Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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AHgSnQ4 (A = Sr, Ba; Q = S, Se): A Series of Hg-Based Infrared Nonlinear-Optical Materials with Strong Second-HarmonicGeneration Response and Good Phase Matchability Yangwu Guo,†,‡,§ Fei Liang,†,§ Zhuang Li,†,‡,§ Wenhao Xing,†,‡,§ Zhe-shuai Lin,† Jiyong Yao,*,†,‡ Arthur Mar,|| and Yicheng Wu†,⊥
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†
Center for Crystal Research and Development, Key Laboratory of Functional Crystals and Laser Technology, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China ‡ Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China § University of Chinese Academy of Sciences, Beijing 100190, People’s Republic of China || Department of Chemistry, University of Alberta, Edmonton, Alberta T6G 2G2, Canada ⊥ Institute of Functional Crystal Materials, Tianjin University of Technology, Tianjin 300384, People’s Republic of China S Supporting Information *
ABSTRACT: Four Hg-based IR nonlinear-optical materials, AHgSnQ4 (A = Sr, Ba; Q = S, Se), were discovered and investigated systematically. Their structures are built of two-dimensional [HgSnQ4]2− layers, which are assembled alternately by distorted (HgQ4 and SnQ4) tetrahedra and separated by eight-coordinated A2+ cations. The two sulfides AHgSnS4 (A = Ba, Sr) exhibit large second-harmonic-generation (SHG) responses (2.8 and 1.9 × AgGaS2 at 2.09 μm), as well as large band gaps (2.77 and 2.72 eV). The two selenides AHgSnSe4 (A = Ba, Sr) show even stronger SHG responses, about 5 times that of AgGaS2. Furthermore, all four compounds show phase-matching behavior, and the results of first-principles calculation elucidate the key role of the HgQ4 group in the enhanced SHG effect in β-BaHgSnS4 and BaHgSnSe4.
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INTRODUCTION IR nonlinear-optical (NLO) materials, as irreplaceable devices for expanding the laser wavelengths by parametric frequency conversion technology, have aroused extensive research interest for their wide applications in laser technology fields.1−4 Currently, the commercial IR NLO materials AgGaS2, AgGaSe2, and ZnGeP25−7 suffer from some inherent defects such as phase mismatching, low laser damage thresholds (LDTs), and two-photon absorption (TPA) of the conventional 1−2 μm pumping source that limit their applications. In general, there are many factors involved in the search for promising IR NLO materials for practical applications, including high second-harmonic-generation (SHG) coefficients, phase matchability, wide IR transparency, large band gap, and high thermal stability.8−12 However, meeting these requirements at the same time is very difficult because they are © XXXX American Chemical Society
interrelated and even mutually restrictive; for example, the band gap usually has an inverse proportion with NLO coefficients.13 Despite the fact that a large number of new metal chalcogenides with NLO properties have been discovered, only a handful of them can satisfy the above requirements, and, among these, even fewer [BaGa4Q7, BaGa2GeQ6 (Q = S, Se), Li2In2GeSe6] could obtain large single crystals for further study.14,15 Therefore, the development of new IR NLO materials with excellent comprehensive performance is still a challenge. Metal chalcogenides remain the most promising in the IR range owing to their structural flexibility and wide transparency range. Recently, the Hg-based metal chalcogenides have Received: May 28, 2019
A
DOI: 10.1021/acs.inorgchem.9b01572 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Figure 1. Powder XRD patterns of AHgSnQ4 (A = Sr, Ba; Q = S, Se).
A−Hg−Sn−Q system (A = alkaline-earth metals; Q = chalcogen) and finally discovered four interesting IR NLO materials, AHgSnQ4 (A = Sr, Ba; Q = S, Se). It is interesting that although these compounds possess the same formula and all of them have similar 2D [HgSnQ4]2− layers assembled by distorted HgQ4 and SnQ4 tetrahedra, they adopt different space groups (BaHgSnS4 and SrHgSnS4, Ama2; BaHgSnSe4 and SrHgSnSe4, Fdd2). Note that another phase (Pnn2) of BaHgSnS4 was reported in 1980.24 Thus, according to their structural symmetries, we defined that BaHgSnS4 crystallizes in two different phases (α, Pnn2; β, Ama2). β-BaHgSnS4 and SrHgSnS4 exhibit strong SHG responses (2.8 and 1.9 × AgGaS2 at 2.09 μm), as well as large band gaps. As for BaHgSnSe4 and SrHgSnSe4, they possess even stronger SHG response (5 × AgGaS2). What is more, all of the title compounds exhibit phase-matchable behavior. The syntheses, structures, and thermal, linear, and NLO properties of these materials are reported in detail. Besides, their structure− property relationships are also analyzed by theoretical calculations.
attracted much attention because the Hg cation has various coordination types and thus one can more efficiently discover new compounds with interesting crystal structures and good NLO performance, such as K2Hg3M2S8 (M = Ge, Sn),16 Li4HgGe2S7,17 Na2Hg3M2S8,18 and KHg4Ga5Se12.19 Particularly, the [HgSe3]4− trigonal-planar compound in BaHgSe2 was reported as a new basic functional unit in IR NLO materials to obtain large SHG responses and stable chemical properties.20 Besides, the band gap could be enhanced by introducing ionic bonding between strongly electropositive cations and NLO functional anionic groups. Strong NLO effects can be produced by packing these NLO units, as demonstrated by PbGa2GeSe6 and KCd4Ga5Se12.21,22 One class of IR NLO materials that has attracted much attention recently is the AM′MQ4 (A = alkaline-earth metals; M′ = d10 elements; M = Si, Ge, Sn; Q = chalcogen) family. These compounds crystallize in several related structures as a result of the different combinations of cations and anions. Among them, the two selenides AHgGeSe4 (A = Sr, Ba), crystallizing in the noncentrosymmetric structures (Ama2), exhibit appealing overall properties including large band gap, strong SHG response, a wide transparent region, good physicochemical stability, suitable birefringence, and also the valuable congruent-melting behavior (melting points ∼ 740 °C).23 Theoretical calculations indicate that HgSe4 tetrahedra make the largest contribution to the NLO response, but GeSe4 tetrahedra also make nonnegligible contributions. When the group IVA element M goes from Si to Ge to Sn, the MQ4 tetrahedra will become more easily polarizable and hence generate a larger NLO response. Hence, in this paper, we continue our exploration in Sn-containing compounds of the
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EXPERIMENTS
Syntheses. The following raw materials were used as obtained: Ba, 99.9%; Sr, 99.9%; Sn, 99.99%; S, 99.99%; Se, 99.99%; HgS, 99%; HgSe, 99%. The binary materials BaQ, SrQ, and SnQ2 (Q = S, Se) were prepared by stoichiometric reactions of the elements at high temperature. AHgSnS4 (A = Sr, Ba). To prepare single crystals of AHgSnS4 (A = Sr, Ba), equimolar mixtures of BaS, SrS, HgS, and SnS2 were ground under an Ar atmosphere in a glovebox. Then the mixture of reagents was placed into quartz tubes and sealed under a vacuum of 10−3 Pa. The tubes were heated to 1273 K in 20 h, maintained at that B
DOI: 10.1021/acs.inorgchem.9b01572 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry temperature for 72 h, and cooled to 673 K in 200 h, and then the furnace was shut off. Orange crystals of β-BaHgSnS4 and SrHgSnS4 were found with yields of about 50%. These crystals were air- and moisture-stable. AHgSnSe4 (A = Sr, Ba). To prepare single crystals of AHgSnSe4 (A = Sr, Ba), the same procedure as that above was used except that the maximum heating temperature was 1173 K. Many red crystals were obtained, and the yields for BaHgSnSe4 and SrHgSnSe4 were 70% and 50%, respectively. The crystals were air- and moisture-stable. For structure characterization, the crystals were manually selected, and energy-dispersive X-ray analysis showed that they have compositions of Ba (or Sr) and Hg, Sn, and S (or Se) in the approximate ratio of 1:1:1:4 using a Hitachi S-4800 field-emission scanning electron microscope. For the synthesis of polycrystalline samples, equimolar mixtures of AQ, HgQ, and SnQ2 were ground, loaded into silica tubes, and flamesealed under 10−3 Pa. The samples were heated to 973 K in 12 h and kept for 100 h, and the furnace was finally turned off. By using a Bruker D8 diffractometer (Cu Kα, λ = 1.5418 Å), the powder X-ray diffraction (XRD) analyses were performed, and the patterns are shown in Figure 1. The experimental powder XRD patterns are consistent with the calculated ones. Structure Determination. Single crystals of the title compounds were selected for the single-crystal XRD analysis, which was performed on a Rigaku AFC10 diffractometer using Mo Kα radiation (λ = 0.71073 Å) at room temperature. Data collection, cell refinement, and data reduction were performed by the CrystalClear software. The program XPREP was applied for face-indexed absorption corrections.25 The structures were solved by direct methods in the noncentrosymmetric space group Ama2 (for βBaHgSnS4 and SrHgSnS4) or Fdd2 (for BaHgSnSe4 and SrHgSnSe4) and refined through full-matrix least-squares methods.26 Crystal data and structure refinement details are given in Table 1, and Tables S1
AHgSnQ4 (A = Sr, Ba; Q = S, Se) were sealed in fused-silica tubes under a high vacuum. The samples were heated and cooled at 15 K min−1. SHG Measurements. By means of the Kurtz−Perry method,27 the optical SHG responses for the title compounds were investigated by a 2090 nm Q-switched laser. Before measurement, the samples were ground to powder and sieved into a series of different particle size ranges. Microcrystals of AgGaS2 with the same size range were used as references. Theoretical Calculations. The density functional theory simulations were carried out by using the CASTEP28 software for βBaHgSnS4 and BaHgSnSe4. The generalized gradient approximation (GGA) with a common Perdew−Burke−Ernzerhof (PBE) exchangecorrelation (XC) functional was adopted, and a cutoff energy of 700 eV was utilized with 4 × 4 × 2 and 2 × 2 × 2 Monkhorst−Pack kpoint meshes in the first reciprocal Brillouin zone.29,30 The ion− electron interactions were simulated by the norm-conserving pseudopotentials, including Ba 5s25p66s2, Hg 5d106s2, Sn 5s25p2, S 3s23p4, and Se 4s24p4.31 The cell parameters and atomic positions in the unit cells of β-BaHgSnS4 and BaHgSnSe4 were fully fixed based on the experimental crystal structures. On the basis of the calculated electronic band structure, the SHG coefficients dij of the title compounds were calculated.32
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RESULTS AND DISCUSSION Crystal Structure. AHgSnS4 (A = Sr, Ba). β-BaHgSnS4 and SrHgSnS4 crystallize in the Ama2 (No. 40) space group, and they are isostructural. Herein, β-BaHgSnS4 is chosen as the representative. In its asymmetric unit, there exist one unique Ba site, one unique Hg site, one unique Sn atom, and three unique S atoms. Each Ba atom coordinates with eight S atoms to form the BaS8 bicapped-trigonal prism within a bond length of 3.2196−3.3158 Å. The coordination environment of Hg atoms is perhaps more accurately described as trigonalpyramidal (Figure 2a) instead of tetrahedral, given the contracted angles between the apical and basal Se atoms [92.67(5)−98.47(7)°]. In addition, the Sn atoms exhibit typical SnS4 tetrahedra with the Sn−S distances of 2.3405− 2.4308 Å, which are close to those of other compounds, such as α-K2Hg3Sn2S816 and Li2HgSnS4.33 The calculated bond valence sums (BVSs)34,35 are consistent with the excepted
Table 1. Crystallographic Data for AHgSnQ4 (A = Sr, Ba; Q = S, Se) fw space group a (Å) b (Å) c (Å) V (Å3) Z T (K) ρc (g cm−3) μ(Mo Kα) (mm−1) Flack parameter R(F) for Fo2 > 2σ(Fo2)a Rw(Fo2)b goodness of fit
β-BaHgSnS4
SrHgSnS4
BaHgSnSe4
SrHgSnSe4
584.86 Ama2 10.8450(7) 10.8042(6) 6.6178(4) 775.42(8) 4 293 5.010 28.93
535.14 Ama2 10.4072(13) 10.4873(10) 6.5578(6) 715.74(13) 4 293 4.966 33.33
772.46 Fdd2 22.441(5) 22.760(5) 13.579(3) 6936(2) 32 293 5.918 41.71
722.74 Fdd2 21.9027(8) 21.9059(7) 13.5010(8) 6477.8(5) 32 293 5.928 46.41
0.002(5)
0.035
0.03(1)
0.046(14)
0.021
0.027
0.065
0.082
0.047 1.04
0.057 1.02
0.163 1.13
0.223 1.06
R(F) = ∑||Fo| − |Fc||/∑|Fo| for Fo2 > 2σ(Fo2). bRw(Fo2) = [∑[w(Fo2 − Fc2)2]/∑wFo4]1/2; w−1 = [σ2(Fo2) + (Ap)2 + Bp], where p = [max(Fo2,0) + 2Fc2]/3. a
and S2 list selected interatomic distances. Full crystallographic details, including positional and displacement parameters, are available as Supporting Information. Diffuse-Reflectance Spectroscopy. Optical spectra were measured on a Cary 6000 UV−vis−NIR spectrophotometer from 250 to 2500 nm (5.0−0.5 eV) with BaSO4 used as a reference. Thermal Analysis. By using a Labsys TG-DTA16 (SETARAM) thermal analyzer, the differential scanning calorimetry (DSC) curves of these materials were measured. About 30 mg powder samples of
Figure 2. (a) Heavily distorted HgS4 tetrahedron. (b) Structure of βBaHgSnS4 viewed along the c direction. (c) View of one ∞[Hg−Sn− S] layer along the a direction. C
DOI: 10.1021/acs.inorgchem.9b01572 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
the idealized one-dimensional chains, resulting in a more complex 2D layer than that found in β-BaHgSnS4 (highlighted in the yellow rectangle in Figure S2b). Some similar structural changes have recently been reported in the I2−II−MIV−Q4 system (I = Li, Na, Cu, Ag; II = Sr, Ba; MIV = Si, Ge, Sn; Q = S, Se).38,39 Diffuse-Reflectance Spectroscopy. On the basis of the Kubelka−Munk function40 and Tauc method,41 the direct band gaps are obtained as 2.77, 2.72, 1.98, and 2.07 eV, corresponding to the absorption edges of 448, 456, 626, and 599 nm for β-BaHgSnS 4, SrHgSnS4, BaHgSnSe4, and SrHgSnSe4, respectively (Figure 4). The band gaps of β-
values (Table S7). As shown in Figure 2b,c, the crystal adopts a two-dimensional (2D) layer structure: the distorted HgS4 and SnS4 tetrahedra form [HgSnS6]6− clusters via edge sharing, with the edges aligned in the same direction. These [HgSnS6]6− clusters are further connected by sharing corners to form a 2D layer, and Ba atoms are located between the two adjacent ∞[Hg−Sn−S] layers. AHgSnSe4 (A = Sr, Ba). BaHgSnSe4 and SrHgSnSe4 belong to the orthorhombic Fdd2 (No. 43) space group. These two compounds are isostructural, so BaHgSnSe4 is chosen as the representative. BaHgSnSe4 adopts a 2D layer structure, with the Ba2+ cations located between the ∞[Hg−Sn−Se] layers (Figure 3). By sharing corners and edges, the distorted HgSe4
Figure 4. Optical reflection spectra of AHgSnQ4 (A = Sr, Ba; Q = S, Se). Figure 3. (a) Structure of the [HgSnSe6] dimer. (b) View of one ∞ [Hg−Sn−Se] layer along the b direction. (c) Structure of BaHgSnSe4 viewed along the c direction.
BaHgSnS4 and SrHgSnS4 are larger than that of AgGaS2 and help to avoid TPA of the 1 μm (Nd:YAG) laser pumping (2.33 eV, 532 nm). It is generally accepted that a large band gap can lead to a high LDT; thus, it may possess a higher LDT than AgGaS2. The band gaps for BaHgSnSe4 (1.98 eV, 626 nm) and SrHgSnSe4 (2.07 eV, 599 nm) are not optimal for highefficiency 1 μm pumping, but they are larger than that of ZnGeP2 (1.7 eV), which is currently the best material for 2 μm pumping in mid-IR lasers. Given their larger band gaps, BaHgSnSe4 and SrHgSnSe4 could be suitable candidates for 2 μm pumping. Thermal Stability. The DSC patterns for the title compounds are shown in Figure 5. During the entire cycle, β-BaHgSnS4 has two different endothermic peaks (720 and 808 °C) and one exothermic peak (622 °C), which indicated that β-BaHgSnS4 has high thermal stability. Considering that BaHgSnS4 has two phases (α, Pnn2; β, Ama2), there may be phase transitions during the process. 16 SrHgSnS4 and SrHgSnSe4 are stable up to 710 and 694 °C, respectively, and exhibit incongruent melting behavior. Interestingly, BaHgSnSe4 melts at 712 °C and recrystallizes at 675 °C, showing congruent melting behavior. The differences in the thermal properties may be due to different cations (Ba atom is much heavier than Sr atom).42,43 The bulk BaHgSnSe4 single crystal could be grown by the Bridgman−Stockbarger method, which is critical for further evaluation of the IR NLO crystal. Besides, compared with the melting point of commercial IR NLO materials (ZnGeP2, 1025 °C; AgGaS2, 998 °C; AgGaSe2, 860 °C),44 the relatively low temperature during the crystal
and SnSe4 tetrahedra are connected to build the infinite ∞ [Hg−Sn−Se] layers and stacked along the b axis. The Hg and Sn atoms are all tetrahedrally coordinated, with the Hg−Se bonds ranging from 2.547 to 2.785 Å in good agreement with those in BaHgSe2 (2.467−2.642 Å) and the Sn−Se distances of 2.493−2.567 Å close to those in K2ZnSn3Se8 (2.468−2.584 Å).36 The Ba atoms are linked by eight Se atoms to BaSe8 polyhedra with Ba−Se distances of 3.309−3.435 Å, which are comparable to those in BaCdSnSe4 (3.317−3.392 Å).37 The key differences among AHgSnS4 (A = Sr, Ba), AHgSnSe4 (A = Sr, Ba), and other AM′MQ4 compounds are clarified through a Bärnighausen tree, which reveals group− subgroup relationships in these structures involving klassengleiche reductions in symmetry (i.e., within the same crystal class; Figure S1). The BaZnSiSe4-type structure (Ama2) can be considered to be the aristotype in which the Zn and Si atoms are almost collinear, similar to those found in SiS2 and KFeS2. In β-BaHgSnS4, the Hg atoms move to one side and are connected with chalcogen atoms of adjacent chains (Figure S2a), leading to a 2D layer. The α-BaHgSnS4-type structure (Pnn2) is a transitional one in which the mirror symmetry is lost, although the displacement of atoms from this plane is very slight (
