Strong Infrared Nonlinear Optical Efficiency and High Laser Damage

5 mins ago - Synopsis. Two new sulfides, Na2Ga2GeS6 and Na2Ga2SnS6, were obtained by mixing different typical nonlinear optical (NLO)-active motifs ...
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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Strong Infrared Nonlinear Optical Efficiency and High Laser Damage Threshold Realized in Quaternary Alkali Metal Sulfides Na2Ga2MS6 (M = Ge, Sn) Containing Mixed Nonlinear Optically Active Motifs Shu-Fang Li, Xiao-Ming Jiang,* Bin-Wen Liu, Dong Yan, Hui-Yi Zeng, and Guo-Cong Guo* State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P.R. China S Supporting Information *

motifs, such as GaQ4, InQ4, GeQ4, and SiQ4 (Q = S, Se) tetrahedra, in a single chalcogenide compound to modulate SHG efficiency and LIDT simultaneously.10,11 For instance, the incorporation of GeS2 into AgGaS2 produces quaternary AgGaGeS4 with improved LIDT, making it a substitute to the commonly used AGS in the frequency conversion laser application.12 The structural design via the assembly of typical NLO-active chalcogenide tetrahedral motifs into superpolyhedral clusters may be another effective strategy to explore promising IR NLO materials exhibiting favorable LIDT and SHG responses. For instance, in Na2In4SSe6, NaGaIn2Se5, and NaIn3Se5, their structures consist of superpolydedral chalcogenide clusters, exhibit SHG responses that are 7.0, 2.1, and 0.3 times larger than AGSs, respectively, and LIDTs that are 5.8, 6.4, and 10.7 times higher than those of AGS, respectively.13 Different microscopic NLO-active motifs can be used as the structural building blocks of IR NLO materials, and various assembling types can also be adopted. Thus, mixing different microscopic NLO-active motifs is a flexible way to optimize NLO efficiency. Compared with chalcogenides without alkali metals, alkali metal-containing ones usually have relatively wider band gaps and higher LIDT. Furthermore, these alkali metalcontaining chalcogenides provide an excellent platform for the optimization of NLO efficiency using the mixing method. During the exploration of IR nonlinear optical materials in the alkali metal chalcogenides by the method of mixing different NLOactive motifs MQ4 (M = Ga, Ge, and Sn), we successfully obtained two quaternary compounds, Na2Ga2GeS6 (1) and Na2Ga2SnS6 (2). These compounds exhibit LIDTs that are 18.1 and 17.9 times that of AGS and relatively high SHG response (0.8 × AGS for 1 and 1.1 × AGS for 2). The LIDTs of 1 and 2 are the highest among the reported Na-containing chalcogenides. In addition, compounds 1 and 2 exhibit wide IR transparency and type-I phase-matching behavior. Hence, these compounds are potential IR NLO materials for high-power laser applications. Single crystals of 1 and 2 were synthesized by solid-state reactions with a mixture of Na2S, Ga, Ge (Sn) and S at 800 °C for 4 days (for details, see the Supporting Information). Energydispersive X-ray spectroscopy (EDS) of single crystals of both compounds reveals average Na/Ga/Ge(Sn)/S molar ratios of 2.1/1.8/1.0/6.1 and 2.1/1.9/1.0/5.9 for 1 and 2, respectively; they are consistent with those determined from single-crystal X-

ABSTRACT: Two new infrared (IR) nonlinear optical (NLO) sulfides, Na2Ga2GeS6 and Na2Ga2SnS6, were obtained by mixing different typical NLO-active motifs GaS4 and GeS4/SnS4 in the alkali metal-containing system. The IR NLO sulfides present laser-induced damage thresholds that are 18.1 and 17.9 times that of the reference AgGaS2 (AGS) and second-harmonic generation efficiencies that are 0.8 and 1.1 times that of AGS. These properties originate from the GaS4, GeS4, and SnS4 tetrahedral blocks in the structures of the sulfides. Both compounds also exhibit a broad transparency range and type-I phase-matching behavior, which support their high potential in high-power laser applications. This work sheds new light on the development of promising mid-IR NLO materials by combining different NLO-active motifs.

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olar compounds whose structures lack an inversion center may exhibit various technologically important physical properties, such as ferroelectricity, piezoelectricity, and nonlinear optical (NLO) properties.1 Metal chalcogenides with a secondharmonic generation (SHG) effect may be used as NLO materials in the infrared (IR) region, whereas oxide materials, LiB3O5, β-BaB2O4, KH2PO4, LiNbO3 and so on, are restricted due to their intrinsic IR absorptions.2 To date, some chalcogenide IR NLO materials, including ZnGeP2 (ZGP), AgGaS2 (AGS), and AgGaSe2, are commercially available. However, they suffer from some drawbacks including multiphoton effects and low laser-induced damage thresholds (LIDT),3 which hamper their applications for high power lasers. The wide band gaps of IR NLO materials confer high LIDTs but small SHG efficiency, implying the incompatibility between them.4 Therefore, developing superior IR NLO materials exhibiting both good SHG response and LIDT is challenging. The balance between SHG response and LIDT can be achieved in several ways. One way is to increase the band gap by replacing Ag+ with strong electropositive alkali or alkaline earth metals in classical IA−IIIA−VIA2 type IR NLO materials, such as the LiBQ2 (B = Ga, In; Q = S, Se, Te) family and BaGa4S7.5−7 Their band gaps of up to 4 eV lead to LIDTs exceeding those of Ag analogs, although their SHG efficiencies become slightly lower. For instance, the band gap of LiGaS2 is 3.65 eV, and it is considerably wider than that of AGS (2.62 eV),8 whereas its SHG coefficient is approximately a quarter of that of AGS.9 Another way is to combine two or more different typical NLO-active © XXXX American Chemical Society

Received: April 12, 2018

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DOI: 10.1021/acs.inorgchem.8b00891 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

suggests that 1 and 2 are phase-matchable at 1910 nm. The SHG efficiencies are approximately 0.8 and 1.1 times that of AGS at the particle sizes of 75−100 μm (Figure 2b). As the SHG intensity measured using the Kurtz and Perry power method is proportional to the square of the SHG coefficient deff, and the deff of AGS is 12.5 pm/V,8,22 the derived deff values of 1 and 2 are 11.18 and 13.11 pm/V, respectively. Compared with Na2In2MS6 (M = Si, Ge), the space group changes from Cc of Na2In2MS6 to Fdd2 of 1 and 2, resulting in different tetrahedral distortions and consequently different SHG performance and band gaps. The zcomponents of dipole moments of anionic groups make major contributions to the net dipole moments in 1 and 2 (Table S4). The LIDT of the pressed powdery samples of 1 and 2 with AGS acting as the benchmark were performed using the single pulse LIDT method.23 As illustrated in Table 1, a small spot area

ray structural analyses. The measured powder X-ray diffraction patterns of both compounds agree well with the simulated ones of their single-crystal data (Figure S1), indicating that the samples are pure. Both compounds are isostructural and crystallize in the same polar space group of Fdd2 (No. 43) in the orthorhombic system. Taking 1 as the representative to describe their structures, two crystallographically independent Na atoms, two Ga atoms, one Ge atom, and six S atoms exist in the asymmetric unit of 1. Its 3D framework is composed of two different building blocks, namely, 1D infinite ∞1(GaS3)3− chains (red tetrahedral chains in Figure 1(a)) and isolated (GeS4)4− tetrahedra (green tetrahedra in

Table 1. Measured LIDTs of 1, 2, and Benchmark AGS

Figure 1. (a) 3D structure of 1 viewed in the a direction. (b) Infinite 1 3− chains represented as red columns and isolated (GeS4) ∞ (GaS3) tetrahedra in 1.

compounds

Damage Energy (mJ)

spot area (cm2)

damage threshold (MW·cm−2)

1 2 AGS

63.76 35.63 10.87

0.0616 0.0346 0.1901

103.55 102.87 5.72

of 0.0616 cm2 for 1 and 0.0346 cm2 for 2 are chosen during the measurement, and the damage energies of 1 (35.63 mJ) and 2 (35.63 mJ) are considerably larger than that of AGS (10.87 mJ). The powder LIDTs of 1 (103.55 MW·cm−2) and 2 (102.87 MW· cm−2) are 18.10 and 17.98 times that of AGS (5.72 MW·cm−2), indicating that these are the highest among the reported Nacontaining chalcogenides such as Na2ZnGeS6 (∼6 × AGS),24 Na2BaSnS4 (∼5 × AGS),25 and Na2Hg3Si2S8 (∼4.5 × AGS).23 Thus, the LIDTs of 1 and 2 are considerably improved with maintained favorable SHG efficiency, which is extremely important for the high-power IR-NLO applications. The laser damage of an NLO crystal is a highly complicated process that involves pitting, erosion, melting, delamination, fracture, and discoloration.26 Wider band gaps simultaneously lead to small SHG efficiency and high LIDT. The UV−visible− NIR diffuse reflectance spectra show that the optical band gaps of 1 and 2 are 3.10 and 2.74 eV, respectively (Figure S4), compared with those of AGS (2.62 eV)8 and ZGP (1.75 eV).27 Compounds 1 and 2 have significantly larger band gaps, implying that they can circumvent the two-photon absorption of the typical 1064 nm laser and consequently present the improvement of LIDT. We also attempted to understand the extremely high LIDTs of 1 and 2 from the viewpoint of thermal effect. Thermal expansion anisotropy (TEA) is an important intrinsic parameter of NLO materials that influence the ultimate experimental LIDTs.13 NLO crystals exhibiting small TEA tend to endure larger thermal shock due to optical absorptions and exhibit higher LIDTs. To study the TEA of 1, 2, and the reference AGS, the measurement of temperature-variable lattice parameters was carried out usuing an X-ray diffractometer in the region of 300−500 K with the interval of ∼20 K (Figure S6). The thermal expansion coefficients (TEC, αL = R0−1(dR(T)/dT), R0 is the a, b, c values at T = 0 K) of the cell lengths of 1, 2, and AGS were calculated (Table S5). TEA (δ) is defined as the ratio of the maximum and the minimum TECs. The derived TEC values of 1, 2, and AGS are 1.05, 1.50 and 2.97, respectively, and the measured TEA and TEC of AGS are similar to the reported ones.28 The measured TEA with the δ(AGS) > δ(2) > δ(1) order agrees with the

Figure 1(a)). Each Ga atom is coordinated by four S atoms to form typical (GaS4)5− tetrahedra with Ga−S bond lengths of 2.242−2.283 Å, which then connect with each other by sharing corners to form isolated ∞1(GaS3)3− chains. Analogous to Ga atoms, each Ge atom is also tetrahedrally coordinated by four S atoms to form the (GeS4)4− tetrahedra with Ge−S bond distances of 2.227−2.239 Å. Two groups of ∞1(GaS3)3− chains along the a + c and a − c directions intercross with each other with (GeS4)4− tetrahedra acting as the connections to form the 3D anionic (Ga2GeS6)2− framework of 1 (Figure 1(b)), and tunnels are formed along the a + c directions. These tunnels are embedded with counter cations Na+. The Sn positions in 2, corresponding to Ge positions in 1, are mixed with Ga with half occupancy. The Ga−S, Ge−S, and (Ga/Sn)−S bond lengths (Table S3) in 1 and 2 are close to those in BaGa4S7,7 BaGa2GeS6,14 and Ba2Ga8GeS16,11 respectively. Ga/Ge/SnS4 tetrahedra are typical NLO-active structural units.15−19 Compounds 1 and 2 may exhibit SHG efficiency due to their noncentrosymmetric polar space group. The powder SHG measurements of both compounds were carried out by the modified Kurtz powder method with AGS as the reference.20,21 As shown in Figure 2a, for both compounds, the SHG signal intensities become stronger as the particle size increases, which

Figure 2. (a) Particle size dependence of SHG intensities of 1 and 2 at 1910 nm. (b) SHG intensities at the particle sizes of 75−100 μm. AGS samples sieved in the same particle sizes were used as the reference. B

DOI: 10.1021/acs.inorgchem.8b00891 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry experimental LIDTs with the LIDT(2) ∼ LIDT(1) > LIDT(AGS) order. To gain further insights on the SHG efficiency of 1 and 2, we performed first-principles calculations of band structures, density of states (DOS), and optical properties using the CASTEP package based on density functional theory.29 The calculated band gaps are 2.73 eV for 1 and 2.49 eV for 2 (Figure S7), which are lower than the experimental ones. The underestimated band gaps of both compounds are due to the limitation of DFT that sometimes underestimates the band gaps in semiconductors and insulators. The partial density of states of 1 and 2 (Figure S8) reveal that the valence band top is dominated by S-3p states and that the conduction band bottom mainly originates from empty Ga-4p, Ge-4p, and S-3p orbitals for 1 and Ga-4s, Sn-5s, and S-3p for 2. Therefore, the electronic structures of 1 and 2 close to the Fermi level are mainly derived from the GaS4, GeS4, and SnS4 tetrahedral units, which are responsible for the SHG effect in 1 and 2. Compounds 1 and 2 belong to polar class mm2 and have three nonvanishing independent SHG tensor components d113, d223, and d333 under the Kleinman’s symmetry restriction. The average SHG coefficients of d113, d223, and d333 are calculated to be 7.70 pm/V (1) and 8.58 pm/V (2) at the experimental wavelength of 1910 nm (0.649 eV) (Figure S11). The results agree with the experimentally derived powder SHG efficiency. The calculated birefringence Δn values of 1 and 2 at 1910 nm are 0.004 and 0.02, respectively (Figure S10); they are phase-matchable at room temperature, implying the temperature-dependent behaviors of Δn of 1 and 2. Additionally, the real (ε1) and the imaginary (ε2) parts of the optical dielectric functions of 1 and 2 are illustrated in Figure S9. On the basis of the dispersion feature of the ε2 spectra, the maximum absorptions are located at approximately 6.77 eV for 1 and 6.23 eV for 2, which are mainly originated from the charge transitions from S-3p states to Ga-4p, Ge-4p, and S-3p orbitals in 1 and Ga-4s, Sn-5s, and S-3p states in 2 according to the above DOS analysis. Differential scanning calorimetry analysis showed that compound 2 melts congruently with the melting point 833 °C upon heating and crystallization upon cooling events (Figure S2b), while 1 has no fixed melting point (Figure S2a). For 1, a broad endothermic peak appears at around 750 °C upon heating, and an apparent obvious exothermic peak is absent on the cooling curve, indicating the possible thermal decomposition behavior at high temperatures. For 2, it can be seen from Figure S2b that one endothermic peak and one exothermic peak are on the heating and the cooling curves, respectively, and the powder XRD pattern of the residue after one melting and recrystallization cycle agrees well with that before melting (Figure S3), indicating that compound 2 is congruently melting, compared with known IR NLO materials with high melting temperatures such as AGS (996 °C)30 and BaGa4S7 (1090 °C).7 The lower melting point with the congruent melting feature implies that the large crystal of 2 may be grown by the Bridgman−Stockbarger method at a lower temperature. The IR and optical diffuse reflectance spectra of 1 and 2 show no chemical bonding absorptions in a broad spectral range of 400−4000 cm−1 (Figure S5), demonstrating that both compounds may be suitable for NLO applications in a wide IR spectral range. In summary, two new IR NLO chalcogenides, namely, Na2Ga2GeS6 (1) and Na2Ga2SnS6 (2), were obtained by combing typical NLO-active motifs GaS4 and GeS4/SnS4 in the alkali metal-containing system. They present LIDTs that are 18.1 and 17.9 times that of the benchmark AGS, the highest among

the reported Na-containing chalcogenides, and SHG efficiencies that are 0.8 and 1.1 times that of AGS. The experimental band gap and thermal effect studies show that the high LIDTs of both compounds can be ascribed to their relatively wider band gaps and smaller thermal expansion anisotropy compared with AGS. The first-principles calculations of band structures, DOS, and optical properties of 1 and 2 show that the GaS4, GeS4, and SnS4 tetrahedral units are responsible for their SHG efficiency. Compounds 1 and 2 also exhibit a wide transparency range and type-I phase-matching behavior, showing that they are potential IR NLO materials for high power laser applications. This work sheds new light on the development of practical midIR NLO materials by combining different NLO-active motifs.

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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.inorgchem.8b00891. CIF data and additional figures. Synthesis, experimental details, structural refinement and crystal data, powder XRD, IR spectra and DSC curves, electronic structures, PDOS plots, and calculated frequency-dependent SHG coefficients. (PDF) Accession Codes

CCDC 1838057−1838058 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], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Guo-Cong Guo: 0000-0002-7450-9702 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support by the Strategic Priority Research Program of the Chinese Academy of Sciences, Grant No. XDB20000000, the NSF of China (21701176 and 21401052), the National Postdoctoral Program for Innovative Talents (BX201600163), the China Postdoctoral Science Foundation (2016M600510), and the National Key Laboratory Development Fund (20180026).

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REFERENCES

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DOI: 10.1021/acs.inorgchem.8b00891 Inorg. Chem. XXXX, XXX, XXX−XXX