Article pubs.acs.org/cm
Ba6Zn7Ga2S16: A Wide Band Gap Sulfide with Phase-Matchable Infrared NLO Properties Yan-Yan Li,† Peng-Fei Liu,†,‡ and Li-Ming Wu*,† †
Key Laboratory of Research on Chemistry and Physics of Optoelectronic Materials, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, People’s Republic of China ‡ University of Chinese Academy of Sciences, Beijing 100039, People’s Republic of China S Supporting Information *
ABSTRACT: High-performance infrared (IR) nonlinear optical (NLO) materials with large laser damage thresholds (LDTs) are urgently needed because the current commercially available AgGaS2, AgGaSe2, and ZnGeP2 suffer their very low LDTs which shorten significantly their service lifetimes. Here, a novel sulfide, Ba6Zn7Ga2S16 with a very wide band gap of 3.5 eV, has been discovered. This compound crystallizes in the chiral trigonal R3 space group with a novel 3D framework that is constructed by ZnS4 tetrahedra, Zn3GaS10 supertetrahedra (a T2-type), and Zn3GaS10 quadri-tetrahedral clusters via vertex-sharing. Such a novel structure exhibits desirable features which suggest a promising NLO material: phasematchability (PM), good NLO efficiency (about half that of benchmark AgGaS2), and the highest LDT among PM chalcogenides (28 times that of benchmark AgGaS2). In addition, the density functional theory (DFT) calculations confirm its PM behavior and reveal that the second harmonic generation (SHG) origin is mainly ascribed to the transition process from S-3p to Ga-4p, Zn-3p, Zn-3d, and Ba-5d states; the calculated d11 coefficient of 6.1 pm/V agrees well with experimental values.
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dij value (>10 × KDP) and wide Eg (>3.5 eV), simultaneously.17−21,25 (see Table S1 in the Supporting Information). For instance, BaGa4S721 has the NLO coefficient dij of 33.3 × KDP and LDT of 3 × AgGaS2 under the band gap of 3.54 eV; Na2BaGeS417 with large Eg of 3.7 eV exhibits NLO coefficient dij of 10 × KDP and LDT of 8 × AgGaS2. We notice that a common structural feature of these crystals is they are all 3D frameworks constructed by main group metal-centered tetrahedra with Li+ or Ba2+ as counter cations. And their LDT values of 2.5−11 × AgGaS2 strongly depend on the band gaps of 3.5−4.15 eV.17−21 Zn2+ cation with relatively small covalent radii shows positive roles to increase the Eg and to maintain the dij of a compound, simutaneously.11 On the other hand, Zn2+-containing chalcogenides usually exhibit interesting fluorescence properties.27−29 Therefore, we think it is interesting to introduce a ZnS4 tetrahedral building unit into a 3D network of main group metal tetrahedra in the hope of discovering new compounds with desired NLO properties and interesting fluorescence properties. Herein, a novel chalcogenide, Ba6Zn7Ga2S16, with excellent NLO performance is discovered. This compound possesses a very wide Eg of about 3.5 eV, good NLO coefficient
INTRODUCTION Infrared (IR) nonlinear optical (NLO) materials are the key components in laser frequency conversion technology to produce a new coherent and tunable IR laser source for a wide range of applications.1−3 The current commercially available IR NLO materials are merely chalcopyrite-type AgGaS2, AgGaSe2, and ZnGeP2.4−6 And these crystals feature large NLO susceptibilities but unfortunately suffer from drawbacks of low laser damage thresholds (LDTs) or twophoton absorptions which severely hinder their wider practical applications. Therefore, the exploration of new IR NLO crystals with high LDTs is challenging and of great significance. Chalcogenides is known to be the most promising source of potential IR NLO materials, and many new candidates have been discovered recently.7−10 Although many of these show very large NLO coefficients and wide IR transparent windows, their relatively narrow band gaps (Eg < 3 eV) lower their LDTs significantly.7−10 Ideally, the Eg should be at least wider than 3.0 eV, preferably over 3.5 eV.11−14 On the other hand, the NLO materials must be phase-matchable (PM) to be useful in real applications. Unfortunately, there are only a few PM chalcogenides that can meet the requirement for a useful NLO crystal, that is, a balance between the wide band gaps (>3.0 eV) and the large NLO coefficient dij values (>10 × KH2PO4 (KDP)).11,15−26 And among them, there are only five PM chalcogenides that are able to have large NLO coefficient © 2017 American Chemical Society
Received: March 31, 2017 Revised: May 31, 2017 Published: May 31, 2017 5259
DOI: 10.1021/acs.chemmater.7b01321 Chem. Mater. 2017, 29, 5259−5266
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Chemistry of Materials of about half that of the benchmark AgGaS2 and the highest LDT among PM chalcogenides to date (28-fold that of AgGaS2).7−11 Other than these, this compound shows good chemical stability and good thermal stability (with a melting point of 950 °C). These comprehensive performances distinguish Ba6Zn7Ga2S16 as one promising IR NLO crystal. Systematic studies of syntheses, crystal structures, optical properties, and DFT analyses are reported.
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EXPERIMENTAL SECTION
Syntheses. Ba rod (Alfa Aesar China, 99.9%), Zn rod (Alfa Aesar China, 99.9999%), Ga shot and S powder (Sinopharm, 99.99%), and binary BaS and ZnS powders (Sinopharm, 99.9%) were used as received. The binary Ga2S3 powder was synthesized at 700 °C by a boron−chalcogen method.30−32 These starting reactants were stored in an Ar-filled glovebox, with H2O and O2 contents of less than 0.1 ppm; all manipulations were performed in the same glovebox or under a vacuum of 10−3 Pa. A mixture of Ba (1.134 mmol, 0.158 g), Zn (0.324 mmol, 0.021 g), Ga (0.648 mmol, 0.045 g), and S (2.430 mmol, 0.078 g) was loaded into a graphite crucible and sealed in an evacuated silica tube under vacuum. Then the tube was heated in a high-temperature tubular resistance furnace with the following controlled heating ramp: heated to 850 °C within 25 h, kept at this temperature for 50 h, then heated to 1000 °C within 10 h, left there for 15 h, cooled to 850 °C within 50 h followed by a dwelling for 100 h, and then slowly cooled to 300 °C within 120 h before naturally cooling to room temperature. The reaction tubing was opened in the dry Ar-filled glovebox, and the assynthesized product was fully ground and cold pressed into a pellet. Subsequently, this pellet was put into a graphite crucible, sealed in an evacuated silica jacket, and subjected to the following treatment: heated to 900 °C within 35 h, followed by a dwelling at this temperature for 100 h, and then cooled to 300 °C within 100 h before the furnace was turned off. Some light-orange block-shaped crystals of Ba6Zn7Ga2S16 were found. The homogeneous polycrystalline Ba6Zn7Ga2S16 was obtained by a different route in which binary BaS, ZnS, and Ga2S3 in a 6:7:1 molar ratio with a total weight of 400 mg were used as starting materials. The heating profile was heating to 850 °C in 35 h, left for 100 h, and then cooling to 300 °C in 120 h. The homogeneity of the product was confirmed by the powder XRD patterns shown in Figure 1a. The single crystals of Ba6Zn7Ga2S16 with sizes up to 0.25 mm were obtained by annealing the above-mentioned polycrystalline powder at 920 °C for 100 h and cooling to 300 °C within 120 h. Crystals Ba6Zn7Ga2S16 are stable in air for more than one year so far. Single-Crystal X-ray Crystallography. High quality single crystals of the title compound were selected for structure determination. Data were collected at 293 K on a Mercury CCD diffractometer using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) and corrected for Lorentz and polarization factors, as well as absorption through the multiscan method.33 SHELX-9734,35 was used to solve and refine the structure. All atom assignments were performed through consideration of bonding characteristics and valence bond sums (VBS).36 Any additional symmetry in the final structure of the title compound was examined by using PLATON.37 The final refinement gave an assignment that the tetrahedral sites M(1) (VBS = 2.15), M(2) (VBS = 2.10), M(3) (VBS = 2.05), M(4) (VBS = 3.01), and M(5) (VBS = 2.82) were respectively assigned to Zn(1), Zn(2), Zn(3), Ga(1), and Ga(2). This generated a formula of Ba6Zn7Ga2S16 with R values of R1 = 0.0368 and wR2 = 0.0820, which satisfied the charge-balance requirement and agreed well with the EDX and ICP results. Then, the Zn(3) site with one notable diff. peak of 3.84 e/Å3 exhibited a relatively large atomic displacement parameter (ADP) of 0.035, which was higher than those of the other Zn sites (Zn(1): 0.011, Zn(2): 0.012). The largest diff. peak 3.84 e/Å3 was thus assigned as the site split partner (Zn(3′) atom) of Zn(3). The subsequent refinement gave normal ADP for Zn(3) (0.019) and better R values of R1 = 0.0253 and wR2 = 0.0446 with occupancies of Zn(3): 0.80 and Zn(3′): 0.20, respectively. Crystallographic data are
Figure 1. Properties of Ba6Zn7Ga2S16: (a) experimental powder XRD (×), the calculated (red line) and difference (blue line) results of the GSAS refinements; impurity is not observed. (b) TG (black) and DTA (red) diagrams. (c) UV−vis diffuse−reflectance spectrum. summarized in Table 1, and further data (positional coordinates and isotropic equivalent thermal parameters) of title compound are listed in Table S2 in the Supporting Information. Elemental Analyses. Semiquantitative energy dispersive X-ray spectra (EDX, Oxford INCA) collected on a field emission scanning electron microscope (FESEM, JSM6700F) confirmed the presence of only Ba, Zn, Ga, and S of the title crystal (Figure S1 in the Supporting
Table 1. Crystal Data and Structure Refinements of Ba6Zn7Ga2S16 formula fw crystal system crystal color space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dc (g·cm−3) μ (mm−1) GOOF on F2 R1, wR2 (I > 2σ(I))a R1, wR2 (all data) largest diff. peak and hole (e/Å3) Flack parameter a
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Ba6Zn7Ga2S16 1934.03 trigonal light-orange R3 (No. 146) 9.7723(4) 9.7723(4) 27.006(2) 90 90 120 2233.5(3) 3 4.314 16.203 0.968 0.0231, 0.0424 0.0253, 0.0446 0.854, −0.886 0.00(3)
R1 = ∑||Fo| − |Fc||/∑|Fo|, wR2 = [∑w(Fo2 − Fc2)2/∑w(Fo2)2]1/2. DOI: 10.1021/acs.chemmater.7b01321 Chem. Mater. 2017, 29, 5259−5266
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Figure 2. (a) Crystal packing structure of Ba6Zn7Ga2S16 viewed down the a-direction with the unit cell marked. Black atom: Ba2+; yellow atom: isolated S2− anion; dark blue, blue, and light blue tetrahedra: ZnS4; orange and light orange tetrahedra: GaS4. (b) A single 2D layer perpendicular to the c direction. The asymmetric building units with atom number marked: (c) Zn(1)3Ga(1)S10 T2 supertetrahedron; (d) Zn(3)S4 tetrahedron; (e) Zn(2)3Ga(2)S10 quadri-tetrahedral cluster. Laser Damage Threshold (LDT) Measurements. The LDT of Ba6Zn7Ga2S16 was evaluated on polycrystalline samples by the single pulse measurement method40 with crushed AgGaS2 single crystal as the reference. Samples of the title compound and AgGaS2 in the same particle size range of 150−210 μm were packaged into disks in identical plastic holders of 1 mm thickness and 8 mm diameter. A 1.064 μm pulse laser with the pulse width of 10 ns was used to irradiate the above samples under identical conditions. To determine the damage threshold, the constantly apparent changes with increasing laser energies were observed on an optical microscope. The damage spot radius and the power of the laser beams were respectively measured on a vernier caliper and a Nova II sensor with a PE50-DIF-C energy sensor. Photoluminescence Measurement. Photoluminescence spectra of Ba6Zn7Ga2S16 powder were recorded in the range of 300−750 nm by a FLS920 spectrofluorometer at room temperature. Electronic Structure Calculations. The electronic structures were calculated using the Vienna ab initio simulation package VASP41 to investigate the structure−property relationship in more detail. The following orbital electrons of Ba: 5s25p66s2, Zn: 3d104s2, Ga: 3d104s24p1, and S: 3s23p4 were treated as valence electrons. The exchange-correlation functional was calculated by the local-density approximation (LDA).42 A plane wave basis set energy cutoff of 400 eV with the projector augmented wave (PAW) potentials were used,43,44 and a 5 × 5 × 3 Monkhorst−Pack k-point grid was chosen. The Fermi level was set at 0 eV. Because the occupancies (0.80 vs 0.20) indicated that the Zn(3) site was highly populated, an idealized model of the title compound in which the Zn atom occupied exclusively the Zn(3) site while the Zn(3′) site was empty was used in all calculations. The total energies of the three models for the title compound to probe the atomic distribution of Zn and Ga over the two 9b Wyckoff sites and three 3a Wyckoff sites were calculated, which confirmed that the atom distribution of Zn and Ga suggested in Table
Information). Quantitative inductively coupled plasma (ICP) emission spectra collected on an Ultima-2 inductively coupled plasma emission spectrometer (ICP-OES) gave the corresponding mole percentages of 5.9% Ba, 7.2% Zn, and 2.0% Ga for Ba6Zn7Ga2S16 (Table S3 in the Supporting Information). These EDX and ICP results were in accordance with those established by single crystal diffraction data. Powder X-ray Diffraction. Powder XRD measurement of title compound was performed on a Rigaku MiniFlex II diffractometer with Cu Kα radiation at room temperature. The scanning range of 2θ is 5− 85°, and the scanning step width is 0.02°. The data were analyzed through profile-fitting by a least-squares method with use of the program GSAS implemented with EXPGUI.38 Thermal Analyses. Thermogravimetric analysis (TG) and differential thermal analysis (DTA) were carried out on a NETZSCH STA 449C thermal analyzer in investigating the thermal properties of the title compound. The polycrystalline sample was placed in an Al2O3 crucible and heated from room temperature to 1000 °C at 10 °C/min under a constant flow of nitrogen gas. UV−Vis−NIR Diffuse Reflectance Spectra. The ground powders were used for the optical diffuse reflectance spectrum measurement using a PerkinElmer Lambda 950 UV−vis spectrophotometer equipped with an integrating sphere attachment in the range of 250−1500 nm at room temperature; a BaSO4 plate was used as the reference. Second Harmonic Generation (SHG) Measurements. The SHG response was investigated on a modified Kurtz-NLO system using a 2.05 μm laser radiation with the pulse width of 10 ns.21,39 Polycrystalline samples and crushed AgGaS2 single crystal as a reference (2 × 2 × 2 cm3 single crystal of AgGaS2 was supplied by Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences) were ground to obtain particle size in the ranges of 30−46, 46−74, 74−106, 106−150, and 150−210 μm. 5261
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Figure 3. (a) Phase-matching curves, i.e., particle size versus SHG response for Ba6Zn7Ga2S16 and AgGaS2 (reference) at incident wavelength of 2.05 μm laser. (b) SHG oscilloscope traces of Ba6Zn7Ga2S16 and AgGaS2 (reference) in the particle size of 150−210 μm at incident wavelength of 2.05 μm laser. S2 is energetically favorable. All structural geometries were fully relaxed until the residual forces on each atom was less than 0.01 eV/Å and the total energy varied by less than 1.0 × 10−5 eV. The SHG coefficients and birefringence (Δn) were also calculated. A scissors operator of 1.35 eV was used to agree with the experimental value, because the band gap was underestimated by the discontinuity of the exchange-correlation potential.45,46 The second-order coefficients d11, d15, and d33 of Ba6Zn7Ga2S16 were calculated according to the length-gauge formalism by Aversa and Sipe47,48 and under the restriction of Kleinman’s symmetry.49
compound Yb2ZnS4 (Zn−S: 2.255−2.505 Å and S−Zn−S: 99.09−119.32°).51 The Ga(1) and Ga(2) are also 4-fold coordinated by S atoms in distorted tetrahedra with Ga(1)−S bonds of 2.307−2.252 Å and S−Ga(1)−S angles of 106.81°−111.99° and Ga(2)−S bonds of 2.213−2.289 Å and S−Ga(2)−S angles of 109.20°− 109.74°. These values are comparable with those found in KGaS2.52 The Ba(1)2+ and Ba(2)2+ cations are respectively nine- and seven-fold coordinated, with Ba−S bond distances varying from 3.1383 to 3.672 Å (Table S4 in the Supporting Information), which are comparable to the distances of 3.104−3.679 Å in Ba2Ga2S5.53 Selected bond distances and angles of title compound are respectively listed in Tables S4 and S5 in the Supporting Information. Thermal Analyses. The TG and DTA curves of the title compound are shown in Figure 1b. The TG results show that compound Ba6Zn7Ga2S16 experiences no obvious weight loss from room temperature to 1000 °C under a N2 atmosphere. Meanwhile the DTA data show an obvious endothermic peak around at 950 °C. Powder XRD data on the residue after the thermal analysis measurements indicate that Ba6Zn7Ga2S16 is mainly decomposed into ZnS and Ba2ZnS354 (Figure S3 in the Supporting Information). Therefore, title compound has high thermal stability, and it is incongruently melted at temperatures as high as 950 °C. UV−Vis−NIR Diffuse-Reflectance Spectroscopy. Diffuse-reflectance UV−vis/near-IR spectra reveal the large band gap of 3.50 eV for Ba6Zn7Ga2S16 (Figure 1c). This result is larger than that of the commercial IR NLO AgGaS2 (2.56 eV, Figure S4 in the Supporting Information), AgGaSe2 (1.8 eV),55 and ZnGeP2 (1.75 eV),6 which indicates the title compound may carry higher laser damage thresholds than these commercial materials. NLO Properties. The SHG signals of polycrystalline Ba6Zn7Ga2S16 have been measured using the Kurtz and Perry method39 under the 2.05 μm Q-switch laser with crushed AgGaS2 single crystals as the reference. As shown in Figure 3a, the SHG intensities increase with the particle size increasing in the range of 30−210 μm, indicating Ba6Zn7Ga2S16 is a phasematchable material at the 2.05 μm laser. As shown in Figure 3b, compound Ba6Zn7Ga2S16 shows the SHG intensity of about 0.5 times that of AgGaS2 at large particle size (150−210 μm) and is as strong as that of AgGaS2 at small particle size. These intensities are about 17 times that of KDP according to the relation of dpowder (AgGaS2) = 33 × dpowder(KDP).11,16,17 These results show that the title compound satisfies the critical
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RESULTS AND DISCUSSION Crystal Structure. Compound Ba6Zn7Ga2S16 represents a new structure type that crystallizes in the chiral space group R3 (No. 146) of the trigonal system, with unit cell parameters of a = b = 9.7723(4) Å, c = 27.006(2) Å, V = 2233.5(3) Å3, and Z = 3. The asymmetric unit contains two crystallographically unique Ba atoms, two Ga atoms, eight S atoms, and three Zn atoms. As the Wyckoff site 3e of Zn splits into Zn(3) (occu.: 0.80) and Zn(3′) (occu.: 0.20), only the Zn(3) (occu.: 0.80) atom is shown in the following structure discussion (Table S2 and Figure S2 in the Supporting Information). The 3D network of Ba6Zn7Ga2S16 is stacked by 2D layers that are constructed by Zn(3)S4 tetrahedra, Zn(1)3Ga(1)S10 T2 supertetrahedra, and Zn(2)3Ga(2)S10 quadri-tetrahedral clusters. The Ba2+ cations and isolated S(8)2− anions locate between the layers (Figure 2a). Interestingly, within each 2D layer, the base of each Zn(3)S4 tetrahedron is connected via the S(3) apexes in an alternating fashion to the Zn(1)3Ga(1)S10 T2 supertetrahedron and the Zn(2)3Ga(2)S10 quadri-tetrahedral cluster (Figure 2b). Note that, as shown in Figure 2a, the T2 supertetrahedron and the quadri-tetrahedral cluster are aligning along the c axis but point in the opposite directions. And the adjacent layers are joint\ed along the c axis via the S(6) atom, which serves as the apex of each Zn(3)S4 tetrahedron in each layer, as well as the apex of the T2 supertetrahedron from the neighboring layer. In this way, a 3D network of Ba6Zn7Ga2S16 is eventually formed (Figures 2c−e). The Zn−S bond distances and the S−Zn−S angles of Zn(1)S4 and Zn(2)S4 are ranging from 2.298 to 2.346 Å and 99.24° to 123.0°, respectively, which are comparable to the corresponding values (Zn−S: 2.275−2.356 Å and S−Zn−S: 100.99−136.21°) in Ba6Zn6ZrS14.50 The Zn(3) atom has a highly distorted tetrahedral environment with three identical shorter Zn(3)−S distances of 2.290 Å and one relatively longer Zn(3)−S distance of 2.54 Å. And the S−Zn(3)−S angles are ranging from 98.8° to 117.7°. These data are similar to those in 5262
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Chemistry of Materials balance of large dij values (>10 × (KDP)) and large band gaps (>3.0 eV) for practical use as suggested recently.11−14,16,17 Anionic group theory is accurate and successful in the UV and deep-UV regions.56 By analyzing the dipole moment orientations and magnitudes of the anionic building units, our group has recently found that the microstructures can determine the phase-matchable behavior in the AM3Q6 (A = Ba, Cs; M = Ga, In, Si, Ge, Sn; Q = S, Se) system.57 Herein, we estimate the magnitudes and directions of the local dipole moments of the anionic tetrahedral groups by utilizing a bond valence method.58,59 As listed in Table S6 in the Supporting Information, the direction of the total static dipole moments of anionic tetrahedral groups in Ba6Zn7Ga2S16 is along the c-axis, with the x or y directions canceling each other to be zero, which is consistent with the symmetries of the R3 (No. 146) space group in the trigonal system. The static dipole moments in the z direction of basic building units (BBUs) of Zn(1)S4, Zn(2)S4, Zn(3)S4, Ga(2)S4, and Ga(2)S4 are respectively 1.26, −1.13, 0.65, −1.98, and 1.37 D. Note that the Zn(1)3Ga(1)S10 T2 supertetrahedron and the Zn(2)3Ga(2)S10 quadri-tetrahedral cluster orient oppositely along the c axis, of whom the dipole moments are canceled mostly. And thus the net local dipole moment of a basic repeating unit [Zn(1)3Zn(2)3Zn(3)Ga(1)Ga(2)S22] is 0.43 D. This remanent local dipole moment may be mainly responsible for the SHG effect of Ba6Zn7Ga2S16 under the external perturbation. Laser Damage Threshold (LDT) Properties. As generally accepted, the LDT of a material is influenced by several factors, including band gap, thermal conductivity, anisotropy, and crystal quality.57,60,61 Among these, the band gap is usually the main factor for both single crystal and polycrystalline samples. To assess the LDT of a material, two methods are widely accepted. For materials with single crystals in large sizes (larger than about 3 × 3 mm2 after cutting, grinding, and polishing), the test method according to the ISO 21254-2:2011 standard is most suitable to measure directly on the polished single crystals. For a new NLO compound for which the growth of large-sized single crystals cannot be realized or for which the possibility of growing large-sized single crystals is undetermined, the single pulse powder LDT measurement method on polycrystalline samples proposed by Guo et al. is an effective method.15−17,40,62 Results of LDTs using a single pulse measurement method for polycrystalline Ba6Zn7Ga2S16 and crushed AgGaS2 single crystal (as a reference) in the same particle size range of 150− 210 μm are summarized in Table S7 in the Supporting Information. The LDT of Ba6Zn7Ga2S16 is 40.47 MW/cm2 at the 1.064 μm incident laser with 10 ns and 1 Hz, which is about 28 times that of AgGaS2 (1.44 MW/cm2) under the same measurement conditions. This reads the highest LDT value among the known PM NLO chalcogenides with Eg > 3.0 eV as listed in Table S1 in the Supporting Information. This good performance indicates that title compound can be a good candidate for high power NLO application in the IR region. Photoluminescence Properties. The photoluminescence properties of Ba6Zn7Ga2S16 are investigated at room temperature (Figure 4). Ba6Zn7Ga2S16 exhibits a strong emission behavior of λem = 600 nm with excitation wavelength of λex = 340 nm. This 600 nm photoluminescence output is similar to that of CsCd4Ga5S12 (584 nm).63 This emission peak (600 nm) may be related to the electron transition in various defect levels or low-lying states within the band gaps63−65 and suggests that
Figure 4. Photoluminescence spectra of Ba6Zn7Ga2S16.
the title compound may has good potential as light emitters in optical devices.64 Theoretic Calculation. The electronic band structures reveal direct band gap of 2.15 eV for Ba6Zn7Ga2S16, which is smaller than the experimental observations (Figure 5a). This
Figure 5. (a) Calculated band structure of Ba6Zn7Ga2S16 with the Fermi level is set at 0 eV. (b) Total and partial densities of states of Ba6Zn7Ga2S16.
band gap underestimation is a common problem caused by the discontinuity of the exchange-correlation potential.45,46 In the densities of states (DOS) curve, the top of the valence band (VB) from −4.0 to 0 eV shows obvious hybridizations of Zn3p, Zn-3d, Ga-4p, and S-3p orbitals, indicative of the Zn−S and Ga−S chemical bonds (Figure 5b). The valence band from −8.0 to −4.0 eV contains mainly Zn-3d states with some contributions of Zn-4s, Ga-4s, and S-3p states. Above the Fermi level, the conduction band from 2.0 to 5.0 eV is dominated by Ba-5d, Zn-4s, and small contributions of Ga-4s and S-3p states. The 5.0−14.0 eV conduction band is mainly from Ba-5d with minor amounts of Zn-3p, Ga-4p, and S-3p states. Thus, the band gap absorptions of Ba6Zn7Ga2S16 are mainly determined by the charge transitions from the S-3p states to Zn-3p, Zn-3d, Zn-4s, Ga-4s, Ga-4p, and Ba-5d states. In addition, the detailed comparison between three models of Ba6Zn7Ga2S16, in which Zn(3) atom taking any one of all three 3a Wyckoff sites (model-0: Zn(3), model-1: Ga(1), and model-2: Ga(2)) in the tetrahedral center is listed in Table S8 in the Supporting Information. The total energy of model-0 is 0.67 or 1.17 eV lower than that of model-1 or model-2, showing that the atom 5263
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Figure 6. (a) Calculated birefringence (Δn), (b) calculated frequency-dependent SHG coefficients, and (c) cutoff-energy-dependent static SHG coefficients for Ba6Zn7Ga2S16. (d) Total and partial densities of states (Zn, Ga and S) of VB-1 and (e) CB-2 for Ba6Zn7Ga2S16.
by Zn and Ga atoms, has been discovered and well characterized. Remarkably, Ba6Zn7Ga2S16 exhibits outstanding properties suitable for IR NLO materials. It has a wide band gap of about 3.5 eV, shows phase matchability in the IR region (incident wavelength of 2.05 μm), good NLO coefficient (half that of AgGaS2), and the highest LDT among known PM chalcogenides (28-fold that of AgGaS2, at 1.064 μm laser). The SHG may be mainly ascribed to the transition process from S3p to Ga-4p, Zn-3p, Zn-3d, and Ba-5d states. The good NLO performances together with the other merits of good chemical and thermal stability distinguish Ba6 Zn7Ga 2S16 as one promising IR NLO crystal. Further efforts, especially the growth of large sized single crystals for real applications, are in progress.
distribution of Zn and Ga in model-0 is corrected for the total energy reason. With the scissors corrected method, the linear and nonlinear optical properties of Ba6Zn7Ga2S16 are also calculated. As shown in Figure 6a, the static birefringence of Ba6Zn7Ga2S16 is 0.036, which is close to AgGaS2 (0.039),63 indicating Ba6Zn7Ga2S16 can achieve a phase-matchable condition. Because Ba6Zn7Ga2S16 belongs to the point group 3, there are three independent SHG tensors of d11, d15, and d33 under the restriction of Kleinman symmetry (Figure 6b). Among these tensors, the calculated d11 coefficient of Ba6Zn7Ga2S16 is 6.1 pm/V, which is smaller than that of AgGaS2 (d36 = 18.7 pm/V) at the wavelength of 2.05 μm (i.e., 0.61 eV) and is in accordance with the experimental measurements. Moreover, this value is similar to those of the recent reported Na2ZnGe2S6 (d33 = −5.3 pm/V), Na2ZnGe2S6 (dij = 4.63 pm/V) ,and Na2BaSnS4 (dij = 3.76 pm/V), which have similar band gaps (3.25, 3.27, and 3.7 eV, respectively).16,17 The cutoff-energy-dependent second-order coefficient d11 has been calculated to understand the origin of the SHG components of Ba6Zn7Ga2S16. As shown in Figure 6c, the regions VB-1 and CB-2 mainly contribute to the SHG. As revealed in Figure 6d,e, the VB-1 is dominated by S-3p states hybridized with Zn-3d. The CB-2 has main contributions from S-3p, Ga-4p, Zn-3p, Zn-3d, and Ba-5d states. Thus, the SHG should be mainly ascribed to the transition process from S-3p to Ga-4p, Zn-3p, Zn-3d, and Ba-5d states.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b01321. Details of additional figures and tables (PDF) Crystallographic data (CIF)
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AUTHOR INFORMATION
Corresponding Author
*(L.-M.W.) E-mail:
[email protected].
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ORCID
CONCLUSION In summary, a novel sulfide Ba6Zn7Ga2S16, which crystallizes with a new structure type constructed from tetrahedra centered
Yan-Yan Li: 0000-0002-3572-1858 Peng-Fei Liu: 0000-0002-9170-5238 Li-Ming Wu: 0000-0002-3464-6032 5264
DOI: 10.1021/acs.chemmater.7b01321 Chem. Mater. 2017, 29, 5259−5266
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Chemistry of Materials Notes
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The authors declare no competing financial interest. ICSD: 432837
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ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (21571020, 21233009, and 91422303).
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