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SrCdGeS and SrCdGeSe: Promising Infrared Nonlinear Optical Materials with Congruent-Melting Behaviour Yunwei Dou, Ying Chen, Zhuang Li, Abishek K. Iyer, Bin Kang, Wenlong Yin, Jiyong Yao, and Arthur Mar Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01649 • Publication Date (Web): 07 Jan 2019 Downloaded from http://pubs.acs.org on January 9, 2019

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Crystal Growth & Design

1

SrCdGeS4 and SrCdGeSe4: Promising Infrared Nonlinear Optical Materials with Congruent-Melting Behaviour

Yunwei Dou,† Ying Chen,† Zhuang Li,‡ Abishek K. Iyer,§ Bin Kang,† Wenlong Yin,†,§,* Jiyong Yao,‡ and Arthur Mar§,* †

Institute of Chemical Materials, China Academy of Engineering Physics, Mianyang 621900

People’s Republic of China ‡

Centre for Crystal Research and Development, Technical Institute of Physics and Chemistry,

Chinese Academy of Sciences, Beijing 100190, People’s Republic of China §

Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2

ACS Paragon Plus Environment

Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2 Abstract The quaternary chalcogenides SrCdGeS4 and SrCdGeSe4 were prepared by hightemperature reactions in the form of single crystals (at 1223 and 1123 K, respectively) or microcrystalline samples (at 1023 K). They adopt noncentrosymmetric structures (orthorhombic, space group Ama2, Z = 4; a = 10.3352(14) Å, b = 10.2335(14) Å, c = 6.4408(9) Å for SrCdGeS4; a = 10.8245(8) Å, b = 10.6912(8) Å, c = 6.6792(5) Å for SrCdGeSe4) consisting of anionic [CdGeCh4]2– layers separated by Sr2+ cations. Although the structure is closely related to the BaZnSiSe4-type, the substitution of Cd for Zn is accompanied by a significant displacement away from an ideal tetrahedral site toward a trigonal pyramidal geometry. Relationships to other similar structures are also elucidated. Large optical band gaps of 2.6 eV for SrCdGeS4 and 1.9 eV for SrCdGeSe4 were deduced from the absorption edges, consistent with the light yellow and red colours, respectively, of crystals of these compounds. Nonlinear optical measurements on powder samples indicated that these compounds show intense second harmonic generation signals (2 AgGaS2 for SrCdGeS4 and 5 AgGaS2 for SrCdGeSe4) and type-I phase-matching behaviour using 2090 nm as the fundamental laser wavelength. They also melt congruently at relatively low temperatures (958 ºC for SrCdGeS4 and 851 ºC for SrCdGeSe4).

Keywords: Chalcogenides; Infrared nonlinear optical materials; Crystal structure; Electronic structure; Optical properties


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3 Introduction Infrared lasers are crucial for various chemical, medical, industrial, and military applications, but the commercially available nonlinear optical (NLO) materials that can generate coherent and tunable light in the infrared region are currently limited to a few benchmark materials (AgGaS2, AgGaSe2, ZnGeP2), which suffer from limitations of low laser damage threshold and non-type-I phase matchability at desired wavelengths.1 Given their more covalent bonding character, smaller band gaps, and greater polarizability relative to oxides, metal sulfides and selenides inherently satisfy many of the key criteria for a good IR NLO material, namely large second harmonic generation (SHG) coefficients, high IR transmittance, and wide transparency range.2–5 Other practical requirements (high laser damage threshold, type-I phase matchability) are more difficult to anticipate, and the need to easily grow large single crystals favours compounds that melt congruently. Finally, despite many design strategies to incorporate asymmetric building blocks, there is never any guarantee that they will align appropriately in the crystal structure, although initial attempts in applying machine learning approaches to discover noncentrosymmetric structures appear enticing.6 Systematic investigation of the remaining vast uncharted territory of metal chalcogenides is still essential. Among the many metal chalcogenides that have been explored so far as candidates for IR NLO materials, one of the largest and most promising families contains tetrahedral MCh4 groups centred by main-group (typically group-13 or 14 elements) or d10 transition metal cations (Cu and Zn triad elements), as found in the benchmark materials AgGaS2 and AgGaSe2 themselves. They include ternary compounds such as Cd4GeS6,7 although quaternary compounds in combination with other monovalent or divalent cations are even more numerous, such as



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4 Na2ZnGe2S6,8 Pb5Ga6ZnS15,9 and Ba5CdGa6Se15.10 This brief listing is not exhaustive and many others can be found in reviews.3,4,11 In pursuit of these quaternary chalcogenides, we have been investigating the Ba–M–Tt– Ch (M = Zn, Cd, Hg; Tt = Si, Ge, Sn; Ch = S, Se) system within which the noncentrosymmetric compounds BaZnSiSe4 and BaZnGeSe4 were recently identified.12 Both of these compounds show good attributes for an IR NLO material, with BaZnGeSe4 exhibiting an SHG response as strong as that of AgGaS2 over similar particle sizes. We hypothesize that substitution of Cd for Zn would increase bond polarizability and may improve the SHG response, but at the expense of decreasing the band gap. However, substitution of Sr for Ba may offset this effect and maintain an appropriate band gap to ensure acceptable laser damage thresholds. We report here the synthesis and characterization of the quaternary chalcogenides SrCdGeS4 and SrCdGeSe4, and evaluate their potential as IR NLO materials.

Experimental Section Synthesis. Starting materials were Sr pieces (99%, Sigma-Aldrich), Cd shot (99.95%, Alfa-Aesar), CdSe powder (99.99%, Sinopharm), Ge ingot (99.9999%, Alfa-Aesar), S flakes (99.998%, Sigma-Aldrich), and Se powder (99.99%, Sigma-Aldrich). Single crystals were grown from reactions involving slow cooling from the melt. For SrCdGeS4, a mixture of Sr (61 mg, 0.70 mmol), Cd (79 mg, 0.70 mmol), Ge (51 mg, 0.70 mmol), and S (90 mg, 2.80 mmol) in a molar ratio of 1:1:1:4 was loaded into a fused-silica tube, which was evacuated and sealed. The tube was heated to 1223 K over 30 h, kept at that temperature for 96 h, and then slowly cooled to room temperature over 144 h. Light yellow (almost colourless) air-stable crystals were obtained. For SrCdGeSe4, a mixture of Sr (44 mg,


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5 0.50 mmol), CdSe (96 mg, 0.50 mmol), Ge (36 mg, 0.50 mmol), and Se (118 mg, 1.50 mmol) in a molar ratio of 1:1:1:3 was loaded into a fused-silica tube which was evacuated and sealed. The tube was heated to 1123 K over 30 h, kept at that temperature for 72 h, and then slowly cooled to room temperature over 96 h. Red air-stable crystals were obtained. Energy-dispersive X-ray (EDX) analysis of these crystals, which were examined on a JEOL JSM-6010LA InTouchScope scanning electron microscope, revealed compositions of 15(2)% Sr, 15(2)% Cd, 10(2)% Ge, and 60(4)% Ch, close to expectations (14% Sr, 14% Cd, 14% Ge, 57% Ch (Ch = S, Se)). These crystals were chosen for structure determination. Crystals of both compounds remained visibly unchanged after exposure to air over several months. Polycrystalline samples were prepared from reactions of the same mixtures of starting materials as before. The tubes were heated to 1023 K in 24 h, kept at that temperature for 96 h, and then cooled by shutting off the furnace. The samples were reground and subjected to a repeated heat treatment. Powder X-ray diffraction (XRD) patterns were collected on an Inel diffractometer equipped with a curved position-sensitive detector (CPS 120) and a Cu K1 radiation source operated at 40 kV and 20 mA. These patterns agreed well with the ones simulated from the single-crystal structure determination (Figure 1). The initial discovery of these compounds involved the use of elemental components for the synthesis of SrCdGeS4 and a binary CdSe precusor for the synthesis of SrCdGeSe4. However, subsequent experiments in optimization revealed that both compounds can be prepared easily through either route, involving other combinations of elemental components or binary chalcogenides as starting materials, provided that the correct stoichiometric quantities are used. Structure Determination. Intensity data for SrCdGeS4 and SrCdGeSe4 were collected at room temperature on a Bruker PLATFORM diffractometer equipped with a SMART APEX II



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6 CCD detector and a graphite-monochromated Mo K radiation source, using  scans at 6 (for SrCdGeSe4) or 8 (for SrCdGeS4) different  angles with a frame width of 0.3º and an exposure time of 20 s per frame. Face-indexed numerical absorption corrections were applied. Structure solution and refinement were carried out with use of the SHELXTL (version 6.12) program package.13 The noncentrosymmetric orthorhombic space group Ama2 was chosen over other possibilities (A21am, Amam) on the basis of Laue symmetry, systematic absences, and intensity statistics, as well as the close similarity of the cell parameters to those of the chemically related compounds BaZnSiSe4 and BaZnGeSe4.12 Initial locations of all atoms were found by direct methods.

Atomic positions and labels were standardized with the program STRUCTURE

TIDY.14 The structure refinements for both compounds were straightforward. The Cd and Ge atoms were clearly distinguishable by their disparate distances to surrounding chalcogen atoms. The ordered distribution of these atoms was confirmed by refinements of occupancies in models that presumed site mixing. For example, these occupancies converged to 0.97(1) Cd / 0.03(1) Ge in the site at ¼, 0.17, 0.55 and –0.02(1) Cd / 1.02(1) Ge in the site at ¼, 0.28, 0.01 in SrCdGeS4, and similar results were obtained for SrCdGeSe4.

The absolute configurations in these

noncentrosymmetric structures were established by refining a Flack parameter. No additional symmetry was revealed by the ADDSYM routine in the PLATON suite of programs.15 Table 1 lists crystal data and further details, Table 2 lists positional and equivalent isotropic displacement parameters, and Table 3 lists selected interatomic distances. Diffuse Reflectance Spectroscopy. Optical diffuse reflectance spectra for SrCdGeS4 and SrCdGeSe4 were measured from 350 nm (3.54 eV) to 2000 nm (0.62 eV) on a Cary 5000 UV-vis-NIR spectrophotometer equipped with a diffuse reflectance accessory, with a compacted pellet of BaSO4 used as a 100% reflectance standard. These reflectance spectra were converted


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7 to optical absorption spectra using the Kubelka-Munk function, F(R) = /S = (1–R)2/2R, where  is the Kubelka–Munk absorption coefficient, S is the scattering coefficient, and R is the reflectance.16 Second-Harmonic Generation Measurements. Optical second-harmonic generation (SHG) signals were measured on SrCdGeS4 and SrCdGeSe4 using the Kurtz-Perry method.17 Light with a fundamental wavelength of 2090 nm was generated by a Q-switched Ho:Tm:Cr:YAG laser. The samples were ground and sieved with particle sizes in the ranges of 20–41, 41–74, 74–105, 105–150, and 150–200 m. Microcrystalline AgGaS2 with particle sizes over similar ranges served as the reference. Thermal Analysis. Differential scanning calorimetry (DSC) was performed on a Labsys TG-DTA16 (SETARAM) thermal analyzer and calibrated with Al2O3. Powder samples of SrCdGeS4 and SrCdGeSe4 (about 15 mg each) were sealed within small fused-silica tubes which were evacuated to 10–3 Pa. The heating and cooling rates were 20 K/min. Electronic Structure Calculations. Tight-binding linear muffin tin orbital electronic structure calculations were performed on SrCdGeS4 and SrCdGeSe4 within the local density and atomic spheres approximation with use of the Stuttgart TB-LMTO-ASA program (version 4.7).18 The basis sets consisted of Sr 5s/5p/4d/4f, Cd 5s/5p/4d/4f, Ge 4s/4p/4d, S 3s/3p/3d, and Se 4s/4p/4d orbitals, with the Sr 5p/4f, Cd 4f, Ge 4d, S 3d, and Se 4d orbitals being downfolded. Integrations in reciprocal space were carried out with an improved tetrahedron method over 108 irreducible k points within the first Brillouin zone. Bonding interactions were evaluated through an energy-resolved visualization as quantified by crystal orbital Hamilton populations (COHP).19

Results and Discussion



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8 Within the systems AE–M–Tt–Ch (AE = alkaline-earth metal; M = Zn, Cd, Hg; Tt = Si, Ge, Sn; Ch = S, Se), the previously known quaternary phases were BaCdGeS4,20 BaMSnS4 (M = Zn, Cd, Hg),21–24 and Ba3CdSn2S8 BaCdSnSe4

26

23,25

among the sulfides, and BaZnTtSe4 (Tt = Si, Ge)12 and

among the selenides. It is surprising that these compounds have been limited so

far to Ba-containing ones, with the most prevalent composition being AEMTtCh4.

The

quaternary chalcogenides SrCdGeS4 and SrCdGeSe4 prepared here are the first Sr-containing members in these systems. SrCdGeS4 and SrCdGeSe4 adopt a noncentrosymmetric orthorhombic structure in space group Ama2. Anionic layers [CdGeCh4]2– lie parallel to the bc-plane and separated by Sr2+ cations along the stacking a-direction (Figure 2a). The layers are built up by connecting Cd- and Ge-centred polyhedra through corner- and edge-sharing (Figure 2b).

There is a clear

differentiation between the longer distances of the surrounding Ch atoms to the Cd atoms (2.452(2)–2.663(2) Å in SrCdGeS4, 2.566(1)–2.778(2) Å in SrCdGeSe4) and the shorter distances to the Ge atoms (2.188(2)–2.235(2) Å in SrCdGeS4, 2.327(2)–2.381(2) Å in SrCdGeSe4). Although both Cd and Ge atoms are four-coordinate, the geometry around the Cd atoms is better described as trigonal pyramidal whereas that around the Ge atoms is tetrahedral. The bond angles around the Cd atoms between the apical and any of three basal Ch atoms are greatly contracted (94.46(5)–99.72(7)º in SrCdGeS4, 95.02(3)–98.95(5)º in SrCdGeSe4); in fact, the coordination geometry around the Cd atoms could equally well be described as 3+1, with the longest bond to the apical Ch atom. In contrast, the bond angles around the Ge atoms are closer to the ideal tetrahedral values (104.68(9)–117.14(6)º in SrCdGeS4, 104.67(6)–117.07(4)º in SrCdGeSe4).

The Sr atoms are eight-coordinate in what could be described as tetragonal

antiprismatic or bicapped trigonal prismatic geometry, greatly distorted, of course (Figure 2c).


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9 The structures of the quaternary chalcogenides AEMTtCh4 (AE = Sr, Ba; M = Zn, Cd, Hg; Tt = Si, Ge, Sn; Ch = S, Se) are all closely related but there are subtle differences between them. It is interesting that all of them crystallize in noncentrosymmetric orthorhombic space groups with similar cell parameters which could be doubled and with a polar twofold axis along the cdirection (Table 4).

To draw structural relationships clearly, a good starting point is the

BaZnSiSe4-type structure, which crystallizes in space group Ama2 (Figure 3a).12 Here, onedimensional chains of edge-sharing tetrahedra are centred alternately by M and Tt atoms, which are almost collinear and aligned parallel to the polar c-axis. SrCdGeS4 and SrCdGeSe4 also crystallize in space group Ama2 with similar cell parameters and positions of all atoms as in BaZnSiSe4 and BaZnGeSe4, with one important difference: The M atoms are strongly displaced to one side of the chain so that they link with a chalcogen atom of an adjacent chain, forming a two-dimensional layer (Figure 3b). The shift in atomic position is quite drastic; for example, the Zn atom in BaZnSiSe4 is located at ¼, 0.246, 0.514 but the Cd atom in SrCdGeS4 is located at ¼, 0.169, 0.554, with a considerable difference in the y-coordinate.

This displacement also

elongates the chains, reflected in the larger c-parameters of SrCdGeS4 and SrCdGeSe4 even though smaller Sr atoms have been substituted in place of Ba atoms. Given the significant displacement of the M atom and the altered connectivity of the polyhedra, it is useful to designate SrCdGeS4 as its own structure type, which is a distorted variant distinct from the BaZnSiS4-type structure. Lastly, the BaCdSnS4-type structure crystallizes in space group Fdd2 with all cell parameters doubled relative to the previous structures.22,23 This time, both M and Tt atoms within the ideal one-dimensional chains of edge-sharing tetrahedra are displaced to opposite sides, linking with chalcogen atoms of adjacent chains to form a more complex twodimensional layer lying parallel to the bc-plane (Figure 3c). The repeat distances are lengthened



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10 within this plane, as reflected in the doubled b- and c-parameters, and the stacking sequence of these layers has also changed, as reflected in the doubled a-parameter. In fact, the SrCdGeS4-type structure is most closely related to the BaHgSnS4-type structure.24 As summarized in a Bärnighausen tree,27 SrCdGeS4 (space group Ama2) transforms to BaHgSnS4 (space group Pnn2) through a klassengleiche reduction in symmetry of index 2 (Figure 4). The connectivities of the polyhedra are identical in both structures and there are only very small shifts in atomic positions. Among other symmetry elements, a mirror plane on which the M and Tt atoms lie in SrCdGeS4 has been lost on proceeding to BaHgSnS4, but these atoms are only displaced by less than 0.1 Å off the plane. Moreover, the Sr site (at 4a) in SrCdGeS4 splits into two separate Ba sites (at 2a and 2b) in BaHgSnS4, with a small shift of 0.2 Å of the second site off the ideal position. Relationships can be made to simpler ternary structures. The SrCdGeS4-type structure could be regarded as a quaternary derivative of the Sr2SnS4-type structure (adopted by only one other compound, Sr2GeSe4).28 Figure 5 shows the evolution of the chains in the selenides BaZnGeSe4 (BaZnSiSe4-type), SrCdGeSe4 (SrCdGeS4-type), and Sr2GeSe4 (Sr2SnS4-type). Replacement of Zn with Cd atoms results in a more asymmetric environment, but the covalent bonding network within the chains is retained. Replacement of Cd with highly electropositive Sr atoms, which participate in ionic bonding to the surrounding Se atoms, interrupts the covalent bonding network of the chains. As in other AEMTtCh4 compounds, SrCdGeS4 and SrCdGeSe4 conform to the chargebalanced formula (Sr2+)(Cd2+)(Ge4+)(Ch2–)4, in agreement with the calculated bond valence sums for each atom (Table 5). The valence-precise formulation implies the presence of a band gap. This expectation was confirmed by electronic structure calculations performed on SrCdGeS4 and


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11 SrCdGeSe4. As seen in the density of states (DOS) curves, valence and conduction bands are separated by 2.1 eV in SrCdGeS4 and a narrower gap of 1.4 eV in SrCdGeSe4 (Figure 6). The atomic projections of the DOS reveal that the valence band (extending from –4.8 to 0 eV in both compounds) involves mixing of S 3p or Se 4p states with Cd 4s/4p and Ge 4p states, corresponding to strong covalent bonding interactions between Cd or Ge atoms and chalcogen atoms, as confirmed in the crystal orbital Hamilton population (COHP) curves. Most of the Srbased states are located in empty conduction bands well above the Fermi level, but they also contribute slightly to the valence band. As evaluated by the integrated COHP values (–ICOHP) (Table 6), Ge–Ch interactions are not only inherently the strongest but they also provide the greatest contribution to the covalent bonding stability of the structure. In fact, the magnitude of the band gap is ultimately controlled by the separation of bonding Ge–Ch interactions at the top of the valence band and antibonding Ge–Ch interactions at the bottom of the conduction band. This is an important result because it suggests that the concerns expressed in the introduction about Cd substitution leading to too small a band gap can be allayed. The band dispersion diagrams show that the band gap is indirect (from  to Y) in SrCdGeS4 but direct in SrCdGeSe4 (from Y to Y) (Figure 7). However, given that the energy of the top of the valence band at Y is merely 0.05 eV lower than at , the gap is nearly a direct one in SrCdGeS4. The optical band gaps were measured experimentally from the UV-vis-NIR diffuse reflectance spectra, which were converted to absorption spectra (Figure 8).

Band gaps

extrapolated from the absorption edges were 2.6 eV for SrCdGeS4 and 1.9 eV for SrCdGeSe4, which are larger than the ones obtained from the band structure calculations above but consistent with the light yellow and red colours, respectively, of crystals of these compounds. These values can be compared with those of related quaternary chalcogenides (BaZnSiSe4, 2.7 eV;



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12 BaZnGeSe4, 2.5 eV; BaCdSnS4, 2.3 eV; BaCdSnSe4, 1.8 eV)12,23,26 and benchmark IR NLO materials (AgGaSe2, 1.8 eV; AgGaS2, 2.7 eV; ZnGeP2, 2.0 eV).1 The reasonably large band gaps found for SrCdGeS4 and SrCdGeSe4 thus portend well for their ability to withstand laserinduced damage, although this expectation will need to be confirmed separately. Optical SHG measurements were performed on SrCdGeS4 and SrCdGeSe4 using fundamental light with a wavelength of 2090 nm, and compared to AgGaS2 as a reference material. The SHG responses are impressively strong: over the same particle size range of 150– 200 m, SrCdGeS4 shows about twice the intensity and SrCdGeSe4 shows about five times the intensity as AgGaS2 (Figure 9a).

More importantly, as indicated by the increase in SHG

intensity with particle sizes (Figure 9b), both compounds exhibit type-I phase matching in the infrared region, which is an essential criterion for an NLO material to be viable in a technological application.29 This achievement represents a marked improvement over the related compounds BaZnGeSe4 (1 AgGaS2 over all particle size ranges and type-I phase matchable),12 BaCdSnS4 (5 AgGaS2 at 38–55 m particle sizes but not type-I phase matchable),23 and BaCdSnSe4 (1.6 AgGaS2 at 38–55 m particle sizes but not type-I phase matchable).26 DSC measurements show that both compounds melt congruently, at 958 ºC for SrCdGeS4 and 851 ºC for SrCdGeSe4 (Figure 10). These melting points are similar to those of benchmark materials, 998 ºC for AgGaS2 and 860 ºC for AgGaSe2.1 The considerable effort which will be required to grow large single crystals of NLO materials is thus facilitated by the congruent melting behaviour and relatively low melting points of these new quaternary chalcogenides.

Conclusions


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13 In the introduction, we had proposed that substitution of Sr for Ba and Cd for Zn in the previously known compound BaZnGeSe4 may improve SHG responses while maintaining a reasonably large band gap. This hypothesis was fulfilled by the characterization of the selenide SrCdGeSe4, as well as the corresponding sulfide SrCdGeS4. Both compounds satisfy many of the essential criteria for practical IR NLO materials: They show intense SHG signals (several times greater than AgGaS2), are type-I phase-matchable, have large and essentially direct band gaps, and melt congruently below 1000 ºC. The enhanced SHG behaviour can be traced not only to greater polarizability of Cd–Ch bonds, but also to the unexpected displacement of the Cd atoms away from ideal tetrahedral sites toward a trigonal pyramidal geometry in the crystal structure, leading to greater polarization along the c-direction. Because the magnitude of the band gap is influenced largely by Ge–Ch interactions, it should be possible to carry out further substitutions (perhaps Hg for Cd) to increase polarization without affecting the band gap. The closely related structures of compounds within the AEMTtCh4 series suggest that size factors play an important role in determining the structure type adopted; further work is in progress to prepare other members of this series.



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14 Table 1. Crystallographic Data for SrCdGeS4 and SrCdGeSe4 SrCdGeS4

SrCdGeSe4

fw (amu)

400.85

588.45

space group

Ama2 (no. 40)

Ama2 (no. 40)

a (Å)

10.3352(14)

10.8245(8)

b (Å)

10.2335(14)

10.6912(8)

c (Å)

6.4408(9)

6.6792(5)

V (Å3)

681.21(16)

772.96(10)

Z

4

4

T (K)

296(2)

296(2)

c (g cm–3)

3.908

5.057

crystal dimensions (mm)

0.06  0.04  0.02

0.04  0.04  0.04

(Mo K) (mm–1)

16.38

32.19

transmission factors

0.479–0.732

0.327–0.407

2 limits

7.48–66.49°

7.19–66.55°

data collected

–15  h  15, –15  k  15, –9  l  9

–16  h  16, –16  k  16, –10  l  10

no. of data collected

4937

5518

no. of unique data, including 1368 (Rint = 0.048) Fo2 < 0

1553 (Rint = 0.045)

no. of unique data, with Fo2 > 2(Fo2)

1219

1345

no. of variables

40

39

Flack parameter

0.004(9)

0.029(10)

R(F) for Fo2 > 2(Fo2) a

0.031

0.031

Rw(Fo2) b

0.057

0.064

goodness of fit

1.08

1.01

()max, ()min (e Å-3)

1.49, –1.48

2.03, –1.32

a

R(F) = ∑||Fo| – |Fc|| / ∑|Fo| for Fo2 > 2(Fo2).

b

Rw(Fo2) = [∑[w(Fo2 – Fc2)2] / ∑wFo4]1/2; w–1 = [σ2(Fo2) +

(Ap)2 + Bp], where p = [max(Fo2,0) + 2Fc2] / 3.


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15 Table 2. Atomic Coordinates and Equivalent Isotropic Displacement Parameters for SrCdGeS4 and SrCdGeSe4 atom

Wyckoff position

x

y

z

Ueq (Å2) a

Sr

4a

0

0

0.00000(14)

0.01687(19)

Cd

4b

¼

0.16892(7)

0.55404(11)

0.0261(2)

Ge

4b

¼

0.28260(8)

0.01054(13)

0.01322(19)

S1

8c

0.57964(14)

0.22862(17)

0.3003(3)

0.0167(3)

S2

4b

¼

0.0962(2)

0.1913(3)

0.0147(4)

S3

4b

¼

0.4379(2)

0.2442(3)

0.0173(5)

Sr

4a

0

0

0.00000(17)

0.0194(2)

Cd

4b

¼

0.83319(10)

0.43856(13)

0.0302(2)

Ge

4b

¼

0.21985(11)

0.48770(17)

0.0151(2)

Se1

8c

0.57611(8)

0.26924(8)

0.20158(12)

0.01886(18)

Se2

4b

¼

0.41010(11)

0.30251(16)

0.0162(2)

Se3

4b

¼

0.06337(11)

0.24560(17)

0.0201(3)

SrCdGeS4

SrCdGeSe4

a

Ueq is defined as one-third of the trace of the orthogonalized Uij tensor.



Page 16 of 38

16 Table 3. Interatomic Distances (Å) in SrCdGeS4 and SrCdGeSe4 SrCdGeS4

SrCdGeSe4

Sr–Ch2 (2)

3.0272(12)

3.1602(7)

Sr–Ch3 (2)

3.1297(13)

3.2362(8)

Sr–Ch1 (2)

3.1453(18)

3.2770(11)

Sr–Ch1 (2)

3.1692(18)

3.2828(10)

Cd–Ch2

2.452(2)

2.5662(13)

Cd–Ch1 (2)

2.5915(17)

2.6921(11)

Cd–Ch3

2.663(2)

2.7779(17)

Ge–Ch3

2.188(2)

2.3268(16)

Ge–Ch1 (2)

2.2240(16)

2.3658(11)

Ge–Ch2

2.663(2)

2.3806(16)


Page 17 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


17 Table 4. Structures of Quaternary Chalcogenides AEMTtCh4 (AE = Sr, Ba; M = Zn, Cd, Hg; Tt = Si, Ge, Sn; Ch = S, Se) Compound

Structure type

Space group

a (Å)

b (Å)

c (Å)

Reference

SrCdGeS4

SrCdGeS4-type

Ama2

10.3352(14)

10.2335(14)

6.4408(9)

This work

SrCdGeSe4

SrCdGeS4-type

Ama2

10.8245(8)

10.6912(8)

6.6792(5)

This work

BaZnSiSe4

BaZnSiSe4-type

Ama2

11.3055(8)

11.2344(7)

6.1994(4)

12

BaZnGeSe4

BaZnSiSe4-type

Ama2

11.3255(6)

11.2527(6)

6.2917(3)

12

BaZnSnS4

BaCdSnS4-type

Fdd2

21.964

21.504

12.701

21

BaCdGeS4

BaCdSnS4-type

Fdd2

21.69

21.25

12.78

20

BaCdSnS4

BaCdSnS4-type

Fdd2

21.86(2)

21.69(1)

13.18(5)

22

BaCdSnS4

BaCdSnS4-type

Fdd2

21.566(13)

21.760(13)

13.110(8)

23

BaCdSnSe4

BaCdSnS4-type

Fdd2

22.381(14)

22.711(14)

13.588(9)

26

BaHgSnS4

BaHgSnS4-type

Pnn2

10.804

10.840

6.613

24



Page 18 of 38

18 Table 5. Bond Valence Sums in SrCdGeS4 and SrCdGeSe4 SrCdGeS4

SrCdGeSe4

Sr

1.94

1.98

Cd

1.90

1.91

Ge

4.03

3.90

Ch1

1.86

1.85

Ch2

2.22

2.17

Ch3

1.92

1.92


Page 19 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


19 Table 6.

Integrated Crystal Orbital Hamilton Populations (–ICOHP) for SrCdGeS4 and

SrCdGeSe4 eV/bond

eV/cell

%

Sr–S

0.50

3.97

16.7

Cd–S

1.46

5.86

24.6

Ge–S

3.51

14.03

58.8

Sr–Se

0.50

3.99

18.6

Cd–Se

1.26

5.06

23.6

Ge–Se

3.09

12.37

57.7

SrCdGeS4

SrCdGeSe4



Page 20 of 38

20 Figure Captions Figure 1.

Powder XRD patterns and SEM images of typical crystals (insets) for (a) SrCdGeS4 and (b) SrCdGeSe4.

Figure 2.

Structure of SrCdGeS4 and SrCdGeSe4 (a) viewed down the c-direction, (b) highlighting an anionic [CdGeCh4]2– layer, and (c) showing the coordination around Sr atoms.

Figure 3.

(a) One-dimensional chains of edge-sharing tetrahedra centred by M and Tt atoms in BaZnSiSe4-type structure. (b) M atoms are displaced to link with neighbouring chains, forming layers in SrCdGeS4-type structure. (c) M and Tt atoms are displaced to link with neighbouring chains, forming layers in BaCdSnS4-type structure.

Figure 4.

Group-subgroup relations between SrCdGeS4- and BaHgSnS4-type structures. The atomic positions for BaHgSnS4 have been transformed after interchanging the a and b axes in the orthorhombic unit cell as originally reported.24

Figure 5.

Linking of GeSe4 tetrahedra via (a) ZnSe4 tetrahedra in BaZnGeSe4, (b) CdSe4 trigonal pyramids in SrCdGeSe4, and (c) Sr atoms in Sr2GeSe4.

Figure 6.

Density of states (DOS) (left panel), atomic projections (middle panels), and crystal orbital Hamilton population (–COHP) curves (right panel) for (a) SrCdGeS4 and (b) SrCdGeSe4.

Figure 7.

Band dispersion diagrams for (a) SrCdGeS4 and (b) SrCdGeSe4.

Figure 8.

Optical absorption spectra, converted from the diffuse reflectance spectra, for SrCdGeS4 and SrCdGeSe4.


Page 21 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


21 Figure 9.

SHG intensity at (a) particle sizes of 150–200 m and (b) different particle sizes for SrCdGeS4 and SrCdGeSe4, relative to AgGaS2, using fundamental light with wavelength of 2090 nm.

Figure 10. DSC curves for (a) SrCdGeS4 and (b) SrCdGeSe4.



Page 22 of 38

22

(a)

(b)

Figure 1. Powder XRD patterns and SEM images of typical crystals (insets) for (a) SrCdGeS4 and (b) SrCdGeSe4.


Page 23 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


23

(a)

b Sr a Ch2

Ge Ch3 Cd

Ch1

(b) Ch3 Ch2 Ge Ch1

Ch1

Ch3

Cd

Ch2 c b

(c)

Ch1

Ch2

Ch1

Ch2 Sr Ch1

Ch1 Ch3

Ch3

Figure 2. Structure of SrCdGeS4 and SrCdGeSe4 (a) viewed down the c-direction, (b) highlighting an anionic [CdGeCh4]2– layer, and (c) showing the coordination around Sr atoms.



Page 24 of 38

24 (a)

Tt

M

BaZnSiSe4-type (Ama2)

(b)

Tt

M

SrCdGeS4-type (Ama2)

(c)

Tt

M

BaCdSnS4-type (Fdd2)

Figure 3. (a) One-dimensional chains of edge-sharing tetrahedra centred by M and Tt atoms in BaZnSiSe4-type structure. (b) M atoms are displaced to link with neighbouring chains, forming layers in SrCdGeS4-type structure. (c) M and Tt atoms are displaced to link with neighbouring chains, forming layers in BaCdSnS4-type structure.


Page 25 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


25

Ama2 SrCdGeS4

Sr: 4a ..2 0 0 0.000

Cd: 4b m.. ¼ 0.169 0.554

Ge: 4b m.. ¼ 0.283 0.011

Hg: 4c 1 0.247 0.173 0.567

Sn: 4c 1 0.257 0.287 0.030

S: 8c 1 0.580 0.229 0.300

S: 4b m.. ¼ 0.096 0.191

S: 4b m.. ¼ 0.438 0.244

S: 4c 1 0.257 0.097 0.226

S: 4c 1 0.250 0.448 0.269

a = 10.34 Å b = 10.23 Å c = 6.44 Å

k2

Pnn2 BaHgSnS4

Ba: 2a ..2 0 0 0.000

Ba: 2b ..2 ½ 0 0.027

S: 4c 1 0.583 0.223 0.297

S: 4c 1 0.924 0.236 0.306

a = 10.84 Å b = 10.80 Å c = 6.61 Å

Figure 4. Group-subgroup relations between SrCdGeS4- and BaHgSnS4-type structures. The atomic positions for BaHgSnS4 have been transformed after interchanging the a and b axes in the orthorhombic unit cell as originally reported.24



Page 26 of 38

26

(a)

(b)

(c)

Figure 5. Linking of GeSe4 tetrahedra via (a) ZnSe4 tetrahedra in BaZnGeSe4, (b) CdSe4 trigonal pyramids in SrCdGeSe4, and (c) Sr atoms in Sr2GeSe4.


Page 27 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


27

(a)

(b)

Figure 6. Density of states (DOS) (left panel), atomic projections (middle panels), and crystal orbital Hamilton population (–COHP) curves (right panel) for (a) SrCdGeS4 and (b) SrCdGeSe4.



Page 28 of 38

28

(a)

(b)

Figure 7. Band dispersion diagrams for (a) SrCdGeS4 and (b) SrCdGeSe4.


Page 29 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


29

Figure 8. Optical absorption spectra, converted from the diffuse reflectance spectra, for SrCdGeS4 and SrCdGeSe4.



Page 30 of 38

30

(a)

(b)

Figure 9. SHG intensity at (a) particle sizes of 150–200 m and (b) different particle sizes for SrCdGeS4 and SrCdGeSe4, relative to AgGaS2, using fundamental light with wavelength of 2090 nm.


Page 31 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


31

(a)

(b)

Figure 10. DSC curves for (a) SrCdGeS4 and (b) SrCdGeSe4.



Page 32 of 38

32 Associated Content Accession Codes CCDC 1876032–1876033 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.

Author Information Corresponding Authors * E-mail: [email protected] (W.Y.). * E-mail: [email protected] (A.M.). ORCID Wenlong Yin: 0000-0003-2660-0891 Arthur Mar: 0000-0003-0474-5918 Notes The authors declare no competing financial interest.

Acknowledgments This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC, through Discovery Grant RGPIN-2018-04294), the NSERC Collaborative Research and Training Experience Program (CREATE, through the Alberta/Technical University of Munich International Graduate School), the National Natural Science Foundation of China (through Grant No. 51402270), the Institute of Chemical Materials (through Grant No. 32203),


Page 33 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


33 and the Key Laboratory of Science and Technology on High Energy Laser at the China Academy of Engineering Physics.

Dedication This paper is dedicated to Professor Xin-Tao Wu in honour of his 80th birthday, the 55th anniversary of his independent career, and his contributions to the development of functional materials.



Page 34 of 38

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36 (19) Dronskowski, R.; Blöchl, P. E. Crystal orbital Hamilton populations (COHP). Energyresolved visualization of chemical bonding in solids based on density-functional calculations. J. Phys. Chem. 1993, 97, 8617–8624. (20) Teske, C. L. Zur Kenntnis von BaCdGeS4 mit einem Beitrag zur Kristallchemie von Verbindungen des Typs BaABS4. Z. Anorg. Allg. Chem. 1980, 468, 27–34. (21) Teske, C. L. Darstellung, Kristallstrukturdaten und Eigenschaften der quaternären Thiostannate(IV) BaZnSnS4 und BaMnSnS4. Z. Naturforsch. B 1980, 35, 509–510. (22) Teske, C. L. Darstellung und Kristallstruktur von Barium-Cadmium-Thiostannat(IV) BaCdSnS4. Z. Anorg. Allg. Chem. 1980, 460, 163–168. (23) Zhen, N.; Wu, K.; Wang, Y.; Li, Q.; Gao, W.; Hou, D.; Yang, Z.; Jiang, H.; Dong, Y.; Pan, S. BaCdSnS4 and Ba3CdSn2S8: syntheses, structures, and non-linear optical and photoluminescence properties. Dalton Trans. 2016, 45, 10681–10688. (24) Teske, C. L. Darstellung und Kristallstruktur von Barium-Quecksilber-Thiostannat(IV), BaHgSnS4. Z. Naturforsch. B 1980, 35, 7–11. (25) Teske, C. L. Darstellung und Kristallstruktur von Ba3CdSn2S8 mit einer Anmerkung über Ba6CdAg2Sn4S16. Z. Anorg. Allg. Chem. 1985, 522, 122–130. (26) Wu, K.; Su, X.; Yang, Z.; Pan, S. An investigation of new infrared nonlinear optical material: BaCdSnSe4, and three new related centrosymmetric compounds: Ba2SnSe4, Mg2GeSe4, and Ba2Ge2S6. Dalton Trans. 2015, 44, 19856–19864. (27) Müller, U. Symmetry Relationships between Crystal Structures; Oxford University Press: Oxford, U.K., 2013.


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37 (28) Pocha, R.; Tampier, M.; Hoffmann, R.-D.; Mosel, B. D.; Pöttgen, R.; Johrendt, D. Crystal structures and properties of the thiostannates Eu2SnS4 and Sr2SnS4 and the selenogermanate -Sr2GeSe4. Z. Anorg. Allg. Chem. 2003, 629, 1379–1384. (29) Zhang, W.; Yu, H.; Wu, H.; Halasyamani, P. S. Phase-matching in nonlinear optical compounds: A materials perspective. Chem. Mater. 2017, 29, 2655–2668.



Page 38 of 38

38 For Table of Contents Only

SrCdGeS4 and SrCdGeSe4:

Promising Infrared Nonlinear Optical Materials with

Congruent-Melting Behaviour Yunwei Dou, Ying Chen, Zhuang Li, Abishek K. Iyer, Bin Kang, Wenlong Yin, Jiyong Yao, and Arthur Mar

SrCdGeS4 and SrCdGeSe4, which adopt noncentrosymmetric orthorhombic structures, show strong SHG responses, are type-I phase-matchable, have large band gaps, and melt congruently.


Promising Infrared Nonlinear Optical Materials with ... - ACS Publications

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