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Effect of Element Substitution on Structural Transformation and Optical Performances in I2BaMIVQ4 (I = Li, Na, Cu, and Ag; MIV = Si, Ge, and Sn; Q = S and Se) Leyan Nian, Kui Wu,* Guijie He, Zhihua Yang, and Shilie Pan* CAS Key Laboratory of Functional Materials and Devices for Special Environments; Xinjiang Technical Institute of Physics & Chemistry, CAS; Xinjiang Key Laboratory of Electronic Information Materials and Devices, 40-1 South Beijing Road, Urumqi 830011, China S Supporting Information *

ABSTRACT: In the exploration of new infrared nonlinear optical (IR NLO) materials, element substitution has been developed as an effective way to adjust the structural features and material performances. A series of new IR NLO materials have been discovered in the I−Ba−MIV−Q system (I = Li, Na, Cu, and Ag; MIV = Si, Ge, and Sn; Q = S and Se), and they undergo interesting structural transformation with different element substitution except Li analogues. Herein, we have successfully synthesized three selenides with different space groups (Ag2BaSiSe4: I4̅2m; Ag2BaGeSe4 and Ag2BaSnSe4: I222) in the above system and studied their properties through experimental and theoretical methods. Remarkably, the detailed analysis on the structural changes and properties comparison was also systematically investigated in the I−Ba−MIV−Q system and the results indicate that the distortion degrees of different IQ4 tetrahedra play the critical role to cause the structural transformation with the M or Q elements substitution. More importantly, we have also found that the structural changes have the close relationship with the distance d(I− I) between adjacent I cations in the I2BaSnSe4 system, which makes the four-membered rings formed by edge-sharing BaSe8 units change from the square to rhombus with the increase of d(I−I). The properties comparisons (band gap and NLO effect) in this system have been also systematically studied.



structures and properties of Ag2BaGeS4 and Ag2BaSnS4.78 Note that interesting structural transformations among the I−Ba− MIV−Q system have been found with the different MIV or Q atoms except Li analogues (Table 1). For example, Na2BaMIVQ4 shows the structural transformation from tetragonal (I4̅2d) to trigonal (R3c) and Cu2BaMIVQ4 undergoes the structural changes (Ama2 to P3121 or P3221). And only the crystal structures of Ag2BaMIVSe4 have been reported so far.79,80 An overall observation for the I2BaMIVQ4 system shows that they exhibit the special structural transformations by the element co-substitution; thus, based on the above analysis, the reasons for their structural and properties changes should be systematically investigated. In this work, we have successfully synthesized three compounds (Ag2BaSiSe4, Ag2BaGeSe4, and Ag2BaSnSe4) in the Ag−Ba− MIV−Se system. In addition, their structural changes and optical properties have also been studied for the first time. More importantly, we have also analyzed the structural changes in the I−Ba−MIV−Q system and the effect of I cations on the structural changes in the I2BaSnSe4 system. The properties comparisons (band gap and NLO effect) in this system have also been systematically studied.

INTRODUCTION Exploration of a new family of compounds is important to enrich the structural chemistry.1−37 Recently, increasing attention has been focused on metal chalcogenides because of their diverse structural chemistry and excellent performances.38−67 In review of the structural features for known chalcogenides, they commonly possess the same structures by the substitution of the same main group elements, for example, LiGaS2 vs LiInS268 (orthorhombic, Pna21), AgGaS269 vs AgGaSe270 (tetragonal, I4̅2m), and Li2BaGeS471 vs Li2BaSnS471 (tetragonal, I4̅2m), etc. However, it should be noted that obvious structural transformations have also occurred with the replacement of different alkali or alkaline earth metals in the crystal structures, such as Ba2GeSe472 (P21/m) vs Mg2GeSe473 (Pnma) and Li2CdSnS474 (Pmn21) vs Na2CdSnS474 (C2). More importantly, seen from the structures in a series of A2Hg3Ge2S8 (A = Na−Cs)43,75 compounds, it can be also interestingly found that their structural symmetries show a gradually rising tendency from Cs to Na analogues (Cs, P1̅; Rb, P21/c; α-K, Aba2; Na, P4c̅ 2) because of the cation size effect. Up to now, we have done some works on the discovery of new chalcogenides and several of them have been verified as promising infrared nonlinear optical (IR NLO) materials, such as I2BaMIVQ4 (I = Li, Na, Cu; MIV = Ge, Sn; Q = S, Se).71,76,77 Recently, Wu et al. have given the detailed research on the © XXXX American Chemical Society

Received: January 25, 2018

A

DOI: 10.1021/acs.inorgchem.8b00220 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Table 1. Space Group and Crystal System for I2−Ba−MIV− Q4 (I = Li, Na, Cu, and Ag; MIV = Si, Ge, and Sn; Q = S and Se) compounds

space group

crystal system

ref

Cu2BaSnSe4 Cu2BaSnS4 Cu2BaSiSe4 Cu2BaGeS4 Cu2BaGeSe4 Ag2BaSiSe4 Ag2BaGeS4 Ag2BaGeSe4 Ag2BaSnS4 Ag2BaSnSe4 Na2BaSnS4 Na2BaGeS4 Na2BaGeSe4 Na2BaSnSe4 Li2BaGeS4 Li2BaGeSe4 Li2BaSnS4 Li2BaSnSe4

Ama2 P3221 P3221 P3121 P3121 I4̅2m I4̅2m I222 I222 I222 I4̅2d R3c R3c R3c I4̅2m I4̅2m I4̅2m I4̅2m

orthorhombic trigonal trigonal trigonal trigonal tetragonal tetragonal orthorhombic orthorhombic orthorhombic tetragonal trigonal trigonal trigonal tetragonal tetragonal tetragonal tetragonal

76 76 76 76 76 this work 78 80, this work 78 79, this work 77 77 77 77 71 71 71 71



Table 2. Crystal Data and Structure Refinement for Ag2BaMIVSe4 (MIV = Si, Ge, and Sn) Ag2BaSiSe4b fw crystal system space group a (Å) b (Å) c (Å) Z, V (Å3) Dc (g/cm3) μ (mm−1) GOF on F2 R1, wR2 (I > 2σ(I))a R1, wR2 (all data)a absolute structure parameter largest diff. peak and hole (e Å−3) a b

Ag2BaGeSe4b

Ag2BaSnSe4b

697.01 tetragonal I4̅2m 7.066(3) 7.066(3) 8.233(7) 2, 411.1(5) 5.631 27.205 1.080 0.0318, 0.0817 0.0334, 0.0846 0.12(5)

741.51

787.61 orthorhombic I222 I222 7.117(16) 7.101(10) 7.316(16) 7.469(10) 8.316(18) 8.302(11) 2, 433.0(16) 2, 440.3(10) 5.687 5.941 29.093 28.037 0.952 1.088 0.0476, 0.1199 0.0379, 0.0814 0.0589, 0.1278 0.0417, 0.0847 0.15(8) 0.06(5)

0.715, −1.541

2.617, −2.031

1.690, −2.034

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

crystals after washing with N,N-dimethylformamide (DMF). Powder XRD data for the three compounds were recorded on a Bruker D2 PHASER X-ray diffractometer equipped with a diffracted beam monochromator set for Cu Kα radiation (λ = 1.5418 Å) with a scan step width of 0.02° and a fixed counting time of 1 s/step in the angular range of 2θ = 10−70° at 298 K. From the powder XRD patterns of Ag2BaMIVSe4 (Figure 1), it can be found that the experimental X-ray diffraction patterns match well with the calculated ones that are generated through the Mercury program with CIF data except for Ag2BaSiSe4. Seen from the impurity peaks in the XRD diagram of Ag2BaSiSe4, such as 15, 26, and 43°, they should be attributed as the XRD peaks of Ba2SiSe4. IR Spectrum. The IR spectra of Ag2BaMIVSe4 were recorded on a Shimadzu IR Affinity-1 FTIR spectrometer covering the wavenumber range of 500−4000 cm−1. The crystal samples were mixed with dried KBr at a mass ratio of about 1:100 and ground into a fine powder and then pressed into a transparent sheet on the tablet machine. After the above process, the sheet was loaded in the sample chamber, and then the IR spectrum was measured (Figure S1 in the SI). UV−vis−near-IR Diffuse-Reflectance Spectroscopy. Optical diffuse-reflectance spectra of Ag2BaMIVSe4 were collected at room temperature by a Shimadzu SolidSpec-3700DUV spectrophotometer. Spectral data were collected with a wavelength range of 190−2600 nm. The band gaps of Ag2BaMIVSe4 were estimated via converting the data of reflectance spectra to absorbance by the Kubelka−Munk function.83 Second-Harmonic Generation Measurement. The powder SHG response of title compounds was investigated by the Kurtz and Perry method84 since all of them crystallize in the NCS space group. A 2090 nm Cr:Tm:Ho:YAG Q-switch laser was chosen as the light source. The polycrystalline samples were ground and sieved into several particle size ranges (38−55, 55−88, 88−105, 105−150, and 150−200 μm) to accomplish the size-dependent SHG signals. And the intensity of the frequency-doubled output emitted from the samples was detected by a photomultiplier tube. In addition, the high-quality AgGaS2 bulk crystal was ground and sieved into the same size ranges as the reference material. Raman Spectroscopy. Raman measurements were performed on a LABRAM HR Evolution spectrometer equipped with a 532 nm laser in backscattering configuration. The crushed crystals of title compounds were placed on a small glass slide, and we chose the area of the crystal specimens on a 50× objective lens. The maximum laser power was kept at 60 mW and the beam diameter was 35 mm. The grating was set to be 600 gr per mm. Each spectrum was collected using an integration time of 15 s.

EXPERIMENTAL SECTION

Reagents and Synthesis. Ba block (99%), Ag grain (99.99%), Si (99.99%), Ge powder (99.99%), Sn powder (99.99%), and Se powder (99.99%) were used as purchased from Shanghai Aladdin Biochemistry Technology Co., Ltd. Since the Ba metal reacts with oxygen easily, the loose oxide layer on the surface of the Ba block was first scraped thoroughly. An Ar-filled glovebox was used to avoid the effects of oxygen and moisture in the whole process preparation. A conventional high temperature solid-state method was also used to synthesize the title compounds. Ag2BaGeSe4, Ag2BaSnSe4, and Ag2BaSiSe4. For the synthesis of target compounds, a stoichiometric mixture of the starting materials Ag, Ba, MIV (MIV = Si, Ge, and Sn) and Se in a molar ratio of 2:1:1:4 was loaded into a silica tube (length 20 cm, diameter 1 cm), which was flame-sealed with a mixture of methane and oxygen flame under a high vacuum of 10−3 Pa. After that, the tube was moved into a computercontrolled furnace. The furnace was programmed by the following steps: heated to 600 °C during 30 h and kept at this temperature for 40 h; then, heated to 1000 °C within 20 h and dwelled at this temperature for 100 h; finally, followed by slow cooling to 400 °C at a rate of 5 °C/h and then cooled to room temperature with 10 h. After the reaction, the target products with red color were successfully obtained, and they are stable in air for several months. Structure Determination. High-quality single crystals of Ag2BaSiSe4, Ag2BaGeSe4, and Ag2BaSnSe4 were picked and fixed on the top of a glass fiber with epoxy under a 40× microscope for single crystal X-ray crystallography. All the single-crystal X-ray diffraction data of title compounds were collected by a Bruker SMART Apex II CCD single-crystal X-ray diffractometer employing graphite-monochromatized Mo Kα radiation (λ = 0.71073 Å) with a tube F2 using the SHELXTL crystallographic program package.81 The XPREP program was also employed for multiscan absorption corrections.82 Final structures were checked for missing symmetry elements with the PLATON program,82 and no other higher symmetries elements were found. Table 2 lists the details of crystal data and structure refinements of the three compounds. Isotropic displacement parameters and atomic coordinates, as well as the results of the bond valence sum (BVS) calculations, are summarized in Table S1 in the Supporting Information (SI). Moreover, the related angles and bond lengths are shown in Table S2 in the SI. Powder X-ray Diffraction. For phase characterization and identification, the microcrystalline powders of title compounds used for various performance tests were prepared by grinding the single B

DOI: 10.1021/acs.inorgchem.8b00220 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. Powder XRD patterns of Ag2BaSiSe4 (a), Ag2BaGeSe4 (b), and Ag2BaSnSe4 (c). Theoretical Calculation. By using density functional theory (DFT) calculation tests, we studied the electronic structures, total and partial density of states (T/PDOS), and optical properties of Ag2BaMIVSe4. The ab initio DFT method calculations were performed by the plane-wave pseudo potential implemented in the CASTEP package.85 The local density approximations (LDA) with the CA−PZ functional,86,87 a 700 eV cutoff energy for the plane-wave basis set, and norm-conserving pseudo potentials (NCP)88 were employed in all computations. The Brillouin zone was integrated using Monkhorst− Pack-generated sets of k-points, and we determined 5 × 5 × 5 k-point meshes for the title compounds. The configurations for diverse electron orbital generating pseudo potentials were Ba 5p6 6s2, Ag 4p6 4d10 5s1, Si 3s2 3p2, Ge 4s2 4p2, Sn 5s2 5p2, and Se 4s2 4p4, and the other calculating parameters used in the calculations and convergent criteria were set by the default values of the CASTEP code. To explore the contributions from different structural units to the NLO coefficients, the SHG density method was performed by using the effective SHG: Virtual-Electron (VE) and Virtual-Hole (VH). At a zero frequency, the formula of second-order NLO coefficients can be derived as89 χ αβγ = χ αβγ (VE) + χ αβγ (VH)

As for its structure, highly distorted [AgSe4] tetrahedra with two different Ag−Se bond lengths (2.626 and 2.855 Å) first interconnect together by sharing corners to form twodimensional (2D) layers (Figure 2a) in the ab plane. Then,

(1)

where χ αβγ (VH) =

e3 2ℏ2m3

∑∫ vv ′ c

d3k P(αβγ )Im[pvvα ′ pvβ′ c pcvγ ] 4π 3

⎛ 1 2 ⎞ × ⎜⎜ 3 2 + 4 ⎟⎟ ωvc ωcv ′ ⎠ ⎝ ωcvωv ′ c χ αβγ (VE) =

e3 2ℏ2m3

∑∫ vcc ′

Figure 2. (a) The two-dimensional (2D) [AgSe4]∞ layer in Ag2BaGeSe4. (b) The 2D [AgSe4]∞ layers are bridged with [GeSe4] ligands to make up 4-MR tunnel structure. (c) The Ba cations located inside the 4-MR tunnels. (d) The [BaSe8] polyhedra are isolated in the same tunnel. (e) Adjacent [BaSe8] polyhedra interconnect by cornersharing to form 3D framework structure.

(2)

d3k P(αβγ )Im[pvcα pccβ′ pcγ′ v ] 4π 3

⎛ 1 2 ⎞⎟ × ⎜⎜ 3 2 + 4 ⎟ ωvc ωc ′ v ⎠ ⎝ ωcvωvc ′

the 2D [AgSe4]∞ layers are further bridged with isolated [GeSe4] ligands to make up a four-membered-ring (4-MR) tunnel structure (Figure 2b) along the b-axis with the Ba cations located inside the tunnels (Figure 2c). The isolated [GeSe4] ligands are approximately regular with the same Ge− Se bond lengths (2.37 Å) and three types of similar angles of Se−Ge−Se 109.17°, 107.76°, and 111.51°) and the same angles are located at the symmetrical positions. Note that the Ba cations within the tunnels are surrounded by selenium atoms in eight-coordination to form a [BaSe8] polyhedron within quite a narrow range of the Ba−Se distances (3.41−3.44 Å), which are similar with those of other related compounds containing Ba− Se bonds, such as BaGa4Se790 (3.43−3.86 Å), Ba7Sn3Se1391 (3.18−3.76 Å), Ba2SnSe428 (3.20−3.79 Å) and BaGa2SnSe692 (3.59−3.81 Å). From a geometric point of view, it could be viewed as either bicapped trigonal prismatic or distorted tetragonal antiprismatic. Interestingly, the [BaSe8] polyhedra are isolated in the same tunnel (Figure 2d), but interconnect by corner-sharing with the adjacent [BaSe8] polyhedra to form a 3D framework structure (Figure 2e). Ag2BaSiSe4 crystallizes in the space group I4̅2m of the tetragonal system with a = b = 7.066 (3) Å, c = 8.233 (7) Å, and Z = 2. Ag2BaSiSe4 includes

(3)

In the above equation, v and v′ represent valence bands, α, β, γ are Cartesian components, conduction bands are represented by c and c′, and P(αβγ) denotes full permutation. The band energy difference and momentum matrix elements are expressed by ℏωij and Pijα.



RESULTS AND DISCUSSION Crystal Structure. Compounds in Ag2BaMIVSe4 crystallize in two noncentrosymmetric (NCS) space groups: I42̅ m for Ag2BaSiSe4, and I222 for Ag2BaGeSe4 and Ag2BaSnSe4. Herein, Ag2BaGeSe4 and Ag2BaSiSe4 were chosen as the representatives to illuminate their crystal structures. Ag2BaGeSe4 crystallizes in the orthorhombic space group I222 with a = 7.117(16) Å, b = 7.316(16) Å, c = 8.316(18) Å, and Z = 2 (number of molecules in a unit cell). In its asymmetric unit, there exist one crystallographically independent Ba, Ag, Ge, and Se atom at the Wyckoff positions 2a, 4j, 2c, and 8k, respectively (Table S1 in the SI). Moreover, the results of BVS (Ba = 1.83; Ag = 1.14; Ge = 3.77; Se = 1.97) are in agreement with +2, +1, +4, and −2 for Ba, Ag, Ge, and Se atoms, respectively (Table S1 in the SI). C

DOI: 10.1021/acs.inorgchem.8b00220 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. (a1) [BaSe8] polyhedra connect with each other to form 4-MR in Ag2BaSnSe4; (a2) [AgSe4] tetrahedra reside in the 4-MR; (a3) the distance between two Ag atoms in the 4-MR. (b1) [BaSe8] polyhedra connect with each other to form 4-MR in Li2BaSnSe4; (b2) [LiSe4] tetrahedra reside in the 4-MR; (b3) the distance between two Li atoms in the 4-MR. (c1) [BaSe8] polyhedra connect with each other to form 4-MR in Cu2BaSnSe4; (c2) [CuSe4] tetrahedra reside in the 4-MR; (c3) the distance between two Cu atoms in the 4-MR.

structures, Na2BaSnSe4 shows obviously discriminated structural features (Figure S3) because the Na atoms possess the sixfold coordination environment, which is different from that the I (Ag, Cu, Li) atoms exhibit the four-fold coordination environment in the other three I2BaSnSe4 compounds. Herein, we have systematically analyzed the I cation effect on the structural transformation of I2BaSnSe4 (I = Ag, Cu, Li) compounds. Seen from Figure 3a1, b1, and c1, similar tunnel structures formed by the interconnection of corner-sharing [BaSe8] dodecahedra are observed in the above system, and all of the I cations are located within the respective tunnels formed by the 4-MRs (Figure 3a2, b2, and c2). Note that the distortion degree of the ISe4 tetrahedra can obviously affect the arrangement mode of the ISe4 units and tunnel shapes; for example, they have the different average bond lengths of I−Se including d(Ag−Se) = 2.74 Å, d(Li−Se) = 2.64 Å, and d(Cu− Se) = 2.48 Å, respectively (Table S3). In view of the relatively smaller d(Cu−Se) bonds in the structure of Cu2BaSnSe4, the adjacent two Cu cations with short d(Cu−Cu) = 2.70 Å could be located at the center of regular 4-MR squares (Figure 3c3). However, in Ag2BaSnSe4 and Li2BaSnSe4, the adjacent two monovalent cations (Li+ and Ag+) are located at the long diagonals in the 4-MR rhombus since the longer bond lengths of d(Li−Li) = 4.22 Å and d(Ag−Ag) = 5.091 Å would be difficult for the adjacent two I cations to exist in the center of squares (Figure 3a3 and b3), which make the previous 4-MR squares change to the new 4-MR rhombus with more space

one crystallographically Ba, Ag, Si, and Se (Table S1 in the SI) in its asymmetric unit, respectively. Through the results of BVS (Ba = 2.03; Ag = 1.24; Si = 3.94; Se = 2.11), it indicates that the valence states of Ba, Ag, Si, and Se atoms agree with +2, +1, +4, and −2, respectively (Table S1 in the SI). In the structure of Ag2BaSiSe4, both of the Ag and Si atoms coordinate with four Se atoms to make up typical [AgSe4] and [SiSe4] tetrahedra with d(Ag−Se) = 2.69(12) Å and d(Si−Se) = 2.27(12) Å, respectively. First, the regular [AgSe4] tetrahedra connect with each other via corner Se atoms to form 2D [AgSe4]∞ flat layers in the ac plane (Figure S2a in the SI). Then, the [AgSe4] layers are connected by isolated [SiSe4] tetrahedra to further form a tunnel structure along the b-axis with the Ba atoms resided in (Figure S2b in the SI). The Ba atoms possess 8-fold coordination to form a [BaSe8] polyhedron with d(Ba−Se) = 3.36(16)−3.41(2) Å. Each [BaSe8] polyhedron is isolated in the same tunnel but connects together by corner-sharing to form a 3D framework structure, which is similar with that of Ag2BaGeSe4. Analysis of the Structural Transformation in the I2− Ba−MIV−Q4 (I = Li, Na, Cu, and Ag; MIV = Ge and Sn; Q = S and Se) System. In the recent work, we have reported that interesting structural changes exist in the I2−Ba−MIV−Q4 system with the simple substitution from Ge to Sn or S to Se atoms. Besides, it should be also noted that their structures are also changed with different I atoms; taking the I2BaSnSe4 as example, all of them crystallize in the different space groups (Li: I4̅2m; Ag: I222; Cu: Ama2; Na: R3c). In view of their D

DOI: 10.1021/acs.inorgchem.8b00220 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Table 3. Changing Value of I−Q Bond Lengths (Å) and Q−I−Q Bond Angles (deg) for I2−Ba−MIV−Q4 (I = Li, Na, Cu, and Ag; MIV = Ge and Sn; Q = S and Se) compounds

space group

bond lengths of I−Q (Å)

Δd (Å)

bond angles of Q−I−Q (deg)

Δθ (deg)

ref

Cu2BaSnSe4 Cu2BaGeS4 Cu2BaGeSe4 Cu2BaSnS4 Ag2BaGeS4 Ag2BaGeSe4 Ag2BaSnS4 Ag2BaSnSe4 Na2BaSnS4 Na2BaGeS4 Na2BaGeSe4 Na2BaSnSe4 Li2BaGeS4 Li2BaGeSe4 Li2BaSnS4 Li2BaSnSe4

Ama2 P3121 P3121 P3221 I4̅2m I222 I222 I222 I4̅2d R3c R3c R3c I4̅2m I4̅2m I4̅2m I4̅2m

2.431−2.546 2.316−2.443 2.427−2.541 2.311−2.423 2.589 2.626−2.855 2.494−2.791 2.560−2.920 2.910−3.103 2.800−3.313 2.890−3.494 2.917−3.445 2.519 2.631 2.527 2.643

0.115 0.114 0.114 0.112 0 0.229 0.297 0.360 0.193 0.513 0.604 0.528 0 0 0 0

96.01−115.30 95.26−134.35 95.34−132.96 95.79−134.06 94.90−146.02 92.19−161.91 90.91−166.29 89.42−172.09 69.61−167.48 79.28−171.80 79.20−173.10 80.63−176.50 94.86−146.14 94.40−147.83 93.93−149.65 93.54−151.22

19.29 39.09 37.62 38.27 51.12 69.72 75.38 82.67 97.87 92.52 93.90 95.87 51.28 53.43 55.72 57.68

76 76 76 76 78 80 78 79 77 77 77 77 71 71 71 71

Figure 4. (a) Experimental band gaps of Ag2BaMIVSe4. (b) Raman spectra of Ag2BaMIVSe4.

along the diagonal direction to satisfy the existence of Ag+ or Li+ cations. Moreover, we have also investigated the effect of different Δd (d(longest) − d(shortest)) of I−Q bonds and Δθ (θ(largest) − θ(smallest)) of Q−I−Q angles on the structural changes for the I2−Ba−MIV−Q4 system. Seen from Table 3, it is clear that the Δd or Δθ values show obvious changes while those compounds undergo the structural transformations. Take Ag2−Ba−MIV−Q4 as the representatives, their Δd and Δθ are changed from Ag2BaGeS4 (I42̅ m: Δd = 0 Å; Δθ = 51.12°) to (I222: Δd = 0.23 Å, 0.30 Å, 0.36 Å; Δθ = 69.72°, 75.38°, 82.67°) in the other three compounds, respectively, and this phenomenon is also found in Cu- or Na-containing systems. Note that Li-containing compounds crystallize in the same space groups and do not exhibit the structural changes, which can be attributed to the similar Δd and Δθ for Li−Q and Q− Li−Q angles. Therefore, it can be concluded that the structures of analogues have the intimate relationship with different I and Q atoms that should be devoted considerable attentions in the future structural prediction. Optical Properties of I2BaMIVQ4. Critical optical parameters have been systematically measured on the microcrystal powders for title compounds. IR spectral results indicate that there exhibit no obvious absorption peaks from 4000 to 500 cm −1 . The diffuse-reflectance UV−vis−NIR spectra of Ag2BaMIVSe4 were measured, and the spectral results show

that the experimental band gaps are 1.83 eV for Ag2BaSiSe4, 1.57 eV for Ag2BaGeSe4, and 1.42 eV for Ag2BaSnSe4, respectively (Figure 4a). In addition, measured Raman spectra show that the highest absorption peaks for the three title selenides are located at the vicinity of 200 cm−1, including Ag2BaSiSe4 (222 cm−1), Ag2BaGeSe4 (205 cm−1), and Ag2BaSnSe4 (190 cm−1), which can be assigned to the characteristic absorptions of Si−Se, Ge− Se, and Sn−Se modes, respectively (Figure 4b). When the Raman spectra of title compounds are compared, it can be found that the absorption peak positions shift toward the short wavelength from S to Se or Si to Sn particle sizes at room temperature. Measured results show that they exhibit the weak SHG responses about 0.1−0.2 times that of benchmark AgGaS2 and nonphase-matching behavior. Moreover, we have also summarized the optical properties (NLO effect and band gap) of the I2BaMIVQ4 system (∼18 compounds) (Figure 5). In this system, note that the optical band gaps show the decreasing tendency with different I atoms along the Na, Li, Cu, to Ag analogues. And the SHG effects of selenides are larger than those of sulfides, which can be attributed to the stronger polarization of the external optical field for Se than that for the S atom. According to the present study, the potential IR NLO materials require the performance demand: band gap (>3.0 eV) and strong NLO effect (>0.5 × AgGaS2). In the I2BaMIVQ4 E

DOI: 10.1021/acs.inorgchem.8b00220 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Ag2BaSiSe4, which is different from that (wavelike layers) in the other two compounds. Their properties were also systematically studied through experimental and theoretical methods for the first time. It is worth noting that the structural changes and properties comparison were also investigated in the I−Ba− MIV−Q system and the results indicate that the distortion degrees of different IQ4 tetrahedra play the critical role to cause the structural transformation with the M or Q elements substitution. Moreover, in the I2BaSnSe4 system, we have also found that the structural changes are closely related to the distance d(I−I) between adjacent I cations. In I2BaSnSe4, the 4MR formed by edge-sharing BaSe8 units changes from the square to rhombus while the adjacent I cations vary from Cu to Ag or Li. The properties comparisons (band gap and NLO effect) in the I−Ba−MIV−Q system have been also systematically studied, and the results indicate that the optical band gaps show the decreasing tendency with different I atoms along the Na, Li, Cu, to Ag analogues and the SHG effects of selenides are larger than those of sulfide analogues.

Figure 5. Comparison on band gap and SHG response among the I2BaMIVQ4 (I = Li, Na, Cu, and Ag; MIV = Si, Ge, and Sn; Q = S and Se) system.71,76−80

system, only three compounds including Na 2 BaSnS 4 , Li2BaGeS4, and Li2BaSnS4 show promising application prospect as IR NLO candidates. Theoretical Studies. To further investigate the electronic structures of title compounds, first-principles computations were adopted to complete the relative calculation. As seen from their theoretical electronic structures (Figure S4), the highest point of the valence band (VB) and the lowest point of the conduction band (CB) locate at different points, which indicate that all of them are indirect band gap compounds. The calculated band gaps are about 1.20, 0.70, and 0.61 eV for Ag2BaSiSe4, Ag2BaGeSe4, and Ag2BaSnSe4, respectively. Note that the calculated results are smaller than the experimental observations, which can be attributed to the discontinuity of exchange−correlation energy of the generalized gradient approximation (GGA) functional.93,94 Seen from the PDOS of the three selenides (Figure S5), the region in the VB below the Fermi level is mainly occupied by Ag 3d and Se 4p states and the bottom of the CB is mainly derived from the Si 3s/Ge 4s/Sn 5s and Se 4p states. Therefore, the optical band gaps of the title compounds mainly depend on the [AgSe4] and [MIVSe4] units. Meanwhile, the SHG density method was also used to investigate the origin of SHG responses for Ag2BaSiSe4, Ag2BaGeSe4, and Ag2BaSnSe4. The contributions of VE and VH to the total SHG coefficients were obtained using the bandresolved method. The results show that the VE contribution is greater than 80% for the above four compounds; thus, we chose the occupied and unoccupied of VE to investigate the main origin of the SHG responses (Figure S6). Taking Ag2BaSnSe4 as an example, as shown in Figure S6c, its SHG effect is mainly derived from the [AgSe4] and [SnSe4] units; this result can be also extended to other compounds in this system that their origins of SHG effects are mainly derived from the cooperative contribution of the [AgSe4] and [MIVSe4] units.



ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00220. Atomic coordinates, isotropic displacement parameters, selected bond lengths and angles, IR spectra, absorption spectra, calculated BS and PDOS, calculated SHG density (PDF) Accession Codes

CCDC 1564161, 1564164, and 1564172 contain 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 [email protected]. uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.



AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (K.W.). *E-mail: [email protected] (S.P.). ORCID

Zhihua Yang: 0000-0001-9214-3612 Shilie Pan: 0000-0003-4521-4507 Notes

The authors declare no competing financial interest.





ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 51425206 and 91622107), Ten Thousand People Plan Backup Project (Grant No. QN2016YX0340), the National Key Research Project (Grant Nos. 2016YFB1102302, 2016YFB0402104), the National Basic Research Program of China (Grant No. 2014CB648400), Xinjiang Key Laboratory of Electronic Information Materials and Devices (Grant No. 2017D04029), West Light Foundation of the Chinese Academy of Sciences (Grant No. 2016-YJRC-2), Xinjiang scientific and technological innovation talents project (Grant No. QN2016YX0339).

CONCLUSIONS In summary, element substitution has been regarded as an effective way to adjust the frame structures and performances in the investigation of IR NLO materials. As for the I−Ba−MIV− Q system (I = Li, Na, Cu, and Ag; MIV = Si, Ge, and Sn; Q = S and Se), a series of new IR NLO materials have been found and they undergo structural transformation with different element substitution except the Li2BaMIVQ4 system. In this work, three selenides with different space groups (Ag2BaSiSe4: I4̅2m; Ag2BaGeSe4 and Ag2BaSnSe4: I222) in the above system have been successfully synthesized by a solid-state method. Seen from their structures, the flat [AgSe4]∞ layer exists in F

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Inorganic Chemistry



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DOI: 10.1021/acs.inorgchem.8b00220 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b00220 Inorg. Chem. XXXX, XXX, XXX−XXX