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New Compressed Chalcopyrite-like Li2BaMIVQ4 (MIV = Ge, Sn; Q = S, Se): Promising Infrared Nonlinear Optical Materials Kui Wu,† Bingbing Zhang,† Zhihua Yang,* and Shilie Pan* Key Laboratory of Functional Materials and Devices for Special Environments of CAS, Xinjiang Technical Institute of Physics & Chemistry of CAS, Xinjiang Key Laboratory of Electronic Information Materials and Devices, 40-1 South Beijing Road, Urumqi 830011, China S Supporting Information *
simultaneously maintain good dij with phase-matching ability. Considering the performance features of chalcopyrites, we use the MIVQ4 units to substitute the GaQ4 units to maintain the good dij of AgGaQ2 since MIVQ4 (MIV = Ge, Sn; Q = S, Se) tetrahedra are also verified as the important “NLO active units”.9 Moreover, substitution of highly electropositive elements (alkali/alkaline-earth metal) for the Ag atoms can also increase Eg and further overcome the low LDTs of AgGaQ2.10 Guided by this design strategy, we focused our research on the Li−Ba−MIV−Q (MIV = Ge, Sn; Q = S, Se) system and four new IR NLO materials with compressed chalcopyrite-like structures, Li2BaMIVQ4, were successfully synthesized. Note that previous reports also deduce that chalcopyrite-like compounds can be expected as the potential IR NLO materials.8 Excellent performances measured for Li2BaGeS4 (Eg = 3.66 eV, dij = ∼0.5 × AgGaS2, Δn = 0.031) and for Li2BaSnS4 (Eg = 3.07 eV, dij = ∼0.7 × AgGaS2, Δn = 0.033) have been found, which indicate that they can be expected as promising IR NLO candidates and also effectively eliminate the performance defects (low LDTs and TPA) of commercial materials. Moreover, title selenides possess the phase-matching abilities and simultaneously avoid the nonphase matching behavior in AgGaSe2. Title compounds are isostructural and crystallize in the 4̅2m point group as well as that of chalcopyrite-type AgGaQ2. Reasonable crystal structures for title compounds are also verified by the calculated bond valences and global instability index (Tables S1, S2). Herein, we have chosen Li2BaSnS4 to be representative to discuss its structural features. In its structure, highly distorted LiS4 tetrahedra with d(Li−S) = 2.527 Å and two different S−Li−S angles (149.65(8)° and 93.93(2)°) interconnect together by sharing corners to form two-dimensional (2D) layers (Figure 1b) that are further bridged with isolated SnS4 units to form a 3D tunnel structure, in which the Ba cations are located (Figure 1a). The BaS8 dodecahedra have two types of similar bond lengths of Ba−S (3.173 and 3.328 Å), and the same-length bonds are located at the symmetrical positions; namely, one BaS8 dodecahedron can be viewed as the combination of two interpenetrating tetrahedra. In addition, seen from the c-axis, it also exhibits the 2D layers composed of interlinked BaS8 and SnS4 units that are stacked up by sharing common S atoms (Figure 1c, 1d). Remarkably, in comparison with the structures of classical chalcopyrites,11 it can be
ABSTRACT: Chalcopyrite-type AgGaQ2 (Q = S, Se) and ZnGeP2 are the main commercial infrared nonlinear optical (IR NLO) crystals. Unfortunately, performance defects including low laser damage threshold (LDT), harmful two-photon absorption (TPA), or small birefringence limit their application. With this background, four new compressed chalcopyrite-like IR NLO materials Li2BaMIVQ4 (MIV = Ge, Sn; Q = S, Se) were successfully synthesized with the typical AgGaQ2 as templates. Remarkably, Li2BaGeS4 and Li2BaSnS4 not only maintain the good NLO responses (0.5 and 0.7 × AgGaS2) but also overcome low LDTs and TPA of commercial chalcopyrites, demonstrating that they satisfy critical demands as promising IR NLO candidates. All of them exhibit phasematching abilities. Furthermore, the discovery of chalcopyrite-like compounds also provides a feasible design strategy to explore new promising IR NLO materials.
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ecently, adopting the frequency-conversion technology on nonlinear optical (NLO) materials to achieve the tunable laser output has been well developed.1−4 As for the middle and far-infrared (IR) region, current commercially available NLO materials are mainly the chalcopyrite-type compounds AgGaQ2 (Q = S, Se) and ZnGeP2.5 Note that all of them exhibit wide IR transmission ranges and good second-order harmonic generation (SHG) responses. However, their application ranges are seriously hindered by inherent performance defects, such as low laser damage thresholds (LDTs) for AgGaQ2, harmful twophoton absorption (TPA) at 1 μm for ZnGeP2, and nonphase matching behavior (small birefringence) for AgGaSe2. Therefore, eliminating the above performance defects of commercial chalcopyrites is prerequisite in the exploration of new IR NLO materials.6 Generally, a wide band gap (Eg) corresponds with a high LDT but is inversely proportional to the large SHG coefficient (dij) in one material.7 In addition, suitable birefringence (Δn) is helpful to achieve the phase-matching condition and avoid the destructive optical behaviors. To sum up, an outstanding IR NLO material must satisfy the following conditions: wide Eg (>3.0 eV), large dij (≥0.5 × AgGaS2), and suitable Δn (0.03−0.10).8 On account of the contradictory relationship between Eg and dij, satisfying the suitable balance of key parameters (Eg, dij, and Δn) in one IR NLO material is still an enormous challenge.4a With this in mind, we propose an efficient design strategy that could enhance the LDT and © 2017 American Chemical Society
Received: August 22, 2017 Published: September 25, 2017 14885
DOI: 10.1021/jacs.7b08966 J. Am. Chem. Soc. 2017, 139, 14885−14888
Communication
Journal of the American Chemical Society
reported I2−Ba−MIV−Q4 (I = monovalent cations, such as Cu, Ag, Na) system, their crystal structures can be changed with the simple replacement (Ge to Sn or S to Se)(Figures S2−S4).14,13c,6a Interestingly, title compounds are isostructural while I is the Li cation, which is different from the known results in the I2−Ba−MIV−Q4 system (Table S3). Therefore, it can be viewed as the first example that isostructural structures have been found in the I2−Ba−MIV−Q4 system with different MIV (MIV = Ge, Sn) or Q (Q = S, Se) atoms. Millimeter-level single crystals of Li2BaGeS4 (colorless) and Li2BaSnS4 (pale-yellow) were successfully synthesized by spontaneous crystallization, and they are stable in air over half of the year without any surface changes (see the Supporting Information). The purity of title compounds was also verified by the PXRD measurement, and the results show that experimental patterns match well with the simulated ones derived from the CIF data. Important optical properties were also systematically studied (Figures 3 and S5). Diffuse Figure 1. (a) See the crystal structure of Li2BaSnS4 from the b-axis (Ba−S bonds omitted for clarity). (b) A layer is composed of the interconnection with corner-sharing LiS4 units. (c) See the crystal structure of Li2BaSnS4 from the c-axis (Li−S bonds omitted for clarity). (d) The BaS8 dodecahedra are connected with isolated SnS4 units by sharing edges to form a layer in the ac plane.
interestingly found that title compounds exhibit the obvious distortion as tetragonal compression along the c axis to form the expanded coordination sphere (distorted dodecahedron) for the Ba atoms (Figure 2). Note that the structural distortion
Figure 3. Experimental results of Li2BaGeS4 and Li2BaSnS4: (a) PXRD patterns of Li2BaGeS4, inserted is the photograph of the Li2BaGeS4 crystals; (b) PXRD patterns of Li2BaSnS4, inserted is the photograph of the Li2BaSnS4 crystals; (c) optical band gaps; (d) SHG intensities versus particle sizes with AgGaS2 as the reference at 2.09 μm radiation. Figure 2. Structural transformation from classic chalcopyrite AgGaS2 (I4̅2d) to the compressed chalcopyrite-like structure of Li2BaSnS4 (I4̅2m).
reflection spectra indicate that Li2BaGeS4 and Li2BaSnS4 have larger Eg (3.66 and 3.07 eV), respectively, than that of AgGaS2 (2.73 eV).15a Generally, laser damage can be attributed to the internal thermal effect induced by the strong optical absorption that is closely related to Eg for one material.15b,c Thus, in view of their wide Eg, they can be expected to avoid the TPA of traditional laser radiation and exhibit high LDTs for potential application. Besides, relatively narrow Eg (2.40 and 2.18 eV) are also found in title selenides (Li2BaGeSe4 and Li2BaSnSe4), respectively, but are still larger than those of other commercial chalcopyrites including AgGaSe2 (1.83 eV) and ZnGeP2 (1.65 eV). To better state the relationship between structure and performance, electronic structures using the DFT method16 are studied, and the results indicate that all of them are direct band gap compounds and their Eg are mainly affected by the MIVQ4 tetrahedral units (Figures S6, S7). Recently, estimating the LDT on a powder sample can be viewed as one feasible test method for IR materials.17 Herein, we have investigated their LDTs with powder AgGaS2 as an reference by a 1.064 μm pulse laser. Metal sulfides Li2BaGeS4 and Li2BaSnS4 have consid-
degree (Δd) of chalcopyrite-like compounds can be calculated with the formula (Δd = 2 − (c/a)) where a and c are the cell constants.12 Calculated results (Table S4) show that classical chalcopyrite-type AgGaS2 and AgGaSe2 have small Δd (0.21 vs 0.18) and the Δd for stannite (Cu2FeSnS4 and Cu2CdSnS4) are close to zero.13 However, title compounds and other compounds (Li2PbGeS4, Li2EuGeS4, Ag2BaGeS4) exhibit a severe compression along the c axis, then leading to the larger Δd values (∼0.8).12,13c This high Δd can be explained by the following rationale that the larger cations (Pb or Ba) prefer a coordination number 8 than only 4 in the structures of chalcopyrites; thus, their structures have to further compress themselves to meet the demand for higher coordination spheres, which is in agreement with the previous investigation result that the sizes of different cations can create different coordination environments and further lead to various framework structures.2e Moreover, seen from the previously 14886
DOI: 10.1021/jacs.7b08966 J. Am. Chem. Soc. 2017, 139, 14885−14888
Communication
Journal of the American Chemical Society erable LDTs about 325 and 192 MW/cm2 that are about 11 and 6.5 times that of AgGaS2 (29.6 MW/cm2), respectively. The above results give good confirmation that title sulfides possess promising potential for application in high-energy laser systems. Powder SHG responses of title compounds were systematically investigated with AgGaS2 as the reference using the Kurtz and Perry method.19 The SHG intensities increase with increasing particle sizes under 2.09 μm laser, showing type I phase-matching behavior. Besides, all of them exhibit good SHG responses which are about 0.5, 0.7, 1.1, and 1.3 times that of AgGaS2 for Li2BaGeS4, Li2BaSnS4, Li2BaGeSe4, and Li2BaSnSe4 at 200−250 μm particle size, respectively (Figures 3 and S8). Note that title selenides avoid the nonphase matching behavior in AgGaSe2 by element cosubstitution. Theoretical NLO coefficients (d14) and Δn were also calculated to be 4.52 pm/V, Δn = 0.031 for Li2BaGeS4; 7.61 pm/V, Δn = 0.033 for Li2BaSnS4; 9.64 pm/V, Δn = 0.038 for Li2BaGeSe4; and 14.17 pm/V, Δn = 0.066 for Li2BaSnSe4, which are consistent with experimental results. Note that the dij has an inverse relationship with Eg and also has been determined by the structural distortion and arrangement of the NLO-active units. According to calculated results (Table S6), the smaller dij of title sulfides can be attributed to their larger band gaps and smaller packing density of active units in the unit cell compared with those of AgGaS2. Remarkably, in comparison with critical optical parameters of famous IR NLO materials (Table 1), title compounds also display comparable properties and can be expected to be applied in the high-power laser frequencyconversion region.
chalcopyrite-like structures provides a predictive guide to explore new promising IR NLO candidates.
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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b08966. Experimental details, calculated structural distortion, BVS and GII, structural comparison, powder XRD, IR and Raman spectra, SHG responses, electronic structures, PDOS plots (PDF)
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materials
dij (×AgGaS2)
LDT (×AgGaS2)
Δn
ref
AgGaS2 LiGaS2 LiInS2 Li2BaGeS4 Li2BaSnS4 Li2BaGeSe4 Li2BaSnSe4 LiGaSe2 LiInSe2 AgGaSe2
2.73 4.15 3.59 3.66 3.07 2.40 2.18 3.34 2.86 1.83
1 0.45 0.56 0.50 0.70 1.1 1.3 0.76 0.9 2.5
1 11 2.5 11 6.5 ∼1 ∼1 − 2 1
0.039 0.040 0.040 0.031 0.033 0.038 0.066 0.045 0.050 0.020
15a 15a, 18a 15a, 18b This work This work This work This work 15a, 18a 15a, 18c
AUTHOR INFORMATION
Corresponding Authors
*
[email protected]. *
[email protected]. ORCID
Shilie Pan: 0000-0003-4521-4507 Author Contributions †
K.W. and B.Z. contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 51402352, 51425206, and 91622107), Ten Thousand People Plan Backup Project (QN2016YX0340), and National Key Research Project (Grant Nos. 2016YFB1102302, 2016YFB0402104).
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Table 1. Property Comparison for Famous IR NLO Materials and Title Compounds Eg (eV)
ASSOCIATED CONTENT
S Supporting Information *
REFERENCES
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In summary, four new IR NLO materials, Li2BaMIVQ4, were successfully designed and synthesized by element cosubstitution with classical AgGaQ2 as templates. All of them are isostructural and exhibit compressed chalcopyrite-like structures along the c-axis compared with those of typical chalcopyrites. Note that title compounds can be viewed as the first example that isostructural structures are found in the I2−Ba−MIV−Q4 system with different MIV or Q atoms. Moreover, all of them exhibit phase-matching abilities and excellent NLO performances; especially Li2BaGeS4 and Li2BaSnS4 satisfy the critical performance demands including wide Eg, high LDTs, good dij, and suitable Δn, which indicates that they are promising candidates for IR NLO application and eliminate the inherent defects (low LDT and strong TPA) of commercial chalcopyrite-type materials. Furthermore, this work also demonstrates that designing the new materials with 14887
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