LiGa2PS6 and LiCd3PS6: Molecular Designs of Two New Mid

Article Cite This: Chem. Mater. 2018, 30, 3901−3908

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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*,† †

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, People’s Republic of China ‡ University of Chinese Academy of Sciences, Beijing 100039, People’s Republic of China

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ABSTRACT: To simultaneously maintain large second harmonic generation (SHG) and high laser damage thresholds (LDTs) for mid- 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 the 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 the 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 53−71 μm) and high LDTs (5.5−10.4 × AgGaS2); therefore both crystals are new promising mid-IR NLO materials.

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INTRODUCTION Nonlinear optical (NLO) materials have attracted intensive scientific attention as a significant component of optical parametric oscillation (OPO) to produce tunable coherent laser sources.1−7 In the mid- 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 mid-infrared (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 (Δn) (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. However, because of the inverse relationship of LDT and SHG efficiency, it is still a great challenge to develop an MIR NLO crystal with a suitable balance among the above qualifications. During the past decades, metal chalcogenides have been ecstatically investigated as a rich family of MIR NLO materials, because of their wide compositional flexibility and high transmittance in the MIR range,1−3 as well as a large number of noncentrosymmetric (NCS) structures induced by the © 2018 American Chemical Society

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 KHg4 Ga 5Se12,18 RbGeP4 Se 12 ,19 Ba 8Sn4S 15 ,20 BaGa 4Se7 ,21 Ba3CsGa5Se10Cl2,22 Cu2HgGeS4,23 Ag2CdGeS4,24 MGa2S4 (M = Zn, Cd),25,26 etc. However, materials that meet all of the fundamental conditions of practical applications are still scarce. On the basis of 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 growth, as exemplified by LiZnPS4,27 AgZnPS4,27 KZrPS6,28 and [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 exemplified by LiGaS2 (4.2 eV)31 versus 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 Received: April 9, 2018 Revised: May 16, 2018 Published: May 17, 2018 3901

DOI: 10.1021/acs.chemmater.8b01470 Chem. Mater. 2018, 30, 3901−3908

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Chemistry of Materials

Single-Crystal Structure Determinations. Needle-shaped (LiGa2PS6) or cubic-shaped (LiCd3PS6) single crystals were glued onto glass fibers and mounted on a SuperNova (Mo) X-ray source (λ = 0.710 73 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 multiscan 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 the 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 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.

to replace the silver in AgGa2PS6, which led to LiGa2PS6 with an enlarged Eg (3.15 eV) and suitable Δn (0.08), but slightly decreased SHG effect. Furthermore, for an improvement in 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.

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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 further purification. Microprobe elemental analyses were performed using field-emission scanning electron microscopy (FESEM, JSM6700F) with energy-dispersive X-ray spectroscopy (EDS, Oxford INCA). Inductively coupled plasma (ICP) measurements were investigated on an Ultima 2 inductively coupled plasma OES spectrometer. Powder Xray diffraction (PXRD) patterns were recorded on an XPERT-MPD θ−2θ diffractometer equipped with Cu Kα radiation (λ = 1.540 598 Å) within the 2θ range 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 330−2500 nm. A BaSO4 plate was used as a standard (100% reflectance). The band gaps were estimated on the basis of the reflectance spectra by using the Tauc plot method.39 The infrared (IR) spectra were recorded as KBr pellets on a VERTEX 70 FTIR spectrophotometer in the range 4000−400 cm−1. Powder LDTs of LiGa2PS6, LiCd3PS6, and AgGaS2 (reference) in the size range 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 densities 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 the Kurtz and Perry method by the incident 1910 and 1064 nm laser.40 LiGa2PS6 and LiCd3PS6 with particle sizes of 53−71 μm were selected, and then 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 hightemperature 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 held for 2 days, subsequently heated to 650 °C for LiGa2PS6 (610 °C for LiCd3PS6) in 10 h and held 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, and have been confirmed by PXRD analyses (Figure S1). The EDS elemental analyses of several single crystals shows 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.

Table 1. Crystallographic Data and Structure Refinements for LiGa2PS6 and LiCd3PS6 formula fw temp, K space group a, Å b, Å c, Å β, deg volume, Å3 Z Dcalc, g/cm3 μ, mm−1 GOF on F2 Flack factor R1, wR2a [I > 2σ(I)] R1, wR2 (all data)

LiGa2PS6 369.71 293(2) Cc, (no. 9) 11.3737(14) 6.9461(7) 11.4010(13) 107.942(13) 856.91(17) 4 2.866 7.844 1.036 0.03(16) 0.0259, 0.0608 0.0269, 0.0616

LiCd3PS6 567.47 293(2) Cc, (no. 9) 12.1780(18) 6.9428(9) 12.2405(15) 110.593(15) 968.8(2) 4 3.891 7.904 1.031 −0.07(12) 0.0597, 0.1598 0.0608, 0.1609

R 1 = ∑||F 0 | − |F c ||/∑|F 0|, wR 2 = {∑w[(F 0 ) 2 − (F c) 2 ] 2 / ∑w[(F0)2]2}1/2.

a

Computational Descriptions. Single-crystal structural data of LiGa2PS6 and LiCd3PS6 were applied for calculations of electronic structures and optical properties by the density function theory (DFT) method within the CASTEP program.46,47 For the exchangecorrelation function, we used the Perdew−Burke−Ernzerhof (PBE) in the 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 and 650 eV for LiGa2PS6 and LiCd3PS6, respectively, and a k-point sampling of 2 × 4 × 2 for both were used to perform numerical integration of the Brillouin zone in the calculations. 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 the 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 on the basis of 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

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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 with the parent AgGa2PS6 (Figure 1), but LiCd3PS6 features a three-dimensional (3D) anionic framework 3902

DOI: 10.1021/acs.chemmater.8b01470 Chem. Mater. 2018, 30, 3901−3908

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Chemistry of Materials

S2a,b). 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 6membered rings (MRs) in the 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 the b-axis. In addition, Li(1)S4 and P(1)S4 tetrahedra alternately connect with each other via corner-sharing S(1) and S(2) to form 1D chains of [LiPS6]6− (Figure 2c), which are 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 the 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 the 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 band lengths fall in the normal range 2.019(3)−2.059(2) and 2.016(8)−2.058(7) Å for LiGa2PS6 and LiCd3PS6, respectively, and the relevant S−P−S angles cover the range 106.23(9)−112.88(10)° and 106.2(3)− 114.0(3)° 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 in the extent of 92.64(6)°−119.89(7)° and 85.56(17)°−127.4(2)°, 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) and 2.813(2) Å with S−Li−S angles of 84.8(4)−125.9(6)°, and the corresponding bond lengths and angles are 2.37(3)−2.82(3) Å and 85.9(9)−121.6(12)°, respectively, for LiCd3PS6. The calculated bond valence sums (BVS) of 0.799 (Li), 2.827−2.838 (Ga), and 4.839 (P) for LiGa2PS6, and 0.917 (Li), 1.994−2.081 (Cd), and 4.934 (P) for

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

of [Cd3S6]6− composed of 2D layers of [Cd2S5]6− and the bridging CdS4 tetrahedra with 1D tunnels along the b-axis filled by 1D chains of [LiPS6]6− (Figure 2a). The asymmetric unit of

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

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 (Figure

Figure 3. UV−vis−NIR diffuse reflectance spectra of LiGa2PS6 and LiCd3PS6, and the relevant band gaps of LiGa2PS6 and LiCd3PS6 (inset). 3903

DOI: 10.1021/acs.chemmater.8b01470 Chem. Mater. 2018, 30, 3901−3908

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Chemistry of Materials 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 °C (or 620 °C) during the heating process, and the exothermic peaks at 615 °C (or 605 °C) in the cooling curves (Figure S3). PXRD analyses of residuals of LiGa2PS6 indicate the existence of some Ga2S3; hence, LiGa2PS6 decomposed at 662 °C 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 in the powder state and does not show a distinct melt phenomenon. To confirm this, more samples in a fused silicon tube were heated to 800 °C for half an hour, and then directly taken out. However, they did not melt. In addition, several single crystals of LiCd3PS6 were also picked and heated to 800 °C in a fused tube, but no change in shape was noticed. Therefore, the single endothermic and exothermic peaks would belong to the reversible phasetransition. Optical Properties. The UV/vis/NIR diffuse reflectance spectra show high reflectance in the ranges 330−2500 nm for LiGa2PS6 and 400−2500 nm for LiCd3PS6, and their UV cutoff edges are around 320 and 370 nm for LiGa2PS6 and LiCd3PS6, respectively, as shown in Figure 3. On the basis of their reflectance spectra, the band gaps of LiGa2PS6 and LiCd3PS6 are estimated to be 3.15 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, and 530 cm−1 for LiGa2PS6, and 572 and 537 cm−1 for LiCd3PS6, can be assigned to the vibrations of the P−S bonds (Figure S5).59 LDT and SHG Properties. Since a 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. Because of the lack of large crystals with high optical quality, crystals in the particle range 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 a much higher LDT value for LiGa2PS6 (Table 3). Powder SHG measurements reveal that both LiGa2PS6 and LiCd3PS6 exhibit strong SHG responses under 1910 and 1064 nm laser radiations. Their SHG efficiencies are estimated to be about 0.5 × AgGaS2 (for LiGa2PS6) and 0.8 × AgGaS2 (for LiCd3PS6) under 1910 nm laser radiation, and 6.0 × AgGaS2 (for LiGa2PS6) and 8.0 × AgGaS2 (for LiCd3PS6) under 1064

Table 3. Comparison of the Important Parameters of Selected Crystals (AgGa2PS6, LiGa2PS6, and LiCd3PS6 Were Tested Using Powder Samples)

damage energy (mJ)

spot area (mm2)

τp (ns)

damage threshold (MW/cm2)

LiGa2PS6 LiCd3PS6 AgGaS2

36.6 19.2 3.5

0.1018 0.1018 0.1018

10 10 10

31.83 16.70 3.05

dij (×AgGaS2)

LDT (×AgGaS2)

Eg (eV)

AgGaS29 LiGaS263 LiInS264 BaGa4S736 AgGa2PS630

1.0 0.4 0.6 1.0 1.0 0.5 0.8

1.0 11 2.5 3.0 5.1 10.4 5.5

2.64 4.15 3.57 3.54 2.75 3.15 2.97

LiGa2PS6 LiCd3PS6

nm laser radiation (Figure 4). The phase-matching experiments have not been performed because of the lack of samples with

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

other particle size ranges. Compared with other famous MIR NLO crystals (Table 3), both LiGa2PS6 and LiCd3PS6 display comparable SHG performances. Dipole Moments. For a polar space group, dipole moments calculated by the bond−valence method is an effective way to evaluate the relationship between the SHG property and related individual functional units.60,61 The ycomponents of the polarizations from all building units canceled out completely for both compounds. x- and zcomponents, specifically, are −15.86 (x) and −8.093 (z) D (D = debye) for LiGa2PS6, and −1.302 (x) and 11.228 (z) D 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 LiGa2PS6, LiCd3PS6 has a smaller net dipole moment but a stronger SHG response, which may arise from their different chemical compositions and crystal structures. Theoretical Studies. For a disclosure of 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 their conduction bands 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 and 0.68 eV for LiGa2PS6 and LiCd3PS6, respectively, have been adopted to match the experimental gaps in the following analyses.

Table 2. LDTs of LiGa2PS6, LiCd3PS6, and Reference AgGaS2 sample

compound

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DOI: 10.1021/acs.chemmater.8b01470 Chem. Mater. 2018, 30, 3901−3908

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Chemistry of Materials

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

corresponding Δn values at 1910 nm are 0.08 (for LiGa2PS6) and 0.02 (for LiCd3PS6). For an investigation of 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 SHGcontributed regions are in the upper part of the VB (−3.5−0 eV) and the lower part of the CB (