Synthesis, Crystal Structure, and Optical Properties of

(41) Diamond-like Li2CdGeS4 has SHG responses that approximate 70 × α-quartz.(34) Na2ZnGe2S6 exhibits good comprehensive properties (NLO coefficient...
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Synthesis, Crystal Structure, and Optical Properties of Noncentrosymmetric Na2ZnSnS4 Jianqiao He,†,‡ Yangwu Guo,‡,§ Wenjuan Huang,∥ Xian Zhang,*,⊥ Jiyong Yao,*,§ Tianyou Zhai,*,∥ and Fuqiang Huang*,†,⊥

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State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, P. R. China ‡ University of Chinese Academy of Sciences, 19 Yuquan Road, Beijing 100049, P. R. China § Center for Crystal Research and Development, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China ∥ State Key Laboratory of Material Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology (HUST), Wuhan 430074, P. R. China ⊥ Beijing National Laboratory for Molecular Sciences and State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, 202 Chengfu Road, Beijing 100871, P. R. China ABSTRACT: A new chalcogenide Na2ZnSnS4 has been successfully synthesized by using Na2S2 as reactive flux. Na2ZnSnS4 crystallizes in the tetragonal system with space group of I4̅. Its cell parameters are a = 6.4835(6) Å and c = 9.134(1) Å. The structure is a derivative of AgGaS2, in which the Ag+ ions are replaced by Na+ ions and the Ga3+ ions are replaced by Zn2+ and Sn4+ ions. All three cations are in seriously distorted tetrahedral geometry with a distortion factor (η = c/a) of 1.4. Optical measurements show that the Na2ZnSnS4 powder sample has a large transparent range from 0.8 to 25 μm and a wide band gap of 3.1 eV. It exhibits large secondharmonic generation intensity of 0.9 × AgGaS2 in the grain size range from 41 to 74 μm. First-principles calculation results reveal that the valence band maximum and conduction band minimum are mainly composed of S 3p, Zn 3d orbitals and Sn 5s, S 3p orbitals, respectively.

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INTRODUCTION

works have been carried out to explore new IR NLO crystals with balanced NLO coefficients and LDTs. To design new IR NLO crystals, one fruitful strategy is to use intrinsic noncentrosymmetric moieties of group IVA elements.25−27 For example, PbGa2GeSe6, constructed of [PbSe4], [GaSe4], and [Ga/GeSe4] tetrahedra, shows extraordinary strong SHG responses (12 × AgGaS2).25 The inclusion of strong ionic moieties can increase the diversity of structures with NLO properties.28−33 For example, Ba8Sn4S15 and Ba7Sn5S15, containing [SnS3] pyramid or [Sn2S3] trigonal bipyramid, show strong SHG responses (10 × AgGaS2).28,29 LiGaGe2S6 manifests a large band gap and a strong NLO response.32 In particular, many IR NLO materials have been found within a combination of A/IIB/IVA/Q (A = alkali metal, Q = chalcogen).34−46 β-K2Hg3Ge2S8 exhibits a large NLO coefficient deff approaching 20 pm/V and a high LDT.41 Diamond-like Li2CdGeS4 has SHG responses that approximate 70 × α-quartz.34 Na2ZnGe2S6 exhibits good comprehensive properties (NLO coefficient: 30 × KDP; LDT: 6 × AgGaS2).45 Na2CdGe2S6 has a large NLO coefficient of 0.8

Lasers with different frequencies are crucial to daily life as they play important roles in communication,1,2 navigation,3,4 and environmental monitoring. 5,6 Nonlinear optical (NLO) materials can alter the frequencies of lasers through NLO effects such as second-harmonic generation (SHG) and optical parametric oscillation (OPO).7−11 Moreover, NLO materials are suitable for compact and portable solid-state laser sources.12 Ideal NLO materials must meet several criteria: large NLO coefficient, phase match ability, large transparent range, high laser damage threshold (LDT), high thermal conductivity, machinability, and ease to obtain large crystals.7,13−15 In the visible to ultraviolet light region, phosphates (KH2PO4 (KDP), KTiOPO4 (KTP)) and borates (BaB 2 O 4 (BBO), LiB 3 O 5 (LBO)) exhibit outstanding comprehensive properties.1617−19 In the infrared (IR) light region, metal chalcogenides are more often used for their large IR transparent ranges.20 Among them, AgGaQ2 (Q = S, Se) and ZnGeP2 have been successfully commercialized.21−24 They possess the chalcopyrite-type crystal structure and have large NLO coefficients. However, these materials are not suitable for high-power applications for their small LDTs.22 Many research © XXXX American Chemical Society

Received: April 14, 2018

A

DOI: 10.1021/acs.inorgchem.8b01025 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry × AgGaS2 and a high LDT of 4 × AgGaS2.44 Therefore, exploring new A/IIB/IVA/Q compounds may lead to NLO materials. The combination of Na/Zn/Sn/S contains only costeffective and nontoxic elements. New cost-effective, environmentally friendly, and scalable IR NLO materials may be discovered within this combination. Recently, a new compound Na2ZnSn2S6 (space group Fdd2) has been synthesized.42,47 Na2ZnSn2S6 is reported to have strong SHG effect (4 × AgGaS2) and high LDT (2 × AgGaS2),47 but the compound cannot realize type-I phase match. Therefore, it is meaningful to explore other compounds within this family. Here we report the synthesis of a new chalcogenide Na2ZnSnS4 using reactive flux method. It crystallizes in a distorted chalcopyrite-like structure. Optical measurements show that the Na2ZnSnS4 powder has a large transparent range (0.8−25 μm) and a wide band gap of 3.1 eV. In addition, evident SHG response (0.9 × AgGaS2, 41−74 μm) was observed in Na2ZnSnS4 powder samples.

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Table 1. Crystallographic Data and Details of the Structure Refinement for Na2ZnSnS4 chemical formula −1

Mr (g·mol ) crystal system, space group temperature (K) a, c (Å) V (Å3) Z radiation type μ (mm−1) crystal size (mm) diffractometer absorption correction Tmin, Tmax no. of measured, independent and observed [I > 2σ(I)] reflections Rint (sin θ/λ)max (Å−1) R[F2 > 2σ(F2)], wR(F2), S no. of reflections no. of parameters Δρmax, Δρmin (e Å−3)

EXPERIMENTAL SECTION

Synthesis of Na2ZnSnS4 Single Crystals. Na2S2 powder were prepared in advance using the liquid ammonia method. Single crystals of Na2ZnSnS4 were obtained using the reactive flux method. The starting materials of Na2S2 (4 mmol, 0.440 g), Zn (1 mmol, 0.065 g), Sn (1 mmol, 0.119 g), and S (2 mmol, 0.064 g) were mixed uniformly and pressed into pallets, followed by flame sealing in silica tubes under vacuum. The tubes were slowly heated to 600 °C in a programmed furnace and held for 50 h. Afterward, the tubes were cooled to 300 °C over a period of 17 h and finally to room temperature by turning off the furnace. The products were washed and sonicated with N,Ndimethyl-formamide, distilled water, and acetone in sequence for several times to remove extra Na2S2 and Na2SnS3 impurities. The obtained crystals were dried in vacuum oven at 60 °C for 1 h. Lightyellow crystals were then obtained, and the yield based on Sn is ∼60%. Powder samples were obtained through a simple solid-state reaction method. The starting materials were stoichiometry Na2S2, Zn, Sn, and S, which were repeatedly ground and calcined at 600 °C for 20 h twice. The yield of the powder sample is ∼90% based on Sn. Single-Crystal X-ray Crystallography. A Bruker D8 Quest diffractometer equipped with graphite-monochromated Mo Kα radiation and a photon detector was used to carry out single-crystal X-ray diffraction measurements at 298 K. The APEX 3 program was used to reduce the data and solve and refine the structure (data reduction: SAINT V8.34A (Bruker AXS, 2013); structure solution: SHELXT 2014/4 (Sheldrick, 2014); structure refinement: SHELXL2016/6 (Sheldrick, 2016)). The crystal data and the structure refinement details are summarized in Table 1. Purity and Composition. A JEOL scanning electron microscope (SEM) was used to observe the morphology of the crystals. An Oxford energy-dispersive X-ray spectrometer (EDX) embedded in the SEM was used to analyze the composition of the samples. Powder Xray diffraction (PXRD) measurements were carried out on a Bruker D8 Focus X-ray diffractometer equipped with graphite-monochromated Cu Kα radiation. Optical Characterizations. UV−vis diffuse reflection spectra of the powder samples were measured using a Hitachi UV-4100 spectrophotometer. BaSO4 powder was chosen to be the 100% reflectance standard. The powder samples were covered with a quartz plate during the tests. Infrared transmission spectra were carried out on a Shimadzu FTIR-8400 spectrophotometer in the range from 4000 to 400 cm−1. Na2ZnSnS4 powder was mixed with KBr with an expected ratio of 1:100. The mixture was ground and pressed into a transparent plate for the tests. Second-Harmonic Generation Measurements. Single-crystal SHG measurements were conducted with an alpha 300 RS + Raman spectrometer. A 800 nm femtosecond laser was introduced into the

Na2ZnSnS4 358.28 tetragonal, I4̅ 298 6.4835 (6), 9.134 (1) 383.94 (8) 2 Mo Kα 7.47 0.06 × 0.03 × 0.01 Bruker D8 QUEST Multi-Scan (SAINT V8.34A, Bruker AXS, Inc., 2013) 0.75, 0.93 3351, 451, 419 0.056 0.647 0.024, 0.027, 1.0 451 19 0.39, −0.49

spectrometer as an excitation source. The laser beam was focused on Na2ZnSnS4 single crystals by a 50× objective, forming a spot with a diameter of ∼1.8 μm. The excitation laser was generated by a modelocked Ti:sapphire with pulse duration of 140 fs and repetition rate of 80 MHz and filtered into optical parametric oscillator (Chameleon Compact OPO-Vis). The SHG effect of Na2ZnSnS4 powder was tested using the Kurtz−Perry method. The laser resource was a Qswitched Ho/Tm/Cr/YAG laser (1 Hz, 50 ns) with a fundamental light of 2090 nm. The particle size of the powder samples ranges from 20 to 200 μm. Microcrystalline AgGaS2 of similar particle size served as a reference. Electronic Band Structure Calculation. The first-principles calculations were performed using the Vienna Ab-initio Simulation Package (VASP). The Perdew−Burke−Ernzerhof (PBE) version of the generalized gradient approximation (GGA)48 was used to describe the exchange correlation functional, and the projector augmented wave (PAW)49 method was used in the present work. Here the cutoff energy of plane wave was chosen at 350 eV. For the structure optimizations, 6 × 6 × 6 Monkhorst−Pack grids were used for the primitive cell and 4 × 4 × 4 k-points were used for the conventional cell, respectively. The relaxation of geometry optimization was performed until the total energy change was within 10−6 eV/atom and the Hellmann−Feynman force on all atomic sites was 3.8 eV).

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ASSOCIATED CONTENT

Accession Codes

CCDC 1836660 contains 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: *E-mail: *E-mail: *E-mail:

[email protected] (X.Z.). [email protected] (T.Z.). [email protected] (J.Y.). [email protected] (F.H.).

ORCID

Yangwu Guo: 0000-0002-2365-5873 Jiyong Yao: 0000-0002-4802-5093 Tianyou Zhai: 0000-0003-0985-4806 Fuqiang Huang: 0000-0001-7727-0488 E

DOI: 10.1021/acs.inorgchem.8b01025 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research project was financially supported by the National Key Research and Development Program (Grant 2016YFB0901600), CAS Center for Excellence in Superconducting Electronics, the Key Research Program of Chinese Academy of Sciences (Grants QYZDJ-SSW-JSC013 and KGZD-EW-T06), and Science and Technology Commission of Shanghai (Grants 16JC1401700 and 16ZR1440500).

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DOI: 10.1021/acs.inorgchem.8b01025 Inorg. Chem. XXXX, XXX, XXX−XXX