LiGaGe2S6: A Chalcogenide with Good Infrared Nonlinear Optical

Oct 19, 2017 - Synopsis. The new chalcogenide LiGaGe2S6 has been synthesized from the existing IR material LiGaS2 by partially substituting Ga with Ge...
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Article Cite This: Inorg. Chem. XXXX, XXX, XXX-XXX

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LiGaGe2S6: A Chalcogenide with Good Infrared Nonlinear Optical Performance and Low Melting Point Dajiang Mei,*,† Shiyan Zhang,† Fei Liang,‡,§ Sangen Zhao,∥ Jianqiao Jiang,† Junbo Zhong,⊥ Zheshuai Lin,*,‡,§ and Yuandong Wu*,† †

College of Chemistry and Chemical Engineering, Shanghai University of Engineering Science, Shanghai 201620, China Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China § University of Chinese Academy of Sciences, Beijing 100190, China ∥ State Key Laboratory of Structural Chemistry and Key Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China ⊥ Key Laboratory of Green Catalysis of Higher Education Institutes of Sichuan, College of Chemistry and Environmental Engineering, Sichuan University of Science and Engineering, Zigong 643000, China ‡

S Supporting Information *

ABSTRACT: In this work, we design and synthesize a new chalcogenide LiGaGe2S6 on the basis of known infrared (IR) material LiGaS2 by partially substituting Ga with Ge. This compound possesses very strong nonlinear (NLO) response (2.5 × LiGaS2) and large band gap (3.52 eV), manifesting a better balance between band gap and NLO response compared with that for LiGaS2. Moreover, LiGaGe2S6 exhibits a much lower melting point (663 °C) than that of LiGaS2 (1050 °C). This would result in the much smaller vapor pressure of sulfur in the fused quartz vessels used for the crystal growth, and thus, it should be greatly beneficial to obtain the large stoichiometric LiGaGe2S6 single crystal. Our studies demonstrate that LiGaGe2S6 is a good candidate material for IR NLO applications.

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an IR NLO material,48−50 only a handful of IR NLO materials satisfy the above condition. Among the IR NLO materials, the lithium-containing sulfides, e.g., LiGaS2 and LiInS2, often have good optical performances by achieving the good balance of band gap and SHG response [LiGaS2 (4.15 eV and >10 × KDP), LiInS2 (3.57 eV and >10 × KDP)].51,52 Nevertheless, the sulfides are air sensitive compounds and would react with oxygen under high temperature. The consequence is that their crystallizations are performed in the sealed vacuum fused quartz vessel to avoid contact with air. Unfortunately, the lithium-containing sulfides usually have quite high melting points. For LiGaS2 and LiInS2 their melting points are 1050 and 1037 °C,52 respectively. The high temperature leads to the rapid elevation of the sulfur vapor pressure in the quartz vessels (see Figure S1), which often results in quartz vessel explosion and crystal growth failure. For a good IR NLO sulfide, it is very favorable to have both a good balance of optical properties and low melting points for crystallization. As LiGaS2 exhibits a quite good IR NLO performance, it would be efficient and convenient to design

INTRODUCTION Nonlinear optical (NLO) materials are commonly used to extend laser frequency and have been widely applied in many laser science and technology fields.1−26 According to their applied spectral regions, the NLO materials can be catalogued into ultraviolet, visible, and infrared (IR) NLO materials. The IR NLO crystals are of great significance since they play important roles in various military regions and civil segments, including laser guidance, laser collimation, IR remote sensing, IR radar, etc. Nowadays, all of the commercial IR NLO materials, such as AgGaS2, AgGaSe2, GaSe, and ZnGeP2,27−30 exhibit eximious second harmonic generation (SHG) responses and high NLO conversion efficiency, but still are deficient for the high-power IR lasers generation on account of their low laser damage threshold (LDT). Therefore, to be of great value for practical applications, the IR NLO materials which simultaneously have large LDT and strong SHG effect are greatly demanded.31−47 It is well-known that the LDT is intrinsically dependent on the energy band gap in the material, but a larger band gap usually results in a smaller SHG effect; hence, the achievement of their perfect balance is a remaining major challenge. As the good balance between band gap (Eg > 3.0 eV) and SHG effect (dij > 10 × KDP) is highly required for © XXXX American Chemical Society

Received: August 8, 2017

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DOI: 10.1021/acs.inorgchem.7b01989 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Å)]. The powder pattern measured by the powder diffraction instrument is in agreement with the simulation of the single-crystal data (Figure 1).

new NLO sulfides starting from the composition and structure of LiGaS2. Thus, the next key issue is how to decrease the melting point of the new materials. It is well-known that the melting process has microscopically originated from the breaking of chemical bonds, and the collapse of the threedimensional framework strongly depends on the weakest chemical bonds in the compound due to Liebig’s law of the minimum.53 In LiGaS2, the ionic Li−S bonds are weaker than the covalent Ga−S bonds; hence, the breaking of the former chemical bonds directly determines the melting point of this compound. If the Li−S bond lengths could be elongated by increasing the interstice size around the lithium cations (or even removing the Li−S bonds), the Li−S chemical bonds would become weaker, and then, the melting point of the compound would be decreased. In fact, the Li content in sulfides can also influence the single-crystal growth processes. By introducing Ge into the lithium-containing sulfides, Kim et al. successfully synthesized a compound Li2Ga2GeS6,54,55 which showed good NLO properties. However, generally, the compounds with high Li content may cause the corrosion of the silica vessel during crystal growth experiments.37 Therefore, for a particular application of NLO materials, it would be favorable to exploit the sulfides with lower Li content. On the basis of the above considerations, in this work, we successfully synthesize a new sulfide LiGaGe2S6 with relatively low Li content from the prototype compound LiGaS2. This compound possesses a large band gap (3.52 eV) and very strong NLO effect (2.5 × LIGaS2), thus reaching a good balance. Moreover, the melting point of LiGaGe2S6 (663 °C) is significantly lower than that of LiGaS2 (1050 °C).52

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Figure 1. Powder X-ray diffraction patterns of LiGaGe2S6. In the LiGaGe2S6 structure, Ga and Ge were treated as disordered over the M2 and M3 positions. The chemical formula LiGaGe2S6 was verified by the consistency of the powder X-ray diffraction (XRD) pattern of the compound synthesized in a stoichiometric ratio and the simulated XRD pattern from single-crystal data. It is highly difficult to differentiate between the Ga and Ge only through the X-ray measurement. By means of the Shimadzu XRF 1800 wavelength dispersion spectrometer, it was determined that the presence of Ga and Ge in the approximate molar ratio of 1:2 (Li is undetectable in XRF), as shown in Figure 2a. Moreover, the analysis of the crystal with inductively coupled plasma (ICP) measurement showed the presence of Li and Ga in the approximate molar ratio of 1:1 (Figure 2b). Therefore, the chemical formula LiGaGe2S6 is confirmed for the crystal. Thermal Analysis. Thermal analysis was carried out by an SDTQ600 TG-DSC thermal analyzer. About 7 mg of LiGaGe2S6 powder sample was placed into an alumina crucible under nitrogen protection. The heating rate was 10 °C/min. UV−Vis−NIR Diffuse Reflectance Spectroscopy Experiment. The UV−vis−NIR diffuse reflectance spectroscopy experiment was conducted with the Shimadzu UV-3600 spectrophotometer. The tested sample and BaSO4 (totally reflected) were ground together at room temperature, collecting information in the 200−800 nm range, and thus the calculated band gap was 3.52 eV. Laser Damage Threshold Test. The laser damage threshold test was carried out with a pulsed YAG laser (1.06 μm, 10 ns, 10 Hz), with a size similar to that of AgGaS2 as the reference.49 Second Harmonic Generation Measurement. The optical SHG test was measured by using the Kurtz and Perry58 method with the 1.06 and 2.09 μm Q-switch lasers. The samples were ground and sieved using a series of mesh sizes in the range 75−380 μm. The 1.06 and 2.09 μm laser tests were performed on the 75−380 and 325−380 μm samples, respectively. As a reference material, a sample of LiGaS2 was prepared in identical fashion. First-Principles Calculations. The first-principles calculations for LiGaGe2S6 were performed by the plane-wave pseudopotential method implemented in the CASTEP package.59 In order to calculate the disordered structure, the virtual crystal approximation (VCA) method was used.60 The ion−electron interactions were modeled by the norm-conserving pseudopotentials61 for all elements. In this model, Li 2s1, Ga 3d104s24p1, Ge 4s24p2, and S 3s23p4 electrons are treated as the valence electrons, respectively. The local density approximation (LDA)62 was adopted to describe the exchange and correlation (XC) potentials. The kinetic energy cutoffs of 880 eV and Monkhorst−Pack k-point meshes63 with a density of (4 × 4 × 4) points in the Brillouin zone were chosen. Meanwhile, the SHG

EXPERIMENTAL SECTION

Single-Crystal Growth. The mixture containing 2 g of commercial Li2S (99.9%), Ga2S3 (>95%), and GeS2 (>95%) was ground and loaded in a molar ratio of 1:1:4 into a carbon-coated silica tube under an Ar atmosphere in a glovebox. The tube was placed into the computer-controlled furnace and heated to 750−850 °C, kept there for 20 h, and slowly cooled at 3−5 °C/h to room temperature. Structure Determination. Single-crystal X-ray diffraction data were collected with the use of graphite-monochromatized Mo Kα radiation (λ = 0.71073 Å) at −140 °C on a Rigaku AFC10 diffractometer equipped with a Saturn CCD detector. The structure was solved with the package SHELXS,56 and refined against F using the program Jana 2006.57 In the LiGaGe2S6 structure, three S atoms occupied the permanent positions (Wyckoff position 16b), and the other metal elements occupied the three tetrahedral positions, which are M1 (Wyckoff position 16b), M2 (Wyckoff position 16b), and M3 (Wyckoff position 8a). The electronic density is low at the M1 position, and the bond distance of M1−S is pretty long which is easy to determine the atom at the M1 position as the Li atom. However, the isotropic displacement parameter of Li was large. After the least-squares refinement, the isotropic displacement parameter is suitable with the Li occupation being 0.5. Then, it can be concluded that the M2 and M3 positions are occupied by Ga and Ge, respectively. Solid-State Synthesis. The mixture containing 2 g of commercial Li2S (99.9%), Ga2S3 (>95%), and GeS2 (>95%) was ground and loaded into a carbon-coated silica tube under an Ar atmosphere in a glovebox, in the molar ratio 1:1:4. The tube was placed into the computer-controlled furnace and heated to 700−800 °C and kept there for 20 h, and the furnace quickly cooled to room temperature. Powder X-ray Diffraction Determination. The powder X-ray powder diffraction analysis of the crystal powder (selected tiny crystals) was performed at room temperature in the angular range 2θ = 10−80° with a scan step width of 0.02° and a fixed counting time of 0.5 s/step using an automated Bruker D2 phaser [Cu Kα (λ = 1.5418 B

DOI: 10.1021/acs.inorgchem.7b01989 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 3. Transformation of the structure from LiGaS2 to LiGaGe2S6.

Here, ED,q and EH are the total energies of the defect system and the pristine material, respectively. Nα is the number of α atoms added (Nα> 0) or removed (Nα < 0) from the host to create a defect, and Eα is the total energy of one isolated α atom.

in LiGaGe2S6 is slightly different from that in LiGaS2 and the spatial volumes occupied by the remaining Li+ cations increase simultaneously. It should be noted that the cell parameters of LiGaGe2S6 (a = 11.925(2) Å, b = 22.647(5) Å, c = 6.8308(14) Å, V = 1844.8(6) Å3) are very close to those of Li2Ga2GeS6 (a = 11.943(5) Å, b = 22.590(8) Å, c = 6.805(2) Å, V = 1835.9(12)).48 The structure of Li2Ga2GeS6 has also come from LiGaS2, with one-third of Ga3+ replaced by Ge4+ and one-third of Li+ removed to maintain the charge balance. While Ga3+ and Ge4+ have almost identical ionic radii, the Li+ deficiency has little influence on the cell parameters of the compounds. Thus, the topological structures and cell parameters of LiGaGe2S6 and Li2Ga2GeS6 are very similar. In fact, this phenomenon is not occasional; the compounds containing the same elements but in a different ratio may have similar cell parameters. For example, AgGaGe2Se6 (a = 12.4967(6) Å, b = 23.905(1) Å, c = 7.1420(3) Å, V = 2133.5(3) Å3), AgGaGe3Se8 (a = 12.4423(6) Å, b = 23.820(1) Å, c = 7.1403(3) Å, V = 2116.3(3) Å3), AgGaGe4Se10 (a = 12.4126(5) Å, b = 23.7689(9) Å, c = 7.1384(3) Å, V = 2106.1(3) Å3), and AgGaGe5Se12 (a = 12.4107(6) Å, b = 23.767(1) Å, c = 7.1364(3) Å, V = 2105.0(3) Å3) have the same space group Fdd2 and similar cell parameters.67 Thermal Properties. Differential scanning calorimetry reveals that the LiGaGe2S6 melting point is at comparatively low temperature, 663 °C (see Figure 4). From about 550 °C, the curve starts to have a sign of heat absorption, and through appling the tangents, it can be observed that the melting point is 663 °C. The melting point of LiGaGe2S6 is much lower than that of LiGaS2 (1050 °C), and even significantly lower compared with the other lithium-containing NLO sulfides (e.g., LiInS2 1037

RESULTS AND DISCUSSION Structure. LiGaGe2S6 crystallizes in the space group Fdd2 of the orthorhombic system. Its structure can be considered to be deduced from the structure of LiGaS2. As shown in Figure 3, in LiGaS2 , the three-dimensional network structure is constructed by the GaS4 and LiS4 tetrahedra connected with each other via the corner-sharing S2− anions. By introducing the Ge4+ cations to substitute the two-thirds of Ga3+ cations and decreasing the Li+ content (i.e., removing two-thirds of Li+ cations) to compensate for the electronic balance in LiGaS2, the LiGaGe2S6 structure can be formed (selected bond lengths and angles in LiGaGe2S6, see Table S1). The replacement between GaS4 and GeS4 tetrahedra often occurs in the chalcogenide compounds since the ionic radius of the Ga3+ cation (0.61 Å) is very close to that of the Ge4+ cation (0.53 Å).66 Owing to the Li+ vacancy existence, the spatial arrangement of GeS4 groups

Figure 4. DSC curve of LiGaGe2S6.

Figure 2. (a) Wavelength dispersion X-ray intensities (Ga:Ge:S) of LiGaGe2S6 and (b) inductively coupled plasma intensities (Li:Ga) of LiGaGe2S6. coefficients dij are calculated using an expression developed by Lin et al.64 The formation energy (ΔHD,q) can be calculated using the following formula:65

ΔHD,q = (E D,q − E H) +

∑ NαEα α

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C

DOI: 10.1021/acs.inorgchem.7b01989 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry °C, Li2In2GeS6 858 °C, and Li2In2SiS6 780 °C).52,71 Consistent with our consideration that the Li−S bonds greatly affect the melting points, the structural data reveal that indeed the melting points of the Li-containing sulfides are strongly dependent on the interstice size around the Li+ cations, i.e., the Li−S bond lengths. Here we consider five representative Licontaining sulfides, i.e., LiGaS2, LiInS2, Li2In2GeS6, Li2In2SiS6, and LiGaGe2S6. Clearly, the Li−S bond lengths increase in the order LiGaS2 < LiInS2 < Li2In2GeS6 < Li2In2SiS6 < LiGaGe2S6, and the melting points decrease in the same order (Table 2).

Optical Properties. The SHG response of LiGaGe2S6 is investigated by an IR laser with the wavelength of 1.06 μm (see Figure 5). This compound exhibits the type-I phase-matching

Table 1. Crystal Data and Structure Refinement for LiGaGe2S6 LiGaGe2S6 fw a (Å) b (Å) c (Å) V (Å3) space group Z index ranges

414.2 11.925(2) 22.647(5) 6.8308(14) 1844.8(6) Fdd2 8 −8 ≤ l ≤ 9 −17 ≤ h ≤ 17 −31 ≤ k ≤ 27 3.55−30.96 2.9827 106.48 0.0262, 0.0239 0.0366,0.0262

θ range ρc (g/cm3) μ (cm−1) Robs, wRobs Rall, wRall

Figure 5. SHG intensities of LiGaGe2S6 and LiGaS2 with 1.06 μm lasers.

characteristic. The spectra revealed that the SHG signal is close to 2.5 × LiGaS2. As shown in Figure S2, the SHG signal of the LiGaGe2S6 with a 2.09 μm laser is about 1.5 × LiGaS2, which is close to that of BaGa4S7 with 1.4 × LiGaS2. The SHG effect of LiGaGe2S6 is similar to that of Li2Ga2GeS6,54 and smaller than that of LiGaGe2Se6.43 Generally, for isomorph crystals, the SHG responses of sulfur compounds are smaller than those of selenide compounds, and the sulfur compounds possess larger band gaps. The enhanced SHG response in LiGaGe2S6 is attributed to the great distortion of the structural units due to the introduction of Ge4+ cations and the enlargement of the interstitial space around Li+ cations. The UV−vis diffuse reflectance spectrum reveals that the energy band gap of LiGaGe2S6 is 3.52 eV (see Figure 6). The

Table 2. Comparison of the Melting Points (TM’s), Formation Energies (Efor’s), and Average Bond Lengths (Lave’s) of the Li−S Bonds among the Lithium-Containing IR NLO Sulfides compd

TM (°C)

Efor (eV)

Lave (Å)

ref

LiGaS2 LiInS2 Li2In2SiS6 Li2In2GeS6 LiGaGe2S6

1050 1037 858 780 663

6.000 5.975 5.910 5.890 5.516

2.462 2.437 2.521 2.532 2.600

52 52 71 71 this work

The additional first-principles calculations demonstrate that, in these materials, the Li−S bond length increase corresponds to the decrease of formation energies of the Li−S bonds (also listed in Table 1). The smaller formation energies of Li−S bonds result in the lower energy required to break the chemical bonds, thus making the material melt at a lower temperature. In LiGaGe2S6 the substitution of Ga3+ with Ge4+ and the Li content reduction in the crystal lattice provide larger interstitial space around the remaining Li+ cations, and it increases the Li− S bond lengths, which significantly decreases the melting point. Like the LiGaGe2Se6, both of the compounds’ melting points are relatively low (