A High Laser Damage Threshold and a Good Second-Harmonic Generation Response in a New Infrared NLO Material : LiSm 3 SiS 7

A series of new infrared nonlinear optical (IR NLO) materials, LiRe3MS7 (Re = Sm, Gd; M = Si, Ge), have been successfully synthesized in vacuum-sealed silica tubes via a high-temperature solid-state method. All of them crystallize in the non-centrosymmetric space group P63 of the hexagonal system. In their structures, LiS6 octahedra connect with each other by sharing common faces to form infinite isolated one-dimensional ∞[LiS3]n chains along the 63 axis. ReS8 polyhedra share edges and corners to construct a three-dimensional tunnel structure with ∞[LiS3]n chains located inside. Remarkably, LiSm3SiS7 shows promising potential as one new IR NLO candidate, including a wide IR transparent region (0.44–21 μm), a high laser damage threshold (LDT) (3.7 × benchmark AgGaS2), and a good NLO response (1.5 × AgGaS2) at a particle size between 88 μm and 105 μm. Dipole-moment calculation was also used to analyze the origin of NLO responses for title compounds.


Introduction
Nonlinear optical (NLO) crystals play an increasingly critical role in developing new coherent light sources by frequency conversion technology on traditional lasers [1,2].Recently, numerous famous NLO crystals have been discovered and have effectively solved the generation of UV-visible light [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22].However, for the mid-far infrared (IR) region (3-20 µm), outstanding IR NLO materials have been discovered less, and only a few (chalcopyrites AgGaS 2 , AgGaSe 2 , and ZnGeP 2 ) have been commercially applied [23][24][25].Note that some of the self-defects hinder their future development (especially for high-energy laser system), such as low laser-damage thresholds (LDTs) for AgGaS 2 and AgGaSe 2 , and serious two-photon absorption (TPA) for ZnGeP 2 .Nowadays, the development of many important fields, such as laser guidance, infrared remote sensing, and telecommunication, has hardly been realized without the help of high-energy IR sources.Thus, to find new excellent IR NLO materials with a wide IR transparent range, a good chemical stability, a high LDT, and a large NLO coefficient is still an urgent task and a great challenge.In recent decades, research systems have almost covered the entire periodic table, and hundreds of new IR NLO crystals have been discovered .As one NLO material, a non-centrosymmetric (NCS) structure is an essential condition.Previous investigations indicate that tetrahedral M IV S 4 (M IV = Si, Ge, and Sn) with lively alkali metals can increase the opportunity to obtain NCS structures [53,54].Moreover, combining the electropositive elements including alkaline, alkaline-earth, or rare-earth metals with crystal structures can increase the band gaps of compounds and further increase their LDTs [55,56].Based on the above strategy, we have focused our research interests on the Li−Re−M−S system and fortunately obtained three new NCS compounds: LiSm 3 SiS 7 , LiSm 3 GeS 7 , and LiGd 3 GeS 7 , which belong to the well-known ARe 3 MQ 7 family, where A is the monovalent, divalent, or trivalent ions, Re is the rare-earth ions, M is the tetravalent ions (Si 4+ , Ge 4+ , and Sn 4+ ), and Q is S or Se [57][58][59][60][61][62][63][64].To the best of our knowledge, Li-containing ARe 3 MQ 7 compounds has not been reported yet.All of them crystallize in the polar space groups of P6 3 and exhibit similar crystal structures.Among them, LiSm 3 SiS 7 has a high laser damage threshold about 3.7 times that of benchmark AgGaS 2 .In addition, LiSm 3 SiS 7 also shows a wide transmission window in the IR range (up to 21 µm) and a large NLO response (1.5 × AgGaS 2 ) at a particle size between 88 µm and 105 µm.Herein, LiSm 3 SiS 7 can be expected as a new NLO candidate in the IR region.

Crystal Structure
Three compounds including LiSm 3 SiS 7 , LiSm 3 GeS 7 , and LiGd 3 GeS 7 crystallize in the NCS polar space group P6 3 of the hexagonal system.Herein, LiSm 3 SiS 7 was chosen as the representative to illuminate its crystal structure.In its asymmetric unit, there is one unique Sm atom, one Li atom, one Si atom, and three S atoms.As for its structure, Sm atoms are connected with eight S atoms to form the distorted SmS 8 polyhedra with d(Sm−S) = 2.791(5) − 2.993(6) Å. SmS 8 polyhedra connect together by sharing corners or edges to form the three-dimensional tunnel framework (Figure 1a).Si atoms are connected with four S atoms to form the isolated typical SiS 4 tetrahedra with d(Si−S) = 2.089(2) − 2.132(1) Å. Li atoms are linked with six S atoms to form LiS 6 octahedra with three short (2.568(8) Å) and three long (2.623(3) Å) Li−S bonds.The LiS 6 octahedra share faces with each other to form one-dimensional ∞ [LiS 3 ] n chains along the 6 3 axis, and the chains then stretch in the tunnels surrounded by SmS 8 (Figure 1b,c).In addition, title compounds have similar crystal structures to previous reported ARe 3 MQ 7 series of crystals [57][58][59][60][61][62][63][64], where A represents Na, Cu, and Ag.In this work, the new Li-containing compounds were discovered to enlarge the A I Re 3 M IV Q 7 family and enrich the structural chemistry.on the above strategy, we have focused our research interests on the Li−Re−M−S system and fortunately obtained three new NCS compounds: LiSm3SiS7, LiSm3GeS7, and LiGd3GeS7, which belong to the well-known ARe3MQ7 family, where A is the monovalent, divalent, or trivalent ions, Re is the rare-earth ions, M is the tetravalent ions (Si 4+ , Ge 4+ , and Sn 4+ ), and Q is S or Se [57][58][59][60][61][62][63][64].To the best of our knowledge, Li-containing ARe3MQ7 compounds has not been reported yet.All of them crystallize in the polar space groups of P63 and exhibit similar crystal structures.Among them, LiSm3SiS7 has a high laser damage threshold about 3.7 times that of benchmark AgGaS2.In addition, LiSm3SiS7 also shows a wide transmission window in the IR range (up to 21 μm) and a large NLO response (1.5 × AgGaS2) at a particle size between 88 μm and 105 μm.Herein, LiSm3SiS7 can be expected as a new NLO candidate in the IR region.

Crystal Structure
Three compounds including LiSm3SiS7, LiSm3GeS7, and LiGd3GeS7 crystallize in the NCS polar space group P63 of the hexagonal system.Herein, LiSm3SiS7 was chosen as the representative to illuminate its crystal structure.In its asymmetric unit, there is one unique Sm atom, one Li atom, one Si atom, and three S atoms.As for its structure, Sm atoms are connected with eight S atoms to form the distorted SmS8 polyhedra with d(Sm−S) = 2.791(5) − 2.993(6) Å. SmS8 polyhedra connect together by sharing corners or edges to form the three-dimensional tunnel framework (Figure 1a).Si atoms are connected with four S atoms to form the isolated typical SiS4 tetrahedra with d(Si−S) = 2.089(2) − 2.132(1) Å. Li atoms are linked with six S atoms to form LiS6 octahedra with three short (2.568(8) Å) and three long (2.623(3) Å) Li−S bonds.The LiS6 octahedra share faces with each other to form one-dimensional ∞[LiS3]n chains along the 63 axis, and the chains then stretch in the tunnels surrounded by SmS8 (Figure 1b,c).In addition, title compounds have similar crystal structures to previous reported ARe3MQ7 series of crystals [57][58][59][60][61][62][63][64], where A represents Na, Cu, and Ag.In this work, the new Li-containing compounds were discovered to enlarge the A I Re3M IV Q7 family and enrich the structural chemistry.Moreover, in order to ensure the reasonability of crystal structures of these compounds, the bond valence [65,66] and the global instability index (GII) [67][68][69] are systemically calculated (Table 1).The method of bond-valence parameters was used to calculate the bond valences of elements.
The following equation is commonly used to calculate the bond valence ( ij v ): Moreover, in order to ensure the reasonability of crystal structures of these compounds, the bond valence [65,66] and the global instability index (GII) [67][68][69] are systemically calculated (Table 1).The method of bond-valence parameters was used to calculate the bond valences of elements.The following equation is commonly used to calculate the bond valence (v ij ): where r' is an empirically determined bond valence parameter, r ij is the actual bond length, and B is commonly taken to be a universal constant equal to 0.37 Å. Calculated results (Li, 0.993-1.021;Sm/Gd, 3.024-3.039;Si/Ge, 4.060-4.134;S, 1.933-2.128)indicate that all atoms are in reasonable oxidation states.
In addition, GII can be derived from the bond valence concepts, which represent the tension of lattice parameters and are always used to evaluate the rationality of structure.While the value of GII is less than 0.05 vu (valence unit), the tension of the structure is not proper; while the value of GII is larger than 0.2 vu, its structure is not stable.Thus, the value of GII in a reliable structure should be limited to 0.05-0.2 in general.As for title compounds, calculated GII values are in the range of 0.07-0.09vu, which illustrates that the crystal structures of all compounds are reasonable.

Optical Properties
Experimental optical bandgap (Figure 2) of LiSm 3 SiS 7 was measured to be 2.83 eV (440 nm), which is consistent with the crystal color (pale yellow).IR and Raman spectra (Figure 2) show that LiSm 3 SiS 7 has a wide transmission range in the IR region (up to 21 µm) that covers two critical atmospheric windows (3-5 and 8-12 µm), which is comparable to those reported for powdered BaGa 4 Se 7 (~18 µm) [35], AgGaS 2 (~23 µm) [30], Li 2 CdGeS 4 (~22 µm) [70], and Na 2 Hg 3 Si 2 S 8 (~20 µm) [47].Moreover, the bandgap (2.83 eV) of LiSm 3 SiS 7 is larger than that of commercial AgGaS 2 crystal (2.56 eV) [30]; thus, LiSm 3 SiS 7 may have a higher laser damage resistance since the LDT is generally proportional to the bandgap for one material.In this work, a pulse laser (1.06 µm, 10 Hz, and 10 ns) was chosen to measure the LDT of LiSm 3 SiS 7 with AgGaS 2 as the reference on powder samples (Table 2).The LDT value of LiSm 3 SiS 7 is 118 MW/cm 2 , about 3.7 times that of commercial AgGaS 2 (32 MW/cm 2 ), which shows this material has good potential to apply in the high-power laser system.We have also investigated the second-harmonic generation (SHG) response for LiSm 3 SiS 7 , and the particle size versus SHG intensity of LiSm 3 SiS 7 indicates a nonphase-matching behavior at 2.09 µm.As seen from Figure 3 and Table 3, LiSm 3 SiS 7 shows a large SHG response about 1.5 times that of standard AgGaS 2 in the 88-105 µm particle size range.Remarkably, LiSm 3 SiS 7 also achieves a suitable balance of high LDT and good SHG response, which can effectively avoid the drawbacks (low LDT and harmful TPA) of commercially available materials.Thus, LiSm 3 SiS 7 has potential application as a NLO material in the mid-and far-IR region.

Dipole Moment Calculation
The SHG response has a close relationship with the distortion degree of anionic groups in the crystal structure.While the spatial arrangement of all units tends to be uniform, the local NLO effects stack with each other, and then lead to a large NLO effect for material.Thus, in order to obtain a deeper understanding of NLO responses origin of title compounds, the dipole moments for the [LiS6], [ReS8], and [MS4] units were calculated with a bond-valence method [71][72][73], and the calculated results are listed in Table 4. From the table, it can be seen that the polarizations of the x-and y-directions from all building units are almost canceled out, and the polarizations of z-direction are constructively added in a unit cell for title compounds.Moreover, the polarization of

Dipole Moment Calculation
The SHG response has a close relationship with the distortion degree of anionic groups in the crystal structure.While the spatial arrangement of all units tends to be uniform, the local NLO effects stack with each other, and then lead to a large NLO effect for material.Thus, in order to obtain a deeper understanding of NLO responses origin of title compounds, the dipole moments for the [LiS 6 ], [ReS 8 ], and [MS 4 ] units were calculated with a bond-valence method [71][72][73], and the calculated results are listed in Table 4. From the table, it can be seen that the polarizations of the x-and y-directions from all building units are almost canceled out, and the polarizations of z-direction are constructively added in a unit cell for title compounds.Moreover, the polarization of the [ReS 8 ] unit is also found to be much larger than those of the distorted [LiS 6 ] octahedra and [MS 4 ] tetrahedra for all title compounds.As seen from the results for previous reported La 3 Ga 0.5 (Ge 0.5/ Ga 0.5 )S 7 and La 3 In 0.5 (Ge 0.5 /In 0.5 )S 7 on the calculation of cutoff-energy-dependent SHG coefficient [57], their SHG responses mainly originate from the transition processes from S-3p, La-5d states to La-5d, S-3p states, which are consistent with the calculated results of dipole moments for the title compounds in this work.

Synthesis
Raw materials were commercially purchased.Because Li metal is easily oxidized in air, all the preparation processes were completed in an Ar-filled glovebox.

LiSm 3 SiS 7
Elementary reactants of Li, Sm, Si, and S were weighted at the stoichiometric ratio of 1:3:1:7.All the raw materials were firstly loaded into a graphite crucible, the graphite crucible was then put into a silica tube, and the tube was flame-sealed under 10 −3 Pa.The detailed temperature-setting curve for the muffle furnace was heated to 850 • C in 50 h and kept at this temperature for about 100 h, then slowly cooled to 300 • C by 80 h, finally quickly cooled to the room temperature.The product was washed with N,N-dimethylformamide (DMF) to remove the byproducts.Pale-yellow crystals were found and stable in the air.

LiSm 3 GeS 7 and LiGd 3 GeS 7
These reaction processes including starting composition and heating profile are similar to that of LiSm 3 SiS 7 .Finally, yellow crystals were also obtained by washing with DMF, but they would deliquesce when exposed to air for a long time.

Structure Determination
Selected high-quality crystals were used for data collections on a Bruker SMART APEX II 4K CCD diffractometer (Bruker Corporation, Madison, WI, USA) using MoKα radiation (λ = 0.71073 Å) at 296 K.The crystal structures were solved by a direct method and refined using the SHELXTL program package [74].Multi-scan method was chosen for absorption correction [75].As for the LiSm 3 SiS 7 , the space group was determined from the systematic absences was P6 3 .The first run of a routine refinement of the initial structure generated an "un-balanced" formula of "Sm 3 SiS 7 " with R 1 = 2.04%, R 2 = 5.06%.In addition, a high electron density peak with 6.49 e − /Å 3 and 2.55 Å away from S1, was observed.We also tried to assign this position to the other atoms or a mixed occupation with two atoms (Li and Sm), but no refinement results were proper or obtained the balanced formula.Only this position was assigned as Li1, which provided a balanced formula of "LiSm 3 SiS 7 ," and R 1 and R 2 converged to improved values of 1.50% and 3.05%.Rational anisotropic thermal parameters for all atoms were obtained by the anisotropic refinement and extinction correction.Moreover, the maximum and minimum peaks on the final Fourier difference map corresponded to 0.57 and −0.60 e − /Å 3 .Other two compounds (LiSm 3 GeS 7 and LiGd 3 GeS 7 ) have the similar refinement process with that of LiSm 3 SiS 7 .Final structures were also checked with the PLATON program, and no other symmetries were found.Crystallographic data for title compounds are reported in Table 5.

Powder XRD Measurement
Powder X-ray diffraction (XRD) analysis was measured at room temperature using an automated Bruker D2 X-ray diffractometer.However, we did not prepare the pure phase of LiSm 3 GeS 7 and LiGd 3 GeS 7 for the reason of moisture absorption.In this work, only the pure phase of LiSm 3 SiS 7 was obtained, and we systemically studied its physicochemical properties.In addition, in comparison with calculated and experiment results (Figure 4), it can be found that the experimental powder XRD patterns are basically consistent with calculated values.

UV-Vis-NIR Diffuse-Reflectance Spectroscopy
With a Shimadzu SolidSpec-3700DUV spectrophotometer, optical diffuse reflectance spectrum was measured in the wavelength range from 190 nm to 2600 nm.The absorption spectrum was calculated from the diffuse reflectance spectra according to the Kubelka-Munk function: α/S = (1 − R) 2 /2R, where R is the reflectance coefficient, and α and S are the absorption and scattering coefficient, respectively.Based on the absorption spectrum, the optical bandgap was obtained with the absorption edge of material.

Raman Spectroscopy
Hand-picked single-crystals were put on an object slide, and a LABRAM HR Evolution spectrometer (Shimadzu Corporation, Beijing, China) equipped with a CCD detector with a 532-nm laser was then used to record the Raman spectra.The integration time was 5 s.

Infrared Spectroscopy
IR spectra were measured on the powder sample mixed with dried KBr powder.A Shimadzu IRAffinity-1 Fourier transform infrared spectrometer recorded the measurement data in the range of 400-4000 cm −1 with a resolution of 2 cm −1 .

LDT Measurement
A pulse laser (1.06 µm, 10 Hz, and 10 ns) was chosen to estimate the powder LDT with the same powdered AgGaS 2 sample as the reference with a particle size range of 150-200 µm.The detail test procedure is as follows: with increasing laser energy, the color change of the powder sample was constantly observed with an optical microscope to determine the damage threshold.To adjust different laser beams, an optical concave lens was added into the laser path.The damage spot was measured by the scale of optical microscope.

Calculations of Group Dipole Moments
The dipole moments of the LiS 6 , SmS 8 , GdS 8 , SiS 4 , and GeS 4 units were calculated with a simple bond-valence method [71][72][73].Distribution of the electron on the each atom was estimated by the bond-valence theory v ij = exp[(r' − r ij )/B; where r' is the empirical constant, r ij is the actual bond length, and B is commonly taken to be a universal constant equal to 0.37 Å.The geometrical position was taken from the unit cell of the experimental X-ray crystal structure.The Debye equation µ = neR, where µ is the net dipole moment in Debye, n the total number of electrons, e the charge on an electron, and R the difference, in cm, between the "centroids" of positive and negative charges, was used to calculate the dipole moment.

Conclusions
In summary, the first three new Li-containing ARe 3 MQ 7 compounds including LiSm 3 SiS 7 , LiSm 3 GeS 7 , and LiGd 3 GeS 7 are isostructural with a polar space group P6 3 in the hexagonal system.The LiSm 3 SiS 7 , for example, features a 3D tunnel structure composed of isolated SiS 4 tetrahedra, 1D ∞ [LiS 3 ] n chains along the 6 3 axis formed by face-sharing LiS 6 octahedra, and 3D framework by the interconnection of SmS 8 polyhedra, with the 1D ∞ [LiS 3 ] n chains located in the tunnels.Moreover, LiSm 3 SiS 7 shows good SHG efficiency ~1.5 times that of AgGaS 2 in the particle size range of 88-105 µm and a high LDT that is ~3.7 times that of AgGaS 2 , demonstrating that LiSm 3 SiS 7 exhibits a suitable

Figure 1 .
Figure 1.(a) Crystal structure of LiSm3SiS7.All the Li-S bonds are omitted for clarity.(b) Structure of LiSm3SiS7 viewed down the c-axis.The Sm−S bonds were omitted for the sake of clarity.Blue: Sm; Gray octahedron: LiS6; Green tetrahedron: SiS4.(c) One-dimensional ∞[LiS3]n chains in LiSm3SiS7.

Figure 1 .
Figure 1.(a) Crystal structure of LiSm 3 SiS 7 .All the Li-S bonds are omitted for clarity.(b) Structure of LiSm 3 SiS 7 viewed down the c-axis.The Sm−S bonds were omitted for the sake of clarity.Blue: Sm; Gray octahedron: LiS 6 ; Green tetrahedron: SiS 4 .(c) One-dimensional ∞ [LiS 3 ] n chains in LiSm 3 SiS 7 .

Table 1 .
Bond valence sum (vu) and global instability index (GII) of title compounds.

Table 3 .
SHG intensity versus particle size for LiSm3SiS7 and AgGaS2 (as the reference).

Table 3 .
SHG intensity versus particle size for LiSm 3 SiS 7 and AgGaS 2 (as the reference).