LiGa2PS6 and LiCd3PS6: Molecular Designs of Two New Mid

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LiGa2PS6 and LiCd3PS6: Molecular Designs of Two New Mid-infrared Nonlinear Optical materials Jianghe Feng, Chun-Li Hu, Bingxuan Li, and Jiang-Gao Mao Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b01470 • Publication Date (Web): 17 May 2018 Downloaded from http://pubs.acs.org on May 17, 2018

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Chemistry of Materials

LiGa2PS6 and LiCd3PS6: Molecular Designs of Two New Mid-infrared Nonlinear Optical materials Jianghe Feng, †,‡ Chun-Li Hu, † Bingxuan Li, †,‡ Jiang-Gao Mao* † †

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of

Matter, Chinese Academy of Sciences, Fuzhou 350002, PR China ‡

University of Chinese Academy of Sciences, Beijing, 100039, People’s Republic of China

Abstract To simultaneously maintain large second harmonic generation (SHG) and high laser damage thresholds (LDTs) for middle and far infrared (IR) nonlinear optical (NLO) materials is very challenging and significant in laser science and technology. Using a chemical substitution strategy, two new mid-infrared NLO materials based on metal thiophosphates, namely, LiGa2PS6 and LiCd3PS6 have been obtained from AgGa2PS6. They both crystallize in Cc space group but exhibit different structures. LiGa2PS6 is isostructural with the parent AgGa2PS6, displaying a three-dimensional (3D) anionic framework of [Ga2PS6]- composed of [Ga2S6]6- chains and PS4 tetrahedra with the 1D triangular-shaped tunnels filled by Li+ ions. LiCd3PS6 features a 3D anionic framework of [Cd3S6]6− composed of 2D layers of [Cd2S5]6- and the bridging CdS4 units with 1D tunnels along b axis filled by [LiPS6]6- chains. Remarkably, both materials display large band gaps of 3.15 eV (for LiGa2PS6) and 2.97 eV (for LiCd3PS6), and keep a good balance between strong SHG responses (0.5−0.8 × AgGaS2 at 1910 nm in the particle range of 53-71 µm) and high LDTs (5.5−10.4 × AgGaS2), therefore both crystals are new promising MIR-NLO materials.

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INTRODUCTION Nonlinear optical (NLO) materials have attracted intensive scientific attentions as a significant component of optical parametric oscillation (OPO) to produce tunable coherent laser sources.1-7 In middle and far infrared (IR) region, the chalcopyrite-type AgGaS2 (AGS), 8-9 AgGaSe2 (AGSe)10 and ZnGeP2 (ZGP)11 crystals are the main commercial materials due to their large second-order harmonic generation (SHG) responses and wide IR transmission ranges. Unfortunately, the intrinsic drawbacks, such as low laser damage thresholds (LDTs), non-phase matching behavior for AgGaSe2, and harmful two-photon absorption (TPA) at 1.06 µm for ZnGeP2, greatly limited their practical applications. To be a promising MIR-NLO crystal, aside from large SHG response (> 0.5 × AGS) and wide optical transparency range, it must satisfy other basic requirements, including wide band gap (Eg > 3.0 eV) for obtaining the high LDT, and suitable birefringence (△n) (0.04−0.10) for the achievement of phase-matching ability and the avoidance of destructive optical behavior, as well as low melt temperature (Tm) for the bulk crystal growth. But due to the inverse relationship of LDT and SHG efficiency, it is still a great challenge to develop a MIR NLO crystal with a suitable balance among above qualifications. During the past decades, metal chalcogenides have been ecstatically investigated as a rich family of MIR-NLO materials, owing to their wide compositional flexibility and high transmittance in MIR range,1-3 as well as a large number of non-centrosymmetric (NCS) structures induced by the asymmetric MQ4 (M = Ga, In, Ge, Sn, P, etc.; Q = S, Se) tetrahedra. Up to now, numbers of new MIR-NLO crystals based on chalcogenides have been discovered, including Ga2S3,12 LiInS2,13-14 Li2CdGeS4,15 Li2MnGeS4,16 LiGaGe2S6,17 KHg4Ga5Se12,18 RbGeP4Se12,19 Ba8Sn4S15,20 BaGa4Se7,21 Ba3CsGa5Se10Cl2,22 Cu2HgGeS4,23 Ag2CdGeS4,24 MGa2S4 (M = Zn, Cd),25-26 etc. However，materials 2 / 27


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that meet with all of the fundamental conditions of practical applications are still scarce. Based on our previous investigations, metal thiophosphates are able to show relatively low Tm, and thus display advantages in reducing the temperature-dependent sulfur vapor pressure, which would greatly facilitate their large crystal growths, as exampled by LiZnPS4,27 AgZnPS4,27 KZrPS6,28 [Rb3Br][Ga3PS8].29 On the other hand, the strong P-S covalent bonds are also beneficial to the improvement of LDT. Recently, our efforts to introduce the PS4 tetrahedron into AgGaS2 led to the discovery of AgGa2PS6, which displays both large SHG response and high LDT, in addition to the low Tm.30 The use of more electropositive elements than Ag, such as alkali or alkali earth metal as cations in the chalcogenide based NLO materials, can greatly increase the Eg values, as exampled by LiGaS2 (4.2 eV)31 vs AgGaS2 (2.6 eV),8 as well as Li2ZnSiS4 (3.9 eV),32 Li2BaGeS4 (3.7 eV),33 Na2BaGeS4 (3.7 eV),34 KZn4Ge5S12 (3.7 eV),35 BaGa4S7 (3.5 eV),36 Na2ZnGe2S6 (3.3 eV),37 etc. In this work, lithium is used to replace the silver in AgGa2PS6, which led to LiGa2PS6 with an enlarged Eg (3.15 eV) and suitable △n (0.08), but slightly decreased SHG effect. Furthermore, in order to improve the SHG coefficient through increasing the density of NLO-active units according to the anionic group theory presented by Chen,38 LiCd3PS6 with a stronger SHG response and high LDT was also obtained through the replacement of two Ga3+ in LiGa2PS6 by three Cd2+. Herein, we report their syntheses, crystal structures, optical properties, and theoretical calculations.

EXPERIMENTAL SECTION Materials and Methods. Li (99.9 %), Ga2S3 (99.9 %), Cd (99.9 %), P (99.9 %), and S (99.99 %) were purchased from Shanghai Titan Scientific Co. Ltd. All of the chemicals were used without 3 / 27



further purification. Microprobe elemental analyses were performed using field emission scanning electron microscope (FESEM, JSM6700F) with energy dispersive X-ray spectroscope (EDS, Oxford INCA). Inductively coupled plasma (ICP) measurements were investigated on Ultima 2 inductively coupled plasma OES spectrometer. Powder X-ray diffraction (PXRD) patterns were recorded on a XPERT-MPD θ-2θ diffractometer equipped with Cu-Kα radiation (λ = 1.540598 Å) within 2θ range of 10-70° with a step size of 0.02°. The differential scanning calorimetry (DSC) studies were performed on a NETZSCH STA 449F3 unit in N2 atmosphere at 10 °C/min heating rate. Polycrystalline samples were placed in sealed silica tubes evacuated to 10−4 Torr. The room-temperature optical diffuse reflectance spectra were measured on a PE Lambda 950 spectrophotometer in the wavelength range of 330−2500 nm, BaSO4 plate was used as a standard (100 % reflectance). The band gaps were estimated based on the reflectance spectra by using the by Tauc plot method.39 The Infra-red (IR) spectra were recorded as KBr pellets on a VERTEX 70 FTIR spectrophotometer in the range of 4000-400 cm-1. Powder LDTs of LiGa2PS6, LiCd3PS6 and AgGaS2 (reference) in the size range of (53-71 µm) were measured by the single-pulse method.12 During the measurements of LDTs, the power of input laser was increased slowly until damage spots of samples were monitored under a magnifier. The damage thresholds were derived from the equation I(threshold) = E/(πr2τp), in which E is the laser energy of a single pulse, r is the spot radius and τp is pulse width. The average input lasers’ energy density of single pulses and laser spots were recorded when crystals were damaged by the laser with a flat-top laser beam distribution (wavelength λ = 1064 nm and pulse duration τ =10 ns in a 1 Hz repetition). The measurements of the powder frequency-doubling effects were performed by using a modified method of Kurtz and Perry method by the incident 1910 nm and 1064 nm laser.40 LiGa2PS6 and LiCd3PS6 with particle sizes of 53-71 µm were selected, and then 4 / 27


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loaded in the container with a thickness of 1.2 mm and diameter of 6 mm. Sieved AgGaS2 sample in the same size range was used as a reference. Syntheses. Both compounds were synthesized by the high-temperature solid state reactions. A stoichiometric mixture of Li, Ga2S3, P and S for LiGa2PS6 (or Li, Cd, P and S for LiCd3PS6) with a total weight of ~600 mg was thoroughly ground in an argon-filled glovebox and loaded into quartz tubes, which were subsequently flame-sealed under vacuum (∼10−4 Torr). The tubes were heated to 400 °C in 1 day and hold for 2 days, subsequently heated to 650 °C for LiGa2PS6 (610 °C for LiCd3PS6) in 10 h and hold for 30 days, finally cooled to 300 °C slowly before the furnace power was switched off. Air-stable and transparent single crystals of LiGa2PS6 (colorless) and LiCd3PS6 (light yellow) with high purity were obtained, which have been confirmed by PXRD analysis (Figure S1). EDS elemental analysis of several single crystals shown an average Ga/P/S molar ratio of 2.1/1.0/5.9 for LiGa2PS6 and a Cd/P/S molar ratio of 2.8/1.0/5.8 for LiCd3PS6. ICP measurements showed the existences of Li ions in both compounds. These results are consistent with those determined from single-crystal XRD studies. Single Crystal Structure Determinations. Needle (LiGa2PS6) or cubic (LiCd3PS6) shaped single crystals were glued onto glass fibers and mounted on a SuperNova (Mo) X-ray Source (λ = 0.71073 Å), at 293(2) K for reflection data collections. Both data sets were corrected for Lorentz and polarization factors as well as absorption by the Multi–scan method.41 Both structures were solved by direct methods and refined by a full–matrix least–squares fitting on F2 by SHELX–97.42 All of atoms were refined with anisotropic thermal parameters. The structures were checked for possible twinning and missing symmetry elements by using PLATON, but none was found.43 The refined Flack parameters of 0.03(16) (LiGa2PS6) and -0.07(12) (LiCd3PS6) confirmed the correctness of their 5 / 27



absolute structures.44-45 Crystallographic data and structural refinements of LiGa2PS6 and LiCd3PS6 are summarized in Table 1, and important bond lengths and angles are listed in Tables S1 and S2, respectively. Computational Descriptions. Single-crystal structural data of LiGa2PS6 and LiCd3PS6 were applied for calculation of electronic structures and optical properties by the density function theory (DFT) method within CASTEP program.46-47 For the exchange-correlation function, we used the Perdew-Burke-Ernzerhof (PBE) in generalized-gradient-approximation (GGA).48 The normconserving pseudopotential was adopted to treat the core-electron interactions.49 As valence electrons, the following orbital electrons were considered: Li-2s1, Ga-3d104s24p1, Cd-4d105s2, P-3s23p3, S-3s23p4. A cutoff energy of 800 eV and 650 eV for LiGa2PS6 and LiCd3PS6, and a k-point sampling of 2 × 4 × 2 for both were used to perform numerical integration of Brillouin zone in the calculation. During the optical property calculations, more than 270 empty bands were used to ensure the convergence of linear optical properties and SHG coefficients. The other parameters and convergent criteria were the default values of CASTEP code. The refractive indices can be calculated by n2( ω = ε (ω), here, ε (ω) is the dielectric function seen in other references.50-52 The second-order NLO properties were calculated based on length-gauge formalism within the independent particle approximation.53-54 Chen’s static formula was adopted, which has been derived by Rashkeev, et al. and later improved by Chen’s group.55-57

RESULTS AND DISCUSSION Crystal Structures. Both crystals can be viewed to be derived from AgGa2PS6 and belong to the polar monoclinic space group Cc, but they feature different structures. LiGa2PS6 is isostructural 6 / 27


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with the parent AgGa2PS6 (Figure 1), but LiCd3PS6 features a three-dimensional (3D) anionic framework of [Cd3S6]6− composed of 2D layers of [Cd2S5]6− and the bridging CdS4 tetrahedra with 1D tunnels along b axis filled by 1D chains of [LiPS6]6− (Figure 2a). The asymmetric unit of LiGa2PS6 contains one Li, one P, two Ga and six S atoms the asymmetric unit of LiCd3PS6 contains one Li, one P, three Cd and six S atoms, all of them are located at the general positions. All of the Li+, Ga3+, Cd2+ and P5+ cations are four coordinated by four S atoms in distorted tetrahedral geometry (Figures S2a and S2b). In LiGa2PS6, Ga(1)S4 and Ga(2)S4 tetrahedra are alternately interlinked via vertex-sharing to form the zigzag anionic chains of [Ga2S6]6− along the c direction (Figure S2c), which are further bridged by P(1)S4 tetrahedra into the 3D [Ga2PS6]− framework with triangular-shaped tunnels parallel to the c axis, which are filled by Li(1)atoms. In LiCd3PS6, Cd(1)S4 and Cd(2)S4 units are interlinked via corner-sharing to form a 2D layer of [Cd2S5]6− with 6-membered rings (MRs) in ab plane (Figure 2b). These [Cd2S5]6− layers are further bridged by Cd(3)S4 units into a 3D anionic framework of [Cd3S6]6− with 1D irregular tunnels along b axis. In addition, Li(1)S4 and P(1)S4 tetrahedra alternately connect each other via corner-sharing S(1) and S(2) to form 1D chains of [LiPS6]6− (Figure 2c), which is inserted into the above tunnels of the cadmium sulfide. In both compounds, the LiS4 and PS4 units link together to form the chains of [LiPS6]6− along b axis. In LiGa2PS6, the [LiPS6]6− chains connect with the [Ga2S6]6− chains to form the 3D network, whereas in LiCd3PS6, they are located at the tunnels of the 3D [Cd3S6]6−. In addition, the large radius of Cd2+ ion and more metals in LiCd3PS6 resulted in longer a and c axes and larger β angle compared with those of LiGa2PS6. As seen in tables S1 and S2, the P-S bands lengths fall in the normal range of 2.019(3)−2.059(2) 7 / 27



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Å and 2.016(8)−2.058(7) Å for LiGa2PS6 and LiCd3PS6, respectively, and the relevant S-P-S angles cover the range of 106.23(9)o−112.88(10)o and 106.2(3)o−114.0(3)o for LiGa2PS6 and LiCd3PS6, respectively. All of them are comparable with those reported in other metal thiophosphates.27-29 The Ga-S distances vary from 2.2403(17) to 2.3609(18) Å for LiGa2PS6 and the Cd-S bonds span from 2.440(6) to 2.792(6) Å for LiCd3PS6. The corresponding S−Ga-S and S−Cd−S angles lie between 92.64(6)o−119.89(7)o and 85.56(17)o−127.4(2)o, respectively, which are close to those observed in LiGaS2,31

Cs2CdGe3S8,58

CdGa2S4,26

etc.

In

LiGa2PS6,

Li−S

distances

are

between

2.427(16)−2.813(2) Å with S−Li−S angles of 84.8(4)o−125.9(6)o, and the corresponding bond and bond angles are 2.37(3)−2.82(3) Å and 85.9(9)o−121.6(12)o, respectively for LiCd3PS6. The calculated bond valence sums (BVS) of 0.799 (Li), 2.827-2.838 (Ga), 4.839 (P) for LiGa2PS6 and 0.917 (Li), 1.994-2.081 (Cd), 4.934 (P) for LiCd3PS6, respectively, revealed that the oxidation states of Li, Ga, Cd and P atoms are +1, +3, +2 and +5, respectively. Thermal Stability. Both compounds are of high thermal stability according to the differential scanning calorimetry (DSC) studies. LiGa2PS6 (or LiCd3PS6) shows obvious endothermic peaks at 662 oC (or 620 oC) during the heating process, and the exothermic peaks at 615 oC (or 605 oC) in the cooling curves (Figure S3). PXRD analyses of residuals of LiGa2PS6 indicate the existence of some Ga2S3, hence LiGa2PS6 decomposed at 662 oC and recrystallized during the cooling process. The PXRD reflection of LiCd3PS6 residuals is identical with the original one (Figure S4), but after DSC measurement, the sample still remains powder state and does not show distinct melt phenomenon. To confirm it, more samples in a fused silicon tube were heated to 800 oC for half an hour, and then directly taken out. But they did not melt. In addition, several single crystals of LiCd3PS6 were also picked and heated to 800 oC in a fused tube, but no change in shape was noticed. Therefore, the 8 / 27


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single endothermic and exothermic peaks would belong to the reversible phase-transition. Optical Properties. The UV/Vis/NIR diffuse reflectance spectra show high reflectance in the ranges of 330-2500 nm for LiGa2PS6 and 400–2500 nm for LiCd3PS6, and their UV cut-off edges are around 320 nm and 370 nm for LiGa2PS6 and LiCd3PS6, respectively, as shown in Figure 3. Based on their reflectance spectra, the band gaps of LiGa2PS6 and LiCd3PS6 are estimated to be 3.15 eV and 2.97 eV, respectively, which are significantly larger than those of AgGaS2 (2.6 eV) and AgGa2PS6 (2.75 eV). The IR absorption peaks at 601, 587, 564, 543, 530 cm-1 for LiGa2PS6 and 572, 537 cm-1 for LiCd3PS6 can be assigned to the vibrations of the P-S bonds (Figure S5).59 LDT and SHG Properties. Since powder LDT measurement has been applied to preliminarily evaluate the LDT value, the enlarged band gaps imply that these two crystals would display higher LDTs. Subsequently, powder LDTs of LiGa2PS6, LiCd3PS6 and AgGaS2 (as the reference) were performed by a Q-switched pulse laser with flat-top laser beam distribution. Due to the lack of large crystals with high optical quality, crystals in the particle range of 53-71 µm were employed to evaluate the LDT and SHG properties for both compounds. The LDTs of LiGa2PS6 (31.83 MW/cm2) and LiCd3PS6 (16.7 MW/cm2) are estimated to be 10.4 and 5.5 times that of AgGaS2 (3.05 MW/cm2) (Table 2), respectively. It is interesting to note that the substitution of Ag in AgGa2PS6 (5.1 × AgGaS2) by Li resulted in much higher LDT value for LiGa2PS6 (Table 3). Powder SHG measurements reveal that both LiGa2PS6 and LiCd3PS6 exhibit strong SHG responses under 1910 nm and 1064 nm laser radiations. Their SHG efficiencies are estimated to be about 0.5 × AgGaS2 (for LiGa2PS6), 0.8 × AgGaS2 (for LiCd3PS6) under 1910 nm laser radiation, and 6.0 × AgGaS2 (for LiGa2PS6) and 8.0 × AgGaS2 (for LiCd3PS6) under 1064 nm laser radiation (Figure 4). The phase matching experiments have not been performed due to the lack of samples with 9 / 27



other particle size ranges. Compared with other famous MIR NLO crystals (Table 3), both LiGa2PS6 and LiCd3PS6 display comparable SHG performances. Dipole Moments. For polar space group, dipole moments calculated by bond-valence method is an effective way to evaluate the relationship between SHG property and related individual functional units.60-61 The y-components of the polarizations from all building units canceled out completely for both compounds. And x- and z-components, specifically, are -15.86 (x-), -8.093 D (z-) (D = Debyes.) for LiGa2PS6, and -1.302 (x-), 11.228 D (z-) for LiCd3PS (Table S3). The net dipole moments of a unit cell for LiGa2PS6, LiCd3PS6, and AgGa2PS6 are 17.81, 11.30 and 18.04 D, respectively, and those of isostructural LiGa2PS6 and AgGa2PS6 are consistent with their SHG responses observed. However, compared with that of LiGa2PS6, LiCd3PS6 has a smaller net dipole moment but a stronger SHG response, which may be arisen from their different chemical compositions and crystal structures. Theoretical Studies. To disclose the SHG origins of these two compounds theoretically, first-principle calculations were performed. The calculated band structures shown in Figure S6 indicate that their band structures are very similar, the valence bands of both compounds are very dense and flat, but the conduction bands of them are loose and dispersive. The calculated band gaps are 1.84 eV (for LiGa2PS6) and 2.29 eV (for LiCd3PS6); hence the scissors of 1.31 eV and 0.68 eV for LiGa2PS6 and LiCd3PS6, respectively, have been adopted to match the experimental gaps in the following analyses. The partial density of states (PDOS) can be used to assign the band structure and understand the interactions among atoms in the compounds. From Figure S7, we can see that the PDOS diagrams of the two compounds behave very similar: the electronic states of P and Ga/Cd atoms are fully 10 / 27


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overlapped with those of S atoms in the energy regions ranging from VB to CB, showing the strong interactions of P−S, Ga−S and Cd−S bonds in the compounds. For them, the highest valence bands are dominantly constituted by the 3p nonbonding states of S atoms, while the lowest conduction bands are the empty P-3p and a few S-3p orbitals, so the band gaps of LiGa2PS6 and LiCd3PS6 are determined by S and P atoms. The nonlinear optical properties of LiGa2PS6 and LiCd3PS6 were also calculated. It is well-known that, for the monoclinic space group Cc, the dielectric principal axes are not consistent with the crystallographic axes in ac plane, so the principal axes transformation (rotation angles of LiGa2PS6 and LiCd3PS6 are -19.744562° and 24.535867°, respectively) must be performed, according to the formula reported previously.48-49 Then, the SHG coefficients are calculated employing the independent particle approximation (IPA) method and the highest tensors are calculated to be 2.04 × 10-8 esu and 2.34× 10-8 esu for d15 of LiGa2PS6 and d33 of LiCd3PS6, respectively, which are very close to the experimentally measured results (0.5 × AgGaS2 for LiGa2PS6 and 0.8 × AgGaS2 for LiCd3PS6).62 Furthermore, the frequency-dependent refractive indices of two compounds are also calculated and shown in Figure S8, the corresponding △n values at 1910 nm are 0.08 (for LiGa2PS6) and 0.02 (for LiCd3PS6). To investigate which energy levels give the contributions to the strong SHG effects, the spectral decompositions of the largest tensors (d15 for LiGa2PS6 and d33 for LiCd3PS6) are performed, as plotted in the bottommost panels of Figure S7. For d15 of LiGa2PS6, the most SHG-contributed regions are in the upper part of VB (-3.5 ~0 eV) and the lower part of CB (< 6.2 eV), the corresponding electronic states in PDOS graphs are mainly the S-3p and Ga-4p states in VB and the unoccupied S-3p, Ga-4s4p and P-3s3p states in CB. While for d33 of LiCd3PS6, the SHG-contributed 11 / 27



energy region is from -2.7 eV to 6.2 eV, which corresponds to S-3p, P-3s3p, Cd-5s5p and some Li-2s states in PDOS graphs. Furthermore, to accurately exhibit the real electronic orbitals which contribute to the SHG responses, SHG-weighed electron density analyses (SHG- density, for short) were also made (Figure 5). In VB, the main contributions come from the S-3p nonbonding states for both compounds; in CB, the main SHG-contributors are the unoccupied P-3p, some S-3p and Ga-4s states for LiGa2PS6, and the unoccupied P-3p and a bits of S-3p states for LiCd3PS6. In addition, we also calculated the SHG contribution values from the constituted groups/ions for the two compounds. On the basis of pseudopotential radii of atoms and electron density gradient method, each grid point in real space of the system can be distributed to the atoms. So the SHG weighed electron density can also be distributed to different atoms. For the shared atoms, the SHG density is further divided to the different groups based on the bond valence ratio. And the obtained contribution percentages of Li+, GaS4/CdS4 and PS4 groups are 0.11%, 55.43% and 44.24% for LiGa2PS6 and 0.25%, 50.97% and 48.71% for LiCd3PS6, respectively. It is found that the SHG responses of LiGa2PS6 and LiCd3PS6 originate dominantly from PS4 and GaS4/CdS4 groups, while the alkali metal Li+ cations give tiny contributions that can be ignored.

CONCLUSIONS In summary, two mid-infrared nonlinear optical metal thiophosphates of LiGa2PS6 (Cc) and LiCd3PS6 (Cc) with excellent SHG properties have been designed from AgGa2PS6 by a chemical substitution strategy. Although they adopt different structures, the experimental and theoretical analyses indicate that both crystals achieve a good balance between large SHG responses (0.5-0.8× AgGaS2 at 1910 nm) and high LDTs (5.5-10.4 ×AgGaS2). Therefore, both crystals are new 12 / 27


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promising MIR-NLO materials. We are trying to grow larger single crystals for both compounds in order to study their optical properties more deeply and accurately.

ASSOCIATED CONTENT Supporting Information X-ray crystallographic files in CIF format, Powder X-ray diffractions, infrared spectra, DSC curves, band structures, partial density of states, calculated refractive indices, list of the selected bond length and angles, the dipole moment calculations. Accession Codes CCDC 1832280 and 1832281 contain the supplementary crystallographic data of LiGa2PS6 and LiCd3PS6, respectively, 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 Author FAX: (+86)591-63173121; E-mail: [email protected]. ORCID Jiang-Gao Mao: 0000-0002-5101-8898 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS：： This work was supported by the National Natural Science Foundation of China (21701173 and 13 / 27



91622112) and the “Strategic Priority Research Program” of the Chinese Academy of Sciences (XDB20000000) and the “100 Talents Project” of Fujian Province.

DEDICATION Dedicated to Prof. Xin-Tao Wu on the occasion of his 80 thbirthday.

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Table 1. Crystallographic data and structure refinements for LiGa2PS6 and LiCd3PS6. formula LiGa2PS6 LiCd3PS6 fw 369.71 567.47 temp, K 293(2) 293(2) space group Cc, (No. 9) Cc, (No. 9) a /Å 11.3737(14) 12.1780(18) b/Å 6.9461(7) 6.9428(9) c/ Å 11.4010(13) 12.2405(15) β/deg 107.942(13) 110.593(15) Volume/Å3 856.91(17) 968.8(2) Z 4 4 −3 Dcalc, g/cm 2.866 3.891 µ,mm−1 7.844 7.904 2 GOF on F 1.036 1.031 Flack factor 0.03(16) -0.07(12) R1, wR2 [I >2σ(I)] 0.0259, 0.0608 0.0597, 0.1598 R1, wR2 (all data) 0.0269, 0.0616 0.0608, 0.1609 a 2 2 2 R1 = ∑||Fo|-|Fc||/∑|Fo|, wR2 ={∑w[(Fo) -(Fc) ] /∑w[(Fo)2]2}1/2 Table 2. LIDTs of LiGa2PS6, LiCd3PS6 and reference AgGaS2

Sample

damage energy (mJ)

Spot area (mm2)

τp (ns)

LiGa2PS6 LiCd3PS6 AgGaS2

36.6 19.2 3.5

0.1018 0.1018 0.1018

10 10 10

damage threshold (MW/cm2) 31.83 16.70 3.05

Table 3. Comparison of the important parameters of selected crystals (AgGa2PS6, LiGa2PS6 and LiCd3PS6 were tested using powder samples). Compound AgGaS29 LiGaS263 LiInS264 BaGa4S736 AgGa2PS630 LiGa2PS6 LiCd3PS6

dij (×AgGaS2) 1.0 0.4 0.6 1.0 1.0 0.5 0.8

LDT (×AgGaS2) 1.0 11 2.5 3.0 5.1 10.4 5.5

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Eg (eV) 2.64 4.15 3.57 3.54 2.75 3.15 2.97


Figure 1.View of 3D structure of AgGa2PS6 (a), LiGa2PS6 (b), and the transition of their structures.

Figure 2. View of 3D structure of LiCd3PS6 down b-axis (a) (both blue and green tetrahedra represent CdS4 units), a 2D layer of [Cd2S4]4− in ab-plane (b), and a 1D chain of [LiPS6]6− along b-axis.

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Figure 3. UV−vis −NIR diffuse reflectance spectra of LiGa2PS6 and LiCd3PS6, and the relevant band gaps of LiGa2PS6 and LiCd3PS6 (inset).

Figure 4. The measured SHG signals of LiGa2PS6, LiCd3PS6, AgGaS2 and AgGa2PS6 in particle range of 53-71 µm under laser radiations λ = 1910 nm and λ = 1064 nm.

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Figure 5. The calculated SHG density of d15 for LiGa2PS6 (a: VB, b: CB) and d33 for LiCd3PS6 (c: VB, d: CB).

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LiGa2PS6 and LiCd3PS6: Molecular Designs of Two New Mid

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