Midinfrared Nonlinear Optical Thiophosphates from LiZnPS4 to

Mar 25, 2016 - Beijing Center for Crystal R&D, Key Lab of Functional Crystals and Laser Technology of Chinese Academy of Sciences, Technical Institute...
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Midinfrared Nonlinear Optical Thiophosphates from LiZnPS4 to AgZnPS4: A Combined Experimental and Theoretical Study Molin Zhou,†,‡,§ Lei Kang,†,‡,§ Jiyong Yao,*,† Zheshuai Lin,*,† Yicheng Wu,† and Chuangtian Chen† †

Beijing Center for Crystal R&D, Key Lab of Functional Crystals and Laser Technology of Chinese Academy of Sciences, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, Peoples’ Republic of China ‡ University of Chinese Academy of Sciences, Beijing 100190, Peoples’ Republic of China S Supporting Information *

for the mid-IR generation. Among them, due to the parallel polarized and anisotropic covalent framework, LiZnPS429 achieved a superior balance for the good mid-IR NLO performances with a wide energy band gap, large second harmonic generation (SHG) response, and moderate birefringence; thus it was considered as the most outstanding mid-IR NLO candidate. In order to obtain its large single crystal, we have made lots of efforts in the synthesis and growth. However, LiZnPS4 exhibited pungent smell and corrosion to the silica tubes, which were unfavorable for the experimental conditions. In addition, the LiZnPS4 compound was unstable in air, exhibiting heavy deliquescent properties. Moreover, LiZnPS4 displayed incongruent-melting behavior, which was the fatal flaw for the growth of a crystal. Since these defects are probably caused by the elemental properties of lithium, the effective cationic substitution from Li+ to other ions (e.g., Na+, Ag+, etc.) is a way to overcome the problems. With this purpose, we searched in the inorganic crystal structure database (ICSD, 2015-2, version 1.9.7, by Fachinformatiionszentrum) and found only a silver analogue of LiZnPS4, namely AgZnPS4.30 This compound has a different framework structure and might possess a better growth property compared with LiZnPS4. The crystal structures of LiZnPS4 (SG: I4)̅ and AgZnPS4 (SG: Pna21) are shown in Figure 1a and b, respectively. It is

ABSTRACT: Our earlier theoretical calculation and preliminary experiment highlighted LiZnPS4 as a good mid-infrared (mid-IR) nonlinear optical (NLO) material. However, this compound suffers from problems including corrosion of the silica tubes, a pungent smell, deliquescence, and incongruent-melting behavior in the further single crystal growth and applications. In order to overcome these problems, herein, we investigate the analogues of LiZnPS4 and propose that AgZnPS4 would be a good candidate. The combination of experimental and theoretical study demonstrates that AgZnPS4 exhibits a much stronger NLO effect than that of LiZnPS4 despite the relatively smaller energy band gap. More importantly, AgZnPS4 melts congruently with a melting point as low as 534 °C, much lower than those of traditional IR NLO crystals, and is nondeliquescent with enough stability in the air. Such a good crystal growth habit and chemical stability enable AgZnPS4 to possess much better overall performance for the practical mid-IR NLO applications. High-power tunable mid-IR lasers in the range of 3−20 μm have important applications in many civil and military fields, such as atmospheric monitoring, minimally invasive surgery, laser guidance, and laser intrusion.1−3 Frequency conversion with NLO crystals is a common way to convert existing laser sources to mid-IR wavelengths. Over past decades, chalcopyrite-type AgGaQ2 (Q = S, Se),4,5 ZnGeP2,6 and CdSiP27 crystals have been practically used as the benchmark IR NLO materials due to their wide IR transparency and large NLO responses. Nevertheless, low laser damage thresholds and strong twophoton absorption severely limit their applications in the midIR region. Therefore, searching for new mid-IR NLO crystals with good comprehensive performances is urgently demanded, though many attempts have been made in enriching the source of mid-IR NLO materials.8−27 In general, outstanding mid-IR NLO crystals should possess a number of desirable optical performances, such as a large NLO coefficient, wide energy band gap, wide IR transparency, and good IR phase-matchability. In addition to the above requirements, however, it is noteworthy that a good crystal growth habit and good chemical stability may be the prerequisites for mid-IR NLO crystals to become practically usable. In the previous work,28 metal thiophosphates were highlighted as promising NLO crystals with desirable properties © XXXX American Chemical Society

Figure 1. Crystal structures of LiZnPS4 (a) and AgZnPS4 (b). The blue, red, and turquoise polyhedra represent the (PS4)3−, (ZnS4)6−, and (LiS4)7− or (AgS4)7− blocks, respectively.

found that for LiZnPS4 (see Figure 1a) the (PS4)3− tetrahedra are arranged in the cubic closet packing, while Li+ and Zn2+ cations are residing in the tetrahedral voids with an occupancy of 100%. Each (LiS4)7− tetrahedron connects to four (ZnS4)6− tetrahedra by corner-sharing to form a (LiZn4S16)23− layer. Similarly, Each (ZnS4)6− tetrahedron shares corners with four (LiS4)7− tetrahedra, forming a (Li4ZnS16)26− layer adjacent to Received: March 1, 2016

A

DOI: 10.1021/acs.inorgchem.6b00517 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

first-principles calculations on the electronic structures and NLO properties are performed using the plane-wave pseudopotential method implemented in the CASTEP package based on density functional theory (DFT).31 The calculated SHG coefficients are d15 = 10.10 pm/V, d24 = 9.19 pm/V, d33 = −17.04 pm/V, and about 1.3 times the d36 of AgGaS2, which are in a good agreement with the experimental measurements. In addition, the optical birefringence of AgZnPS4 was calculated as about 0.051 (0.073 for LiZnPS4, as a comparison) at 2.09 μm. Such a value is comparable to that of AgGaS2 (∼0.053), indicating that AgZnPS4 is PM for the SHG in the IR region. The above results demonstrate that AgZnPS4 is an interesting mid-IR NLO material. On the other hand, the DSC curves of LiZnPS4 and AgZnPS4 indicate their respective incongruent-melting and congruentmelting behavior (Figure 3). Note that the congruent-melting

the (LiZn4S16)23− layer. Note that all P−S, Zn−S, and Li−S bonds are identical with the same lengths: 2.043(1) Å, 2.337(1) Å, 2.408(1) Å, respectively. In addition, all of the anisotropic covalent anionic groups (i.e., (PS4)3−, (ZnS4)6−, (LiS4)7−) are in parallel arrangements in LiZnPS4, which is mainly responsible for its considerable SHG effect (0.8 × AgGaS2). As for the target AgZnPS4 compound (see Figure 1b), the substitution of cations leads to a novel structure that is greatly different from its matrix LiZnPS4. All anionic groups in AgZnPS4, namely, (PS4)3−, (ZnS4)6−, and (AgS4)7−, are connected with each other via corner-sharing with a molar ratio of 1:1:1 to generate a three-dimensional framework. The (PS4)3− units are almost regular tetrahedra with little variations of the P−S bonds ranging from 2.037(2) Å to 2.058(3) Å. Meanwhile, each Zn atom is coordinated to a slightly distorted tetrahedron of four S atoms. The Zn−S distances range from 2.337(2) Å to 2.371(2) Å, which are only slightly increased compared with those of LiZnPS4. However, the Ag atom is bound to four S atoms with Ag−S bonds ranging from 2.535(2) Å to 2.645(2) Å, forming a severely deformed tetrahedron with large polarization coefficient. Hence, AgZnPS4 exhibits the consistent parallel-polarized covalent framework, and likely its SHG response is much larger than that of LiZnPS4 due to the great contribution of the coparallel-aligned highly polarizable (AgS4)7− blocks. In addition, it is noteworthy that the anionic groups in AgZnPS4 accumulate more densely than those in LiZnPS4. Accordingly, there are less cavities to absorb the water molecules, which would lead to nondeliquescent properties in the air. Furthermore, the experimental and theoretical results are listed to confirm our analysis of the structure−property relationship (the related methods are summarized in the Supporting Information). The diffuse reflectance spectrum (see Figure 2a) shows that the absorption edge of AgZnPS4 is at

Figure 3. Differential Scanning Calorimetry (DSC) curves of LiZnPS4 (a) and AgZnPS4 (b).

property of AgZnPS4 is necessary for the growth of bulk crystals via the Bridman−Stockbarger technique. Moreover, the melting temperature point is 534 °C, much lower than those of traditional IR NLO crystals (e.g., 860 °C for AgGaSe2, 915 °C for LiGaSe2, 998 °C for AgGaS2, 1025 °C for ZnGeP2). The rather low temperature is a very important advantage that would be favorable for the crystal growth by the Bridman− Stockbarger technique. In contrast, LiZnPS4 exhibits the incongruent-melting behavior; its bulk crystal would be difficult to obtain. Compared with LiZnPS4, AgZnPS4 is readily synthesized; it is also nondeliquescent with enough stability in the air. These advantages could be beneficial to its practical application. Note that the single crystal of AgZnPS4 has been grown (see Figure 2c) with a size of about 0.5 mm. Besides, the calculated partial densities of states (PDOS) are plotted in Figure 4. LiZnPS4 and AgZnPS4 exhibit similar electronic structures except for the orbitals of Li and Ag. Clearly, the d orbitals of Ag atoms contribute to the top of valence band and overlap with the p orbitals of S atoms, indicating the production of the bond between Ag and S. As shown, the Ag−S interaction

Figure 2. Diffuse reflectance spectrum (a) and oscilloscope traces of SHG signals with AgGaS2 as a reference at a particle size of 105−150 μm for LiZnPS4 and AgZnPS4 (b), crystal morphology of AgZnPS4 by Scanning Electron Microscope (SEM) (c), and phase-matching curve (i.e., SHG response vesus particle size) for AgZnPS4 (d).

about 450 nm, corresponding to an energy band gap of about 2.76 eV, which is consistent with its yellow color (Figure S2). The value is smaller than that of LiZnPS4 (∼3.38 eV) and comparable to that of AgGaS2 (∼2.64 eV). Additionally, AgZnPS4 is phase-matchable (PM; see Figure 2d) and exhibits good SHG efficiency of about 1.8 times of that of AgGaS2 with a similar particle size (105−150 μm) with the 2.09 μm laser as the fundamental wavelength (see Figure 2b), which is much larger than that of LiZnPS4 (∼0.8 × AgGaS2). Moreover, the

Figure 4. PDOS of LiZnPS4 (a) and AgZnPS4 (b). B

DOI: 10.1021/acs.inorgchem.6b00517 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

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makes AgZnPS4 more chemically stable and more suitable for growth than LiZnPS4. In summary, AgZnPS4 has been investigated by combined experimental and theoretical methods with the purpose of overcoming the disadvantages of LiZnPS 4 , which was previously reported to exhibit outstanding mid-IR NLO performance. The experimental results demonstrate that AgZnPS4 possesses smaller energy band gap but much stronger SHG effect than those of LiZnPS4 (2.76 eV and 1.8 × AgGaS2 for AgZnPS4, 3.38 eV and 0.8 × AgGaS2 for LiZnPS4). In addition, the calculated birefringence (∼0.051) indicates that the AgZnPS4 crystal has a good mid-IR phase-matchability, which has also been elucidated by the experimental measurement. More importantly, AgZnPS4 melts congruently at a relatively low temperature of 534 °C, indicating that the bulk crystal can be grown by the Bridgman−Stockbarger method. In fact, the AgZnPS4 single crystal obtained exhibits good physicochemical stability in the air. The above results demonstrate that AgZnPS4 is an IR NLO material worthy of further investigation. In order to fully evaluate its overall performances, efforts to grow larger high-quality AgZnPS4 single crystals are in progress.

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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.6b00517. Experimental methods for the syntheses and property characterization of LiZnPS4 and AgZnPS4, experimental powder XRD patterns of LiZnPS4 and AgZnPS4, polycrystalline powder morphology of AgZnPS4, Energy Dispersive Spectrometer (EDS) image of AgZnPS4 crystal, and computational methods (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions §

These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the China “863” project (No. 2015AA034203) and the National Natural Science Foundation of China (No. 51132008 and 21271178).

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REFERENCES

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DOI: 10.1021/acs.inorgchem.6b00517 Inorg. Chem. XXXX, XXX, XXX−XXX