Compared with centrosymmetric solids, acentric solids draw growing attention from the research community due to the chance of studying directional physical properties such as second harmonic generation [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] , nonreciprocal responses [16][17][18][19][20][21] , etc. From a chemistry perspective, various strategies have proved efficient to influence the creation of a noncentrosymmetric (NCS) structure, including incorporating second order Jahn-Teller distortion [22][23] and conjugated structure motifs such as distorted FeS4 14,24 or GeS4 tetrahedra 25,26 , WO5 27 , LaO6Br3 28 , etc. Another systematic way to influence a NCS structure is through incorporating atoms with stereochemically active lone pairs (SCALP), including Tl + , Sn 2+ , Pb 2+ , As 3+ , Sb 3+ , Bi 3+ , Te 4+ , etc., with many successful examples such as Tl + in Tl4(OH)2CO3 29 , TlXF3 (X = Be, Sr) 30 ; Sn 2+ in Sn2P2S6 [31][32][33] , Sn2PO4X (X = F, Cl) 34 , Sn[B2O3F2] 35 ; Pb 2+ in Pb2P2S6 36 , Pb3P2S8 15 , Pb13O6Cl4Br10 37 , Pb13O6Cl7Br7 37 , and Pb13O6Cl9Br5 37 ; As 3+ in K3AsS4 38 , Li3AsS3 38 , Pb9As4S15 38 and Ag3AsS3 38 ; Sb 3+ in KSbP2S6 39 , La2CuSbS5 40 ; Bi 3+ in BiB3O6 41 , KBiP2S6 39 , etc. In addition to affecting crystal structure, SCALP also play a role in enhancing second harmonic generation (SHG) response [42][43][44] . Hence, the study of how SCALP contributes to the formation of acentric structures is an important topic. In this work, we report two heteroanionic chalcohalides, La3AsS5Br2 and La5As2S9Cl3. La3AsS5Br2 and La5As2S9Cl3, which exhibit structural similarity, crystallize in an acentric structure and centrosymmetric structure, respectively. The alignment of SCALP plays an important role in the structural difference, which is confirmed by structure analysis and ELF calculations. This work confirms the importance of the alignment of SCALP in influencing crystal structure. The synthesis, crystal growth, crystal and electronic structures, linear and nonlinear optical properties of two heteroanionic chalcohalides, La3AsS5Br2 and La5As2S9Cl3, are summarized in this work.
Synthesis: All starting materials were stored and used in an Arfilled glove box. Starting materials were used as received: La powder (Alfa Aesar, 99.7%), As powder (Fisher Scientific, 99%), S powder (Alfa Aesar, 99.5%), LaBr3 (Alfa Aesar, 99.9%), NaBr (Fisher Scientific, 99+%), LaCl3 (Alfa Aesar, 99.9%), NaCl (Sigma-Aldrich, ≥99%). La2S3 precursor was produced via stoichiometric ratios of La and S sealed under vacuum in a carbonized silica ampule annealed in a muffle furnace at 773K for 96h, then opened and stored in the glovebox.
Synthesis: A molar ratio of La2S3:LaBr3:As:S=7:4:6:9 (total 0.4g) with a mixture of LaBr3:NaBr = 0.66:0.33 (total 0.4g) as a salt flux was placed into a carbonized silica ampule of a diameter of 9mm and flame sealed under high vacuum (<100 mTorr) and placed into a muffle furnace. The ampule was heated to 1123K in 20 hours, annealed at that temperature for 96 hours, and cooled to room temperature in 24 hours. The ampule was opened, and the sample was washed with deionized (DI) water to remove the salt flux, leaving yellow-green mm-sized crystals. La5As2S9Cl3 Synthesis: A molar ratio of La2S3:LaCl3:As:S = 2:1:2:3 (total 0.4g) with a mixture of LaCl3:NaBr = 0.66:0.33 (total 0.4g) as a salt flux was placed into a carbonized silica ampule and sealed under vacuum and heated as the same parameters and temperature profile as La3AsS5Br2. The sample was washed with DI water to remove the flux, leaving green mmsized crystals.
Single Crystal X-Ray Diffraction (SXRD): Data collections were performed at room temperature for La3AsS5Br2 and La5As2S9Cl3 using a Bruker X8 Spectrometer diffractometer equipped with Cu source (λ = 1.5406 Å). Data reduction and integration, together with global unit cell refinements, were performed in the APEX4 software. 45 Multi-scan absorption corrections were applied. 45 The structures were solved by direct methods and refined by full matrix least-squares methods on F 2 using the SHELX package with anisotropic displacement parameters for all atoms. 46 Details of the data collection and structure refinement are provided in Table 1. Atomic coordinates and selected distances are listed in Tables S1 and S2. Crystallographic data for La3AsS5Br2 and La5As2S9Cl3 have been deposited to the Cambridge Crystallographic Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ, UK. Copies of the data can be obtained free of charge by quoting the depository numbers CCDC-2281773 (La3AsS5Br2) and CCDC-2281774 (La5As2S9Cl3). Table 1. Selected crystal data and unit cell parameters for La3AsS5Br2 and La5As2S9Cl3
Empirical Formula
Formula weight 811.77 1239.28 Temperature 296(2) K Radiation, wavelength Cu-K𝛼, 1.54718 Å [47][48] (KM*E) 2 and (KM*E) 1/2 , were applied to estimate direct and indirect band gap, respectively. TB-LMTO-ASA simulations: Electronic structures, including band structures and density of states (DOS), of La3AsS5Br2 were calculated using the tight binding-linear muffin tin orbitalsatomic sphere approximation (TB-LMTO-ASA) program. [49][50] The von-Barth-Hedin exchange potential was employed for the LDA calculations. 49 The radial scalar-relativistic Dirac equation was solved to obtain the partial waves. The density of states and band structures were calculated after converging the total energy on a dense k-mesh of La3AsS5Br2 (16×16×8 points with 1088 irreducible k-points). Second Harmonic Measurements: Using the Kurtz and Perry method, 51 powder SHG responses of La3AsS5Br2 were investigated by a Q-switch laser (2.09 μm, 3 Hz, 50 ns) with various particle sizes, including 38.5-54, 54-88, 88-105, 105-150, and 150-200 μm. Homemade AgGaS2 was selected as the reference. The lab-synthesized AgGaS2 crystals were ground to the same size range as La3AsS5Br2 compound.
Mm-sized crystals of La3AsS5Br2 and La5As2S9Cl3 were successfully grown via a salt-flux method (Figure S1). The mmsized crystals were collected after washing with DI water to remove the salt flux. La3AsS5Br2: La3AsS5Br2 crystallizes in the acentric monoclinic space group Cc (No. 9), which belongs to the known La3AsS5Cl2 structure type. 52 Other isostructural compounds of Pr3AsS5Cl2 53 , La3SbS5Cl2 54 and Ce3SbS5Cl2, 55 Please do not adjust margins Please do not adjust margins full occupancy. The three-dimensional framework of La5As2S9Cl3 is constructed by [La1S4Cl4] bicapped trigonal prisms, [La2S5Cl2] capped trigonal prisms, [La3S6Cl2] bicapped trigonal prisms and [As1S3] trigonal pyramids, which are interlinked to each other. The polyhedral structure model and constructing units of La5Sb2S9Cl3 is shown in Figure 1d and Figure S5, respectively.
The La-S interactions within La3AsS5Br2 fall into the range of 2.87(1)-3.15 (1) Å, which are comparable to the La-S interactions within La5As2S9Cl3 of 2.81(7)-3.14 (4) Å and many lanthanum-sulfide compounds such as La4Ge3S12 (2.864-3.245 Å) 25 , La6Pd0.96Si2S14 (2.817-3.164 Å) 56 , La5Sb2S9Cl3(2.841-3.106 Å) 55 , La3SbS5Cl2 (2.833-3.117 Å) 55 , La3LiSnS7 (2.848-3.297 Å) 57 , etc. The La-Br interactions within La3AsS5Br2 of 3.04 (3)-3.44(3) Å are longer than La-Cl interactions within La5As2S9Cl3 of 2.83(1)-2.97(1) Å, which is expected due to the larger ionic size of Br -than Cl -. The As-S interactions are 2.26(1)-2.27(1) Å and 2.25(5)-2.28(5) Å for La5As2S9Cl3 and La3AsS5Br2, respectively, which are close to many arsenicsulfide compounds such as As4S4 (2.21-2.27Å) 58 , Ba2(As1.5Bi0.5)S5 (2.21-2.43Å) 59 , CsCu2AsS3(2.24-2.27 Å) 60 , Cs2Ag2As2S5 (2.24-2.31 Å) 61 , KCu2AsS3 (2.26-2.30 Å) 62 , etc. In addition to comparable interatomic distances, each unit cell of La5As2S9Cl3 and La3AsS5Br2 is constructed by the same polyhedra: two [LaX8] bicapped trigonal prisms, one [LaX7] capped trigonal prism, and one [AsS3] trigonal pyramid, where the X represents various combinations of S atoms and Cl atoms or Br atoms (Figures S3 and S4). There are two axes of La5As2S9Cl3 with similar length to La5As2S9Cl3 (Table 1). The structural similarity between La5As2S9Cl3 and La3AsS5Br2 is obvious. What is the chemical reason for the structural difference between La5As2S9Cl3 and La3AsS5Br2? Understanding how to form acentric structures is very important for studying directional physical properties . and 2b, respectively. Figure 2a shows the alignment of SCALP of [AsS3] motifs in the same direction in the structure of La3AsS5Br2, producing a directional dipole moment which contributes to its acentric structure. Figure 2b shows the alignment of SCALP of [AsS3] motifs in opposing directions in La5As2S9Cl3, resulting in a cancelled dipole moment, contributing to its centrosymmetric structure. Please also note, the reason for forming acentric structures is complex, the connectivity of polyhedra within La3AsS5Br2 and La5As2S9Cl3 is also distinct, which also contributes to the structural difference. Second harmonic generation measurements were employed to confirm the acentric nature and centrosymmetric nature of La3AsS5Br2 and La5As2S9Cl3, respectively. with AGS measured at the same conditions as a reference. SHG measurements were taken for La3AsS5Br2 and La5As2S9Cl3 in varying particle sizes and compared to AGS (Figure 3a), which confirmed the acentric nature of La3AsS5Br2. La3AsS5Br2 is a Type-I phase matchable compound, in which the SHG intensity increases with increasing particle size. The SHG response of La3AsS5Br2 is about 0.23× AGS for the sample of 150-200 𝜇m particle size. There was no signal of SHG response detected from La5As2S9Cl3 (Figure 3b), which indicates its centrosymmetric nature and agrees well with single crystal X-ray diffraction refinement results. Even though the SHG response of La3AsS5Br2 is not very high, the easily grown nature, excellent air stability, and moderate bandgap (vide infra) still make La3AsS5Br2 attractive for infrared nonlinear application. To further study the properties of La3AsS5Br2, electronic structures were calculated and shown in The band structure of La3AsS5Br2 is presented in Figure 4 top. The top of the valence band is found at the 𝛤 points and the bottom of the conduction band is located at the 𝛧 points. This predicts La3AsS5Br2 as an indirect band gap semiconductor with a theoretical band gap of 2.27 eV. The semiconductor nature of La3AsS5Br2 is also supported by the charge-balanced formula (La 3+ )3(As 3+ )(S 2-)5(Br -)2 by assigning a formal charge of 3+ to the La atoms, 3+ to the As atoms, 2-to the S atoms, and 1-to the Br atoms. The trivalent nature of the As atoms are confirmed with the presence of SCALP (Figure 2). The charge balanced formula (La 3+ )5(As 3+ )2(S 2-)9(Cl -)3 of La5As2S9Cl3 can be established by the same way as La3AsS5Br2. As shown in the DOS in Figure 4 bottom and Figure S6, the top of the valence band is mainly contributed by S 2p orbitals and Br 2p orbitals, with some contributions by La orbitals. The bottom of the conduction band is composed of As 3p orbitals, S 2p orbitals, and La orbitals. The optical properties of La3AsS5Br2 are predominantly contributed by As-S interactions and La-S interactions, with some contributions from La-Br interactions. To improve the SHG response, bandgap engineering such as replacing As by Sb or S by Se would be an applicable way 25 . The semiconducting nature of La3AsS5Br2 was verified by UV-Vis spectrum measurements. Linear Optical Properties The optical bandgaps of La3AsS5Br2 and La5As2S9Cl3 were determined via solid-state UV-Vis diffuse reflectance spectroscopy (Figure 5). La3AsS5Br2 and La5As2S9Cl3 both possess strong absorption edges around 400-475 nm, which agrees well with their green color appearance. La3AsS5Br2 was determined to be an indirect bandgap semiconductor via theoretical calculations (Figure 4). The indirect allowed transition of La3AsS5Br2 was determined to be 2.83(5) eV for La3AsS5Br2, which is close to the theoretically predicted value of 2.27 eV. The indirect allowed transition of La5As2S9Cl was determined to be 2.87(5) eV. For infrared nonlinear optical applications, large laser damage threshold (LDT) is also required, which is proportional to bandgap. The incorporation of electronegative anions into compounds is shown to result in a large band gap 39 . The moderate band gap of La3AsS5Br2 might result in high LDT for La3AsS5Br2, which is undergoing analysis now.
Two arsenic-containing lanthanum chalcohalides have been synthesized for the first time: La3AsS5Br2 and La5As2S9Cl3. Both compounds contain trigonal pyramidal [AsS3] motifs, with the centrosymmetric La5As2S9Cl3 aligning the motifs in opposite directions and the noncentrosymmetric La3AsS5Br2 aligning the motifs in the same direction. The alignment of the motifs contributes to the shifting of the structure from centrosymmetric in the chloride to noncentrosymmetric in the bromide. La5As2S9Cl3 is shown to have an indirect bandgap of 2.87(5) eV. La3AsS5Br2 is shown to have an indirect bandgap of 2.83(5) eV, and a moderate SHG response of 0.23×AGS. These heteroanionic compounds exhibit a good example to study how the alignment of SCALP of [AsS3] motifs affects crystal structure. Moderate SHG response, easy growth of large crystals, excellent ambient stability, and moderate bandgap of La3AsS5Br2 indicates its potential application as an infrared nonlinear optical material.