Modifying Disordered Sites with Rational Cations to Regulate Band

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Modifying Disordered Sites with Rational Cations to Regulate BandGaps and Second-Harmonic-Generation Responses Markedly: Ba6Li2ZnSn4S16 vs. Ba6Ag2ZnSn4S16 vs. Ba6Li2.67Sn4.33S16 Rui-Huan Duan, Rui-An Li, Peng-Fei Liu, Hua Lin, Yue Wang, and Li-Ming Wu Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00927 • Publication Date (Web): 16 Aug 2018 Downloaded from http://pubs.acs.org on August 16, 2018

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Crystal Growth & Design

f School of Biological and Chemical Engineering, Chongqing University of Education, Chongqing 400067, China. KEYWORDS: crystal structures, NLO optical properties, band gaps, sulfides

ABSTRACT: The Infrared (IR) nonlinear optical (NLO) crystals are significant with extensive applications in optoelectronic field. However, it’s still a big task to construct novel IR NLO compounds with at least 3.0 eV band-gaps (Eg) for commercial and academic applications. Herein, three new non-centrosymmetric (NCS) compounds Ba6Li2ZnSn4S16 (1), Ba6Ag2ZnSn4S16 (2) and Ba6Li2.67Sn4.33S16 (3) have been produced by the high-temperature-solid-state reaction. These title compounds are isostructural and feature a three-dimensional (3D) structure that consists of corner-sharing SnS4 and (A/M)4 (A = Li, Ag, M= Zn; A = Li, M= Sn) tetrahedrons with Ba2+ which are located in the holes. Remarkably, by modifying disordered sites with rational cations, Eg and second-harmonic-generation (SHG) intensities of aimed compounds are simultaneously regulated with a wide range, Eg: from 3.30 to 2.23 eV, SHG responses: from 5.2 to 1.45 × AgGaS2 at the same particle size. Meanwhile, compound 1 modified with rational cations (Li/Zn) displays the widest Eg (3.30 eV) and strong SHG response (5.2 × AgGaS2) in these isostructural materials, which illustrates a desired balance between Eg and SHG intensity. Furthermore, the density functional theory (DFT) has been studied to illustrate the origin of optical properties in terms of electronic structures. And the calculations and experiments confirmed each other, which validates that modifying disorder sites with reasonable cations is an effective way to promote properties of materials.

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INTRODUCTION Infrared (IR) nonlinear optical (NLO) matters play a significant role in many advanced technology fields, such as sensors, monitoring, optical communication, with the property of frequency conversion to broaden wavelengths of coherent light.1 Nowadays, only AgGaS2,2 AgGaSe23 and ZnGeP24 are commercially available NLO compounds with indeed large secondharmonic-generation (SHG) responses. Unfortunately, they feature some inherent drawbacks mainly caused by narrow Eg, such as low laser-induced-damage-thresholds (LIDTs) or strong two-photon absorption.5 And these disadvantages acutely hamper their developments. Moreover, numerous IR NLO chalcogenides have been explored in the past decades.6-20 And they usually present stronger SHG intensities than AgGaS2 as possible as they can, such as PbGa2GeSe6 (5 × AgGaS2)6. However, large Eg (> 3.0 eV) were neglected in these NLO crystals, which leads to seriously photon absorption or low LIDTs, and inhibits commercial and academic applications (see Table 1).5e,18 Consequently, it is urgent to design novel IR NLO crystals (Eg > 3.0 eV) to overcome narrow Eg drawback of these commercial compounds.21-25 Previously, our group reported a series of isostructural compounds CsM4In5Se12 (M = Zn, Mn, Cd).15 Modifying the Wyckoff site 9b with (M/In) cations, it achieves the values of Eg vary from 1.62 to 2.11 eV. Meanwhile, our group another work also shows the similar phenomenon, Ba4MGa4Se10Cl2 (M = Mn, Fe, Co, Cu/Ga, Zn, Cd).25b Utilizing different cations to modify the Wyckoff 2c site, the values of Eg vary from 1.88 to 3.08 eV. And Ba4ZnGa4Se10Cl2 shows the largest Eg 3.08 eV. Notably, there are many similar examples, such as CsGaSn2Se6 (1.78 eV) vs. CsInSn2Se6 (1.87 eV),14 BaCu2GeS4 (2.47 eV) vs. BaCu2SnS4 (1.96 eV),19 Na2Hg3Si2S8 (2.86 eV) vs. Na2Hg3Ge2S8 (2.68 eV) vs. Na2Hg3Sn2S8 (2.45 eV).25c Based on analyses above, an inference


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can be drawn that modifying active sites with rational cations may regulate optical properties of materials, especially Eg. Table 1. Comparison on the band-gaps (Eg) and SHG responses (AgGaS2 as reference) of some selected examples.

Compound

SHG intensity (× AgGaS2)

Eg/eV

Ref.

AgGaS2

1

2.32

12

BaHgSe2

1.5

1.56

8

Na2CdGe2Se6

2

2.37

17

RbZn4In5Se12

3.9

2.06

15

CsInSn2Se6

4

1.87

14

PbGa2GeSe6

5

1.96

6

BaGa2SnSe6

5.2

1.95

11

Rb4Ge4Se12

7.5

2.10

7

Ba8Sn4S15

10

2.31

9

Ba7Sn5S15

10

2.29

10

Ba3CsGa5Se10Cl2

100

2.08

13

According to our previous works, Ba6Li2CdSn4S1625f is of special interest. We found this compound has a wide Eg (3.02 eV) and strong SHG response (7.6 × AgGaS2). However, Ba6Li2CdSn4S16 contains cadmium, which is a highly toxic environmental pollutant. In order to create an more environmental friendly NLO material, we introduced Zn2+ cations for substitution Cd2+ to modify the Wyckoff site 12b, and generated a new compound Ba6Li2ZnSn4S16 with relativley

wide

Eg.

Meanwhile,

the

isostructural

compounds

Ba6Ag2ZnSn4S16

and

Ba6Li2.67Sn4.33S16 were also synthesized for comparison. Herein, we report three new IR NLO compounds Ba6Li2ZnSn4S16 (1), Ba6Ag2ZnSn4S16 (2) and Ba6Li2.67Sn4.33S16 (3), whose Eg are regulated by the means of modifying active sites with


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rational caitons, 3.30 eV for 1, 2.85 eV for 2 and 2.23 eV for 3, respectively. Moreover, their SHG responses are simultaneously adjusted, 5.2 vs. 2.6 vs. 1.45 × AgGaS2 at the same particle size, respectively. Furthermore, their syntheses, characterizations and theoretical calculations will be stated. EXPERIMENTAL SECTION Synthesis The starting elements were acquired from Alfa Aesar or Sinopharm Chemical Reagent Co. Ltd. They were deposited in an argon atmosphere glove-box. Ba6Li2ZnSn4S16 (1): With the ratio of 6 : 1 : 1 : 4 : 15, reagents Ba, Li2S, Zn, Sn and S were put in a graphite crucible. Under the high vacuum of 10-3 Pa, the crucible was sealed into a silicon tube, which was loaded in a heater. Then the heater was warmed up to 1203 K in 50 h and kept at 1203 K for 100 h. Next, the heater was refrigerated to 623 K through 100 h before closed. Finally, the buff compounds were successfully synthesized. And the aimed crystals were refined by Single-crystal X-ray crystallography with the formula Ba6Li2ZnSn4S16. It is consistent with the energy dispersive X-ray spectroscope (EDS) analysis results. They can keep stable for at least one year in the air. Ba6Ag2ZnSn4S16 (2): The starting elements were Ba, Ag, Zn, Sn and S with the stoichiometric ratio. And the following procedures were similar to compound 1. However, compound 2 shows yellow color, deeper than 1. They also are very stable, and can be stored in the air for at least one year.


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Ba6Li2.67Sn4.33S16 (3): The starting elements were Ba, Li2S, Sn and S with the ratio of 6 : 1.335 : 4.33 : 8.665. And the following procedures were similar to compound 1. With red color, compound 3 is as stable as 2. Single-crystal X-ray crystallography At 298 K, single-crystal X-ray diffraction were performed on title crystals. The diffractometer is a Rigaku Saturn724 CCD. And it is equipped with graphite-monochromated Mo Kα radiation. The collected information were checked with Lorentz and polarization factors. And absorptioncorrections were operated with the multi-scan method.26 The structures were analysed with the direct-methods by SHELX-97.27 Meanwhile, utilizing the full-matrix-least-squares matching with the structure factor F2, these structures were refined. All structures were checked with PLATON.28 And there aren’t higher symmetry elements. Table 2 listed their crystallographic data and structure refinements information. The positional coordinates were listed in Table S1, accompanying with their isotropic equivalent thermal parameters. Table S2 listed some bond distances and angles of title compounds. Powder X-ray diffraction At 298 K, powdered compound 1, 2 and 3 were operated on a Rigaku MiniFlex II diffractometer with scan steps of 0.02°. The instrument is equipped with Cu Kα radiation (2θmax = 70º). As presented in Figure 1 and Figure S1, the measured XRD patterns are consistent with the simulated shapes. The simulated shapes are derived from single crystal data. Elemental analysis


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To detect the elements of aimed compounds, EDS which is equipped on a field-emissionscanning-electron-microscope (FESEM. JSM6700F) were operated. And the results illustrate the presence of Ba, Ag, Sn, Zn, S atoms (see Figure S2). While, Li atoms are too light, which can’t be detected by EDS.

Table 2. Crystal data of compound 1, 2 and 3.[a]

1

2

3

Formula

Ba6Li2ZnSn4S16

Ba6Ag2ZnSn4S16

Ba6Li2.67Sn4.33S16

fw

1891.01

2092.87

1869.74

crystal system

Cubic

Cubic

Cubic

crystal color

Light yellow

Yellow

Red

space group a (Å)

I 43d

(220)

I 43d

(220)

I 43d

(220)

14.5924(3)

14.6839(4)

14.6034(7)

3107.3(2)

3166.1(2)

3114.3(4)

4

4

4

4.042

4.391

3.988

12.450

13.421

11.931

3312

3664

3267

0.983

1.091

1.069

0.0124, 0.0281

0.0199, 0.0491

0.0161, 0.0344

R1, wR2 (all data) Largest diff. peak and hole (e Å-3)

0.0124, 0.0281

0.0201, 0.0492

0.0203, 0.0476

0.323, -0.528

1.134, -0.608

0.512, -0.969

Absolute structure parameter

0.01(2)

0.02(4)

0.01(3)

3

V (Å ) Z -3

Dc (g.cm ) -1

µ (mm ) F(000) 2

GOOF on F R1, wR2 [a] (I > 2σ(I))

[a] R1 = Σ||Fo| - |Fc||/Σ|Fo|, wR2 = [Σw(Fo2 - Fc2)2/Σw(Fo2)2]1/2


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Figure 1. Powder XRD patterns for compound 1. Thermal analysis Thermogravimetric analyses (TGA) were operated on compound 1, 2 and 3 by the instrument of NETZSCH STA 449C, which is using nitrogen gas with a flow rate of 30 mL/min. Al2O3 crucibles which were filled with 10 mg of title crystals and empty Al2O3 crucibles as the reference were all heated from 273 K to 1423 K. And the heating rate is 10 K/min. Diffuse reflectance spectroscopies To investigate the diffuse reflectance spectra of aimed compounds, Perkin-Elmer Lambda 950 UV–Vis spectrophotometer was used. This instrument was equipped with an integrating sphere attachment. Aimed compounds were operated in the range of 0.2–2.5 µm at 298 K for the optical diffuse reflectance spectrums. And BaSO4 was set as a reference. The equation of Kubelka-Munk: α/S = (1–R)2/2R was utilized to calculate absorption spectra.29 In the equation, R is the reflectance. S is the scattering-coefficient. α is the absorption-coefficient.


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Second-harmonic-generation (SHG) measurements Utilizing a modified method of Kurtz-NLO, the SHG signals at 2.05 µm laser-radiation were measured on title compounds. Aimed samples were cautiously sieved into different sizes, i.e. 30–46, 46–74, 74–106, 106–150 and 150–210 µm, respectively.30 AgGaS2 samples (reference) were also operated as same as title compounds. Electronic structure calculations In compound 1, 12a Wyckoff site is occupied by 66.7% Li and 33.3 % Zn. And a primitive cell was selected with P1 symmetry. Meanwhile, four configurations had been designed. In the structure, 6 sites which matched to the 12a Wyckoff site are randomly occupied by 4 Li and 2 Zn atoms. As the calculation results illustrated, four models almost show the same total energy. Model 2 with the lowest energy is utilized to calculate density of states (DOS) and electronic structures. With the Vienna ab-initio simulation package code,31 the density functional theory (DFT) was carried out. Meanwhile, ionic cores were calculated by projector-augmented-wave (PAW) method.32 In addition, the exchange-correlation potential was calculated by generalizedgradient-approximation (GGA) and the Perdew-Burke-Ernzerhof (PBE).33 In addition, the similar calculation procedures are operated on compound 2 and 3. The valence electrons were chosen from Ba 5s25p66s2, Ag 4d105s1, Li 1s22s12p, Sn 5s25p2, Zn 3d104s2 and S 3s23p4. To calculate properties of aimed compounds, the 5×5×5 Å−1 reciprocal space was selected in the Monkhorst-Pack scheme. To find out the self-consistent charge density, the mesh cutoff energy was set as 400 eV. Less than 0.01 eV/Å of the Hellmann-Feynman force on atoms is fixed. Less than 1.0×10−5 eV of total energy change is also fixed. It means geometries are relaxed. Besides, at less 550 empty bands were adopted to calculate optical properties.


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RESULTS AND DISCUSSION Crystal structure New pentanary chalcogenides Ba6Li2ZnSn4S16 (1), Ba6Ag2ZnSn4S16 (2) and Ba6Li2.6Sn4.33S16 (3) are synthesized. Their unit cell parameters are a = 14.5924 – 14.6839 Å and Z = 4. They all crystallize in the NCS space group 43 (No. 220). They feature the same structure, herein compound 1 is selected as a representative to illustrate their structures. As presented in Figure 2, 1 features a three-dimensional (3D) structure which is constructed by SnS4 and (Li/Zn)S4 tetrahedrons with Ba2+ locating in them. From Figure 3(a), it is obvious that SnS4 tetrahedra are lying in 3-fold axis along the direction of [111]. Meanwhile, with d-glide translation perpendicular to the direction of [110], SnS4 tetrahedra are arranged in the overall framework (see Figure 3(b)). Furthermore, (Li/Zn)S4 tetrahedra are connected with SnS4 tetrahedra by three bridging S atoms, abandoning terminal S2 of SnS4 tetrahedra.

Figure 2. The 3D structure of Ba6Li2ZnSn4S16 (1).


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Figure 3. Patterns of SnS4 tetrahedra in the unit of 1 along [111] (a) and [110] (b).

In the asymmetric unit of compound 1, the crystallographically unique atoms are Ba1 (24d), Sn1 (16c), S1 (16c), S2 (48e), and one disordered metal site (12a), which is occupied by 67% Li and 33% Zn. Comparably, 12a site in compound 2 is occupied by 67% Ag and 33% Zn. Differently, 12a site in compound 3 is occupied by 89% Li and 11% Sn. As shown in Figure S3, the Ba atoms in compound 1 are coordinated by eight S atoms to form BaS8 bicapped trigonal prismatic geometry. The Ba–S distances in BaS8 geometry range from 3.117 to 3.441 Å, which are comparable to those of Ba7Sn5S15 (3.030-3.590 Å),10 NaBa4Ge3S10Cl (2.991-3.561 Å)24 and BaCdSnS4 (3.199-3.302 Å).34a The Sn atoms in 1 are coordinated with four S atoms, forming SnS4 tetrahedrons. The Sn–S distances range from 2.354 to 2.403 Å, comparable to those of αK2Hg3Sn2S8 (2.345-2.417 Å),35 Na2Ba7Sn4S16 (2.365-2.389 Å)36 and K10Zn4Sn4S17 (2.329-2.407 Å).37 The (Li/Zn) positions are also coordinated with four S atoms to build (Li/Zn)S4 twisted tetrahedra with the distance of 2.453 Å for 1. Properties TGA measurements are operated on title compounds to evaluate their thermal stabilities. As shown in Figure S4, title compounds show excellent thermal stabilities before 1173 K, 1100 K


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and 956 K, respectively. In order to evaluate Eg of title compounds, the diffuse-reflectance spectra measurements are operated from 0.2 to 2.5 µm. As shown in Figure 4, Eg of 1 and 2 are 3.30 and 2.85 eV, respectively, largely wider than that of 3 (2.23 eV). And they are consistent with their corresponding colors. Obviously, Eg of compound 1 is larger than the criterion 3.0 eV of IR NLO materials.5e,18 Such value is not only larger than many excellent NLO thiostannate, e.g., Ba8Sn4S15 (2.31 eV),9 and Ba7Sn5S15 (2.29 eV)10 Na2BaSnS4 (3.27 eV)21 and SnGa4S7 (3.10 eV)25d (see Figure 5 and Table 3), but also much wider than those commercial compounds 2.56 eV for AgGaS2, 2.0 eV for ZnGeP2 and 1.8 eV for AgGaSe2. Notably, Eg of compound 1 is the largest in these isostructural compounds (see Figure 5 and Table 3). For another, take account of the NCS structures which title compounds feature, the NLO properties are operated on different size of samples at room temperature. And AgGaS2 with the same size are set as reference samples. The relationship of the SHG response vs. the particle size is shown in Figure S5, which illustrates a non phase-matchable property of title compounds. Additionally, the SHG intensity of compound 1, 2 and 3 are about 5.2 vs. 2.6 vs. 1.45 × AgGaS2 in 30-46 µm size, respectively (see Figure 6).


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Figure 4. Experimental optical band-gaps and the crystal photos (inset) of compound 1 (a), 2 (b) and 3 (c).


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Figure 5. Comparison on the band gaps of known NLO thiostannate. (Pink shows this work) Table 3. Comparison on Eg and SHG responses in 30–46 µm size (AgGaS2 as reference) of isostructural materials.

Compound

Eg/eV

SHG intensity (× AgGaS2)

Wyckoff 12b or 12a

1 (this work )

3.30

5.2

67%Li/33%Zn

Ba6Li2CdSn4S1625f

3.02

7.6

67%Li/33%Cd

2 (this work)

2.85

2.6

67%Ag/33%Zn

Ba3CdSn2S834a

2.75

0.8

67%Cd

Ba6Ag2CdSn4S1625f Na2Ba7Sn4S1636 Li2Ba7Sn4S1636

2.70

5.3

67%Ag/33%Cd

2.50

≈ 1.0

67%Na/33%Ba

2.30

≈ 1.0

67%Li/33%Ba

3 (this work)

2.23

1.45

89%Li/11%Sn

Na2Ba7Sn4Se1636 Li2Ba7Sn4Se1636

2.10

≈ 1.0

67%Na/33%Ba

1.75

≈ 1.0

Ba6Ag4Sn4S1634b

2.37

≈ AgGaSe2 at 800 nm

67%Li/33%Ba 73.9%Ag (14% + 15.6% Ag at 24d)

Ba6Ag3.92Sn4.02S1634b

1.85


74.7%Ag (13.5% + 14.9% Ag at 24d)

Ba6Ag3.21Sn4.2S1634b

1.58


85.7%Ag (7.1% + 6.9% Ag at 24d)


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Figure 6. SHG responses of compound 1, 2, 3 and AgGaS2 (reference) in 30-46 µm size. Obviously, compound 1, 2 and 3 are isostructural to Ba6Ag2.67+4δSn4.33-δS16 (δ = 0.13, 0.31, 0.33),34b Ba6A2CdSn4S16 (A = Li, Ag),25f A2Ba7Sn4Q16 (A= Li, Na; Q = S, Se),36 and Ba3CdSn2S8.34a Interestingly, in the similar structures, the Wyckoff site 12a (or 12b) is occupied by different cations in these isostructural compounds, which show different optical properties (see Table 3). For Ba6Ag3.21Sn4.20S16,34b Ag+ cations locate at 12a site with the occupancy of 85.7%, while, neighboring the 12a site, there are two 24d sites which are occupied by 7.1% and 6.9% Ag+. For Ba6A2CdSn4S16 (A = Li, Ag),25f 12b site is occupied by 67% A+ and 33% Cd2+. In compound Ba3CdSn2S8,34a 12b site is occupied by 67% Cd2+. In A2Ba7Sn4Q16 (A= Li, Na; Q = S, Se),36 12a site is occupied by 67% A+ and 33% Ba2+. Comparing the differences of Eg between these isostructural compounds, it is noteworthy that modifying 12a site with different cations can adjust Eg from 1.58 to 3.30 eV, which illustrates a significant strategy to adjust Eg of materials. Meanwhile, modifying 12a site with the cooperation of Li+ and Zn2+ cations in compound 1 results in the largest Eg in these isostructural compounds. Likewise, modifying 12a site with


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different cations to regulate SHG responses also shows significant potentiality, ranging from 0.8 to 7.6 × AgGaS2. These phenomena can be ascribed to the characteristics of cations locating at 12a or 12b site. For example, comparing compound 1, Ba6Ag2.67+4δSn4.33-δS16 (δ = 0.13, 0.31, 0.33) with Ba3CdSn2S8, Li+ cations with highly electropositivity enlarge Eg of 1. Comparing Ba6A2MSn4S16 (A = Li+, Ag+; M = Zn2+, Cd2+) with A2Ba7Sn4Q16 (A= Li+, Na+; Q = S, Se) on SHG responses, it may be attributed to the smaller radii and larger electronegativities of Zn2+ or Cd2+ than that of Ba2+.25a As a result, the degree of covalency of the Zn–S or Cd–S bonds is larger than Ba–S bonds, which is benefit to enlarge SHG intensities.34b Furthermore, comparing compound 1 and 2, it is obvious that the substitution Li+ for Ag+ can simultaneously enlarge the Eg and the SHG response, which is consistent with Ba6Li2CdSn4S16 vs. Ba6Ag2CdSn4S16. With the substitution of Li+ for Ag+, the non-bonding ingredients increase, which will strengthen SHG responses. To sum up, modifying disorder sites in a framework with rational choice of cations is an effective strategy to regulate and promote properties of materials. Theoretical studies To well explore the relationship between structures and properties of materials, electric structures of title compounds are calculated with VASP software. As shown in Figure 7 and S6, title compounds show direct band gaps with the values of 2.51, 1.76 and 1.68 eV, respectively. And it is consistent to the trend of experimental values, 1 (3.30 eV) > 2 (2.85 eV) > 3 (2.23 eV).


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Figure 7. Electronic band structures for 1 (a) and 2 (b). Ef (the Fermi level) is located at 0 eV.

Figure 8. Total and partial DOSs of compound 1 (a) and 2 (b). Dash lines represent Ef and dotted lines represent apart regions in VB and CB.


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Moreover, the calculated DOSs of title compounds are presented in Figure 8 and S7. In Figure 8a, for compound 1, the S-3p orbitals make main contributions in the peak of the valence band (VB, which is placed below 0 eV), hybridizing with bits of of Ba-5d, Sn-4d5p, Zn-3d4p and Li-2s2p orbitals. The base of the conduction band (CB, which is located up 0 eV), S-3p and Ba-5d orbitals contribute to it, hybridizing with bits of S-3s, Sn-5s5p, Zn-4p and Li-2s2p. Then, the band gap of compound 1 is ascribed to electrons jump from Ba-5d, S-3p, Sn-4d5p, Zn-3d4p, Li-2s2p orbitals to S-3s3p, Ba-5d, Sn-5s5p, Zn-4p, Li-2s2p orbitals. Instead, the DOSs of compound 2 are similar to 1, but some notable differences can be seen in Figure 8b. For compound 2, the peak of the VB is mostly dominated by S-3p and Ag-4d orbitals, hybridizing with bits of Ba-5d, Sn-4d5p and Zn-3d orbitals. The base of the CB in compound 2 is managed by S-3p and Ba-5d, hybridizing with minor Sn-5s5p, Zn-4s and Ag-5s orbitals. Therefore, the band gap for compound 2 is controlled by electrons jump from S-3p, Ag-4d, Ba-5d, Sn-4d5p, Zn-3d orbitals to S-3p, Ba-5d, Sn-5s5p, Zn-4s, Ag-5s orbitals. For compound 3 (Figure S7), the peak of the VB is mostly dominated by S-3p and Sn-5p orbitals, hybriziding with bits of Ba-5d, Sn-4d, Li-2s2p orbitals. In additon, the base of the CB in compound 3 is managed by S-3p, Sn-5s and Ba-5d, hybridizing with minor Sn-5p and Li-2p states. Therefore, the band gap for compound 3 is controlled by the electrons jump from S-3p, Sn-5p4d, Ba-5d, Li-2s2p orbitals to S-3p, Ba-5d, Sn-5s5p, Li-2p orbitals. Meanwhile, the result also can explain the band gaps trend of 1 (3.30 eV) > 2 (2.85 eV) > 3 (2.23 eV). Li+ cations can enlarge band gaps which contributes to the band gap absorptions of compound 1. While, Ag+ cations with its photodarkening effect will narrow Eg, which contributes to the band gap absorptions of compound 2. There are many examples with this trend, such as LiGaS2 (4.15 eV)38 vs. AgGaS2 (2.56 eV), LiGaSe2 (3.34 eV)38 vs. AgGaSe2 (1.80 eV), Li2In2GeS6 (3.45 eV)39 vs. Ag2In2GeS6 (1.96 eV),40 and Li2In2SiS6 (3.61


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eV)39 vs. Ag2In2SiS6 (2.0 eV).40 The narrow Eg of compound 3 may be ascribed to the strong interaction between Sn4+ and S2- in the top of VB.

Figure 9. Calculated SHG coefficients vs. frequency for compound 1, 2 and AgGaS2. Additionally, due to title compounds feature the point group 43, only one nonzero SHG tensor (χ14) restricted with Kleinman’s symmetry. Meanwhile, considering the disordered site of 12a site with 67% A and 33% Zn, a new model was built with P1 symmetry, which shows ten independent SHG tensors (χ11, χ12, χ13, χ14, χ15, χ16, χ22, χ23, χ24 and χ33) restricted with Kleinman’s symmetry. Then, with the biggest value, χ23 which is corresponding to SHG coefficient d23 has been studied. As presented in Figure 9, the calculated SHG coefficient d23 for 1 is 21.58 pm/V at 0.61 eV (the wavelength of 2.05µm), which is evidently larger than 18.21 and 15.55 pm/V of AgGaS2 and compound 2, respectively. It is noteworthy that this calculation results are consistent with the experimental SHG values. Considering the differences in the experimental SHG responses and the calculated SHG coefficients d23 between compound 2 and AgGaS2, it may be attributed to the selected approximate model of P1. As shown in Figure 10, the cutoffenergy dependence of d23 has been calculated, which indicates that the source of SHG signals of


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aimed compounds is all caused by VB-1 regions. However, the interaction of orbitals in VB-1 region for title compounds are relatively different. For 1, the VB-1 region is dominated by S-3p, hybridizing by minor Ba-5d, Sn-4d5p, Zn-3d, Li-2s2p states. For compound 2, VB-1 region is managed by S-3p and Ag-4d orbitals, hybridizing with bits of Ba-5d, Sn-4d, Zn-3d orbitals. Obviously, in VB-1 region of compound 2, there is a strong interaction between S-3p and Ag-4d states, which will hinder the non-bonding S-3p orbitals and weaken SHG intensities. Consequently, the calculated results on SHG responses are consistent with the experimental results, 1 > 2. According to analyses above, to modify disordered sites with rational cations will improve properties of materials.

Figure 10. Static SHG coefficients d23 vs. the cutoff-energy of 1 (a) and 2 (b). Dash lines represent Ef. Dotted lines represent apart regions in VB and CB. In addition, the linear optical properties were also studied, such as the dielectric function ε(ω), refractive index n(ω), which were shown in Figures S8 and S9.


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CONCLUSION In summary, three new NCS compounds Ba6Li2ZnSn4S16 (1), Ba6Ag2ZnSn4S16 (2) and Ba6Li2.6Sn4.33S16 (3) have been successfully designed, synthesized and characterized. They are isostructural and feature a 3D framework which is built by SnS4 tetrahedra and (A/M)S4 (A = Li, Ag, M= Zn; A = Li, M= Sn) tetrahedra via corner-sharing. Notably, modifying 12a sites with different cations, Eg and SHG responses of title compounds are simultaneously regulated with a wide range, Eg: from 3.30 to 2.23 eV, SHG responses: from 5.2 to 1.45 × AgGaS2. And compound 1 modified with rational cations (Li/Zn) displays the widest Eg (3.30 eV) and strong SHG response (5.2 × AgGaS2) in these isostructural materials. Besides, DFT is operated to explore the relationship between electronic structures, linear and nonlinear optical properties. Furthermore, this work demonstrates that modifying disordered sites in a 3D framework with reasonable cations is a successful strategy to improve material properties. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: ******. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].


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*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. #These authors contributed equally to this work. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This research was supported by the NSF of China (Nos. 21771179, 21233009, 91422303, 21571020 and 21301175 and 21671023), Nanhu Scholars Program for Young Scholars of XYNU, the Doctoral Start-up Research Fund of Xinyang Normal University (18074), and Key Laboratory for Green Chemical Technology of Chongqing University of Education (No. 2016xjpt08). We thank Prof. Ning Ye at FJIRSM for helping with the SHG measurements and Prof. Yong-Fan Zhang at Fuzhou University for helping with the DFT calculations. REFERENCES REFERENCES (1) (a) Nikogosyan, D. N. Nonlinear Optical Crystals: A Complete Survey. 1st ed.; Springer: New York 2005. (b) Burland, D. M.; Miller, R. D.; Walsh, C. A. Second-order nonlinearity in


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For Table of Contents Use Only Modifying Disordered Sites with Rational Cations to Regulate Band-Gaps and SecondHarmonic-Generation Responses Markedly: Ba6Li2ZnSn4S16 vs. Ba6Ag2ZnSn4S16 vs. Ba6Li2.67Sn4.33S16 Rui-Huan Duan,∗,#,a,b Rui-An Li,#,c Peng-Fei Liu,d,e Hua Lin,∗,b Yue Wang,f and Li-Ming Wu∗,b,c

Synopsis: Ba6Li2ZnSn4S16 vs. Ba6Ag2ZnSn4S16 vs. Ba6Li2.67Sn4.33S16: Modifying disordered sites with reasonable cations: achieve a significant regulation on band gaps (from 2.23 to 3.30 eV) and SHG responses (from 1.45 to 5.2 × AgGaS2).


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Modifying Disordered Sites with Rational Cations to Regulate Band

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