 1 and 2 through a stoichiometry mixture of Na2S, In, M (M = Si, Ge), S mole ratio of 1:2:1:5, which were held at 620 °C for 72 h and then decreased to room temperature in 20 h. The products were reground and reprocessed by the identical procedure to improve the purity. Single-Crystal Structure Determination. Single crystal with dimensions of 0.133 × 0.120 × 0.103 mm3 for 1 and 0.179 × 0.105 × 0.094 mm 3 for 2 were, respectively, selected for structure determination. The measurements were performed on a Rigaku Pilatus CCD diffractometer employing a graphite-monochromated Mo−Kα radiation (λ = 0.710 73 Å) at 293 K. An ω-scan technique was used for the intensity data sets collection, and CrystalClear software was used for data reduced.26 Structures of 1 and 2 were established through direct methods, and the refinement was done through full-matrix least-squared methods on F2 by the Siemens SHELXL version 5 package of crystallographic software.27 The formulas take collectively into account crystallographically refined compositions and requirements of charge neutrality. ADDSYM/PLATON was used for checking the additional symmetry of the final structure.28 Crystallographic data and structural refinement information are given in Table 1. The isotropic equivalent thermal parameters and the selected bond distances are summarized in Tables S1 and S2 in the Supporting Information.

Table 1. Crystal Data and Structure Refinement Parameters for 1 and 2 formula weight crystal system space group a (Å) b (Å) c (Å) B (deg) V (Å3) Z Dcalcd (g cm−3) μ (mm−1) θ range (deg) GOF on F2 R1a [I > 2σ(I)] wR2b [I > 2σ(I)] R1a (all data) wR2b (all data) Flack parameter x extinction coefficient Δρmax/Δρmin (e Å−3)

1

2

496.07 monoclinic Cc 12.039(3) 7.8077(14) 12.165(3) 108.341(5) 1085.4(4) 4 3.036 5.526 3.53−25.50 1.105 0.0182 0.0486 0.0182 0.0486 0.02(2) 0.0073(2) 0.774/−0.532

540.57 monoclinic Cc 12.169(5) 7.819(3) 12.235(6) 108.645(6) 1103.1(8) 4 3.255 8.001 3.15−27.45 1.092 0.0292 0.0562 0.0329 0.0770 0.00(3) 0.0008(1) 1.149/−1.199

R 1 = ∑∥F o | − |F c ∥/∑|F o |. ∑[w(Fo2)2)]1/2. a

b

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RESULTS AND DISCUSSION Crystal Structures. Compounds 1 and 2 are isostructural and crystallize with space group Cc in the monoclinic phases. Their asymmetric units contain crystallographically independent two Na, two In, one M (M = Si, Ge), and six S atoms. Take 2 for example, where the Na(1) and Na(2) are, respectively, coordinated by four and five S atoms, while the In(1), In(2), and Ge atoms are all connected with four S atoms in a distorted tetrahedral geometry. Each In(1)S4 and In(2)S4 tetrahedra are connected into a 1∞[In2S6]6− chain via sharing S(1) and S(2) atoms along the c direction (Figure 1a). Then 1∞[In2S6]6− chain is further connected with GeS4 tetrahedra by sharing S(3) and S(6) atoms to form a 1∞[In2GeS6]2− chain (Figure 1b). The three-dimensional (3D) anionic framework of 2 is formed by

wR 2 = ∑[(w(F o 2 − F c 2 ) 2 )/ 1481

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Inorganic Chemistry

Figure 1. View of the 1∞[In2S6]6− chain (a) and the chain (b) in 2 down the b axis.

1 2− ∞[In2GeS6]

Figure 3. (a) Phase-matching curves and (b) SHG signals in the particle size of 150−200 μm of 1 and 2 with AgGaS2 as a reference.

are similar to those in Na6MnS4.34 The Na−S distances range from 2.712(3) to 3.080(3) Å in 1 and 2.712(7) to 3.128(6) in 2, which are closed to those in Na6MnS4 (2.717−3.230 Å);34 the In−S bond distances of 2.428(1)−2.492(1) Å for 1 and 2.425(3)−2.483(3) for 2 are comparable to those found in Ba2BiInS5 (2.447(6) to 2.578(6) Å);35 the Ge−S bond distances in the scope of 2.197(3) to 2.232(3) Å are in good agreement with those of 2.207(9)−2.156(6) Å in Li2MnGeS4;36 the Si−S bond length range from 2.115(1) to 2.144(1) Å, comparable to those in ZnY6SiS14 (2.088−2.136 Å).25 UV−vis−near-IR Diffuse Reflectance and FT-IR Spectroscopy. The UV−vis−NIR optical diffuse-reflectance spectra studied indicate that the band gaps were measured at 2.470 eV for 1 and 2.417 eV for 2 (Figure S3), which are larger than those of Ag2In2SiS6 (2.0 eV) and Ag2In2GeS6 (1.96 eV)37 and smaller than those of Li2In2SiS6 (3.61 eV) and Li2In2GeS6 (3.45 eV).19 Compounds 1 and 2 have larger band gaps in comparison with their isostructural compounds, Ag2In2SiS6 and Ag2In2GeS6, which is in accordance with the lower electronegativity of Na to give higher ionicity compared with Ag. Li-containing compounds usually have larger band gap in comparison with their analogues due to its smaller ionic radius and better polarization. The IR spectroscopy indicates that compounds 1 and 2 have no obvious absorption peak appeared in wavelength scope of

Figure 2. 3D anionic framework of 2 viewed down the c axis (a) and down the b axis (b).

the condensation of these 1∞[In2GeS6]2− chains via sharing S(5) atoms (Figure 2a) with the Na(1) and Na(2) atoms being located in the cavities of 3D framework. Different from the tetrahedrally coordinated environments of Li+ in Li2In2MS6 (M = Si, Ge),19 the Na+ cation in 1 and 2 present two kinds of environments: the Na(1) and Na(2) are respectively coordinated with five and four sulfur atoms, which 1482

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Figure 4. Directions of dipole moment in GeS4 tetrahedra (a), In(1)S4 tetrahedra (b), In(2)S4 tetrahedra (c), and the polyhedra (d) in the unit cell of 2. The Na and S atoms are omitted for clarity.

Figure 5. Calculated band structures of 1 (a) and 2 (b).

2.5−25 μm (Figure S4) that covers the important band ranges of 3−5 and 8−14 μm of atmospheric transparent window. Second Harmonic Generation Measurements. The intensities of SHG signals measured on powders of different sizes increased with the increasing particle size, suggesting typeI phase-matching behaviors for 1 and 2 (Figure 3). The

Figure 6. Total and partial DOSs of 1 (a) and 2 (b).

measured SHG signal intensities of 1 and 2 at the particle size of 150−200 μm are ∼0.3 and 0.5 times that of AgGaS2, respectively. As the measured intensity of SHG signal through the Kurtz and Perry powder method is proportional to the 1483

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Inorganic Chemistry square of the SHG coefficient deff, the reported deff for AGS is 13.7 pm/V,38 and therefore the estimated SHG coefficient deff for 1 and 2 are ∼7.5 and 9.69 pm/V, respectively. Dipole Moments. To better understand the SHG effects for 1 and 2, the dipole moments for the polyhedra SiS4, GeS4, and InS4 in the unit cell were calculated using a bond-valence method proposed earlier.39,40 The arrangements of the three kind dipole moments in the unit cell of 2 are displayed in Figure 4. The b-components of the polarizations from all building units canceled out absolutely, and the polarizations of a- and c-components are constructively added in a unit cell for 1 and 2 (Table S3). For 2, the polarization of a-components for the GeS4, In(1)S4, and In(2)S4 tetrahedra are ∼4 × 0.092, ∼4 × (−0.595), and ∼4 × 1.883 D, the polarization of c-components for the GeS4, In(1)S4, and In(2)S4 tetrahedra are ∼4 × 0.123, ∼4 × (−0.569), and ∼4 × 2.760 D, constructively added to a value of 10.78 D for a unit cell. Equally, the summed dipole moments in 1 are 8.10 D, which is a little smaller than that in 2, resulting in the SHG intensities of 1 being weaker than 2. The dipole moments analysis result is consistent with the SHG measurement above. In comparison with Li2In2GeS6, the InS4 and GeS4 tetrahedra in 2 distort derived from the replacement of Li by Na ion, resulting in changing dipole moments of In and Ge tetrahedra. The net dipole moments of the unit cell in Li2In2GeS6 and 2 were calculated to give 16.86 D for Li2In2GeS6 and 10.78 D for 2 (Table S3), which is consistent with their SHG performance. Powder Laser-Induced Damage Threshhold Measurements. The powder LIDTs for 1, 2, and AGS (as a reference) were measured by the single-pulse LIDTs method (Table S4). The damage energy of 1 (24.87 mJ) and 2 (14.51 mJ) is larger than that of AGS (11.54 mJ). As the spot area of 0.2642 cm2 for 1 and 2 is smaller than that of AGS (0.8495 cm2), the average LIDTs of 9.41(22) MW·cm−2 for 1 and 5.49(68) MW·cm−2 for 2 are 6.9 and 4.0 times as much as commercial AGS (1.36(11) MW·cm−2), respectively. It is important to note that the output SHG intensities of 1 and 2 will be effectively increased to ∼4.3 and 4.0 times larger than that of AGS when the intensity of incident laser is increased to close the damage energy for 1 and 2, respectively, as the output SHG intensity is proportional to the square of deff and incident laser intensity. The high LIDTs indicate that compounds 1 and 2 are promising in the infrared NLO region for high-power application. Thermal Analysis. The DSC curves of 1 and 2 show that compounds 1 and 2 melt congruently at the melting points at ∼763 and ∼705 °C, respectively (Figure S5). The relatively low melting point with the congruent melting behavior indicate the bulk crystals of 1 and 2 can be grown through the Bridgman− Stockbarger technique as the bulk crystals are essential in practical application. Electronic Band Structure Calculations. From the calculated band structures along high symmetry points in the first Brillouin zone for 1 and 2 (Figure 5), it is presented that compounds 1 and 2 have direct band gaps of 2.457 and 2.403 eV, respectively, which are consistent with the measured result (2.470 eV for 1 and 2.417 eV for 2). Figure 6 illustrates the DOSs for 1 and 2. The conductive band (CB) is mostly composed of S-3p, In-5s, and In-5p states, mixing minor Na-3s, Na-2p, Si-3s (Ge-4s), and Si-3p (Ge-4p) states, while the valence band (VB) from −8.0 eV to the Fermi level is dominated by S-3p and In-5s states, together with minor In-5p, Si-3s, (Ge-4s), and Si-3p (Ge-4p) states. The region between −20.0 and −8.0 eV is contributed by In-4d and S-3s

states, as well as a fraction of Si-3s (Ge-4s), and Si-3p (Ge-4p) states. The region from −55.0 to −20.0 eV is mainly attributed to Na-2p and Na-3s. Hence, the optical absorption for 1 and 2 are primarily attributed to the charge transfers from S-3p, In-5s to S-3p, In-5s5p states.

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CONCLUSIONS In summary, the first two new Na-containing sulfides 1 and 2 are isostructural with space group Cc in the monoclinic phases, have been synthesized successfully, which feature 3D structures built by the 1∞[In2MS6]2− (M = Si, Ge) chains through cornersharing InS4 tetrahedra and MS4 (M = Si, Ge) tetrahedra, with Na+ cation located in the cavities. Furthermore, compounds 1 and 2 show comparatively SHG efficiencies ∼0.3 and 0.5 times of AgGaS2 with phase-matchable behavior and high LIDTs of 6.9 and 4.0 times as strong as AgGaS2 for 1 and 2, respectively, suggesting that the output SHG intensities of 1 and 2 will be ∼4.3 and 4.0 times larger than that of AGS, respectively. These results indicate that compounds 1 and 2 can be promising IR NLO materials.

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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.5b02211. Atomic coordinates and equivalent isotropic displacement parameters, selected bond lengths, calculated dipole moments for asymmetric units and the unit cells, laser-induced damage thresholds, experimental and simulated XRD patterns, EDX, IR, and UV−vis spectra, DSC curves. (PDF) X-ray crytallographic information. (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (H.-Y.Z.) *E-mail: [email protected]. (G.-C.G.) Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the NSF of China (91222204, 21403231, 21101152, and 91021004) and the NSF of Fujian Province (2014J05025 and 2014J05034).

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REFERENCES

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DOI: 10.1021/acs.inorgchem.5b02211 Inorg. Chem. 2016, 55, 1480−1485

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DOI: 10.1021/acs.inorgchem.5b02211 Inorg. Chem. 2016, 55, 1480−1485