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Hong Yan, Kotaro Fujii, [Yoshitaka Matsushita](https://orcid.org/0000-0002-4968-8905), [Masatomo Yashima](https://orcid.org/0000-0001-5406-9183), [Kazunari Yamaura](https://orcid.org/0000-0003-0390-8244), [Yoshihiro Tsujimoto](https://orcid.org/0000-0003-2140-3362)

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[Flux crystal growth, structure, and optical properties of non-centrosymmetric oxysulfides Ln<sub>3</sub>Ga<sub>3</sub>Ge<sub>2</sub>S<sub>3</sub>O<sub>10</sub> (Ln = La, Ce, Pr, Nd)](https://mdr.nims.go.jp/datasets/d0730f28-f1c9-4ec7-b69c-cd7883562179)

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Flux crystal growth, structure, and optical properties of non-centrosymmetric oxysulfides Ln3Ga3Ge2S3O10 (Ln = La, Ce, Pr, Nd)DaltonTransactionsPAPERCite this: Dalton Trans., 2025, 54,10433Received 9th May 2025,Accepted 4th June 2025DOI: 10.1039/d5dt01100krsc.li/daltonFlux crystal growth, structure, and opticalproperties of non-centrosymmetric oxysulfidesLn3Ga3Ge2S3O10 (Ln = La, Ce, Pr, Nd)†Hong Yan,a,b Kotaro Fujii,c,d Yoshitaka Matsushita, e Masatomo Yashima, cKazunari Yamaura a,b and Yoshihiro Tsujimoto *a,bSingle crystals of the non-centrosymmetric oxysulfides Ln3Ga3Ge2S3O10 (Ln = La, Ce, Pr, Nd) in a hexag-onal space group P6̄2c were grown by a flux crystal growth method using a eutectic BaCl2–NaCl moltensalt. Single-crystal X-ray diffraction analysis of them revealed that the Ga and Ge atoms were located onthe 6g and 4f sites, which were tetrahedrally coordinated with two O and two S atoms, and four O atoms,respectively. In the structure, the GaS2O2 and GeO4 tetrahedra form 11[Ga3S3O3] triangular tubes andGe2O7 tetrahedral dimers aligned along the c axis, which are surrounded by LaS2O6 square prisms in theab plane. Neutron powder diffraction studies on polycrystalline samples of La3Ga3Ge2S3O10 andNd3Ga3Ge2S3O10 which were prepared by high-temperature solid state reactions supported the Ga/Gecation order determined by the single-crystal structure analysis. UV-Vis-NIR absorption spectra revealedband gaps larger than 4.60 eV for La3Ga3Ge2S3O10, Pr3Ga3Ge2S3O10, and Nd3Ga3Ge2S3O10, whileCe3Ga3Ge2S3O10 was found to have a small band gap value of 3.51 eV because of the Ce-4f 1 electronicconfiguration.IntroductionMetal oxychalcogenides, in which oxide and chalcogenide ionscoexist in one structure, have provided a playground for solid-state scientists to explore fascinating properties because of thelarge differences in their ionic radii, electronegativities, oxi-dation states, and polarizabilities.1,2 One recent significantadvantage of oxychalcogenide compounds is the design ofnonlinear optical (NLO) materials.3,4 Stabilizing metal-cen-tered heteroleptic coordination environments containing bothoxide and chalcogenide ligands offers a chance to significantlyincrease the optical band gaps and second harmonic gene-ration (SHG) responses compared with chalcogenides, becauseof the presence of O 2p orbitals in the valence band maximumand acentric polyhedral units with polar atomic displacement.Since the discovery of the first phase-matchable oxyselenideBaGeOSe2 and oxysulfide SrZn2S2O with strong SHGintensities,5,6 a number of new oxychalcogenides have beenreported to show good balanced properties with high SHGresponses, high laser damage thresholds, and wide opticalband gaps.3,7 While most of the reported NLO oxychalcogenidecompounds were focused on applications in the infraredregion because of their relatively small band gap values (<4eV),7 a new oxysulfide La3Ga3Ge2S3O10 consisting of (Ga/Ge)O4and (Ga/Ge)S2O2 tetrahedra exhibited promising properties forapplications in the UV regions (Fig. 1): the exceptionally largeband gap value (4.70 eV), short cutoff edge (250 nm), andstrong SHG response twice that of the KH2PO4 benchmarkcompound.8 The fundamental origin of the large bandgap andstrong SHG response can be attributed to the lack of an all-sulfide coordination environment and the presence of a het-eroleptic coordination environment,9,10 as in [Ba2F2][Ge2O3S2](1.4 × AgGaS2),11 Nd3Ga3Ge2S3O10 (1.7 × KDP, 0.8 × AgGaS2),12and La3Ga3Si2S3O10 (1.7 × KDP, 0.3 × AgGaS2).13Single-crystal X-ray diffraction (SCXRD) analyses ofLa3Ga3Ge2S3O10, La3Ga3Si2S3O10, and Nd3Ga3Ge2S3O10 havedemonstrated that they are isostructural and adopt a hexagonalunit cell in the space group P6̄2c.8,12,13 However, they exhibiteddifferent atomic distribution patterns, as described below. In the†Electronic supplementary information (ESI) available. CCDC 2449159, 2449160,2449238 and 2449239. For ESI and crystallographic data in CIF or other elec-tronic format see DOI: https://doi.org/10.1039/d5dt01100kaResearch Center for Materials Nanoarchitechtonics (MANA), National Institute forMaterials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan.E-mail: TSUJIMOTO.Yoshihiro@nims.go.jpbGraduate School of Chemical Sciences and Engineering, Hokkaido University, North10 West 8, Kita-ku, Sapporo, Hokkaido 060-0810, JapancDepartment of Chemistry, School of Science, Institute of Science Tokyo, 2-12-1-W4-17, Ookayama Meguro-ku, Tokyo, 152-8551, JapandFaculty of Bioscience and Applied Chemistry, Department of Chemical Science andTechnology, Hosei University, 3-5-4 Kajino-cho, Koganei, Tokyo 184-8584, JapaneMaterials Analysis Station, NIMS, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, JapanThis journal is © The Royal Society of Chemistry 2025 Dalton Trans., 2025, 54, 10433–10440 | 10433Open Access Article. Published on 12 June 2025. Downloaded on 7/2/2025 1:47:57 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article OnlineView Journal  | View Issuehttp://rsc.li/daltonhttp://orcid.org/0000-0002-4968-8905http://orcid.org/0000-0001-5406-9183http://orcid.org/0000-0003-0390-8244http://orcid.org/0000-0003-2140-3362https://doi.org/10.1039/d5dt01100khttps://doi.org/10.1039/d5dt01100khttp://crossmark.crossref.org/dialog/?doi=10.1039/d5dt01100k&domain=pdf&date_stamp=2025-06-26http://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d5dt01100khttps://pubs.rsc.org/en/journals/journal/DThttps://pubs.rsc.org/en/journals/journal/DT?issueid=DT054026latter two phases, the Ga and Ge/Si atoms were located onWyckoff positions 6g and 4f, respectively, to take the GaS2O4 and(Ge/Si)O4 tetrahedral geometries (Fig. 1). These tetrahedraformed 11[Ga3S3O3] triangular tubes and (Ge/Si)2O7 tetrahedraldimers, which were separated by (La/Nd)S2O6 square antiprismsin the ab plane. In contrast, La3Ga3Ge2S3O10 exhibited a randomdistribution of Ga and Ge over the 6g and 4f sites at an atomicratio of 3 : 2. It is reasonable for Si ions to adopt an all-oxygenhomoleptic coordination geometry because they rarely form het-eroleptic coordination polyhedra, while the difference in cationordering patterns in La3Ga3Ge2S3O10 and Nd3Ga3Ge2S3O10 isnon-trivial. In previous studies, the bond-valence-sum (BVS)values for Ga/Ge on 6g and 4f were estimated to be 3.04 and3.92, respectively. This implied a Ga/Ge ordered state as in theNd analog, although structural refinements based on the cation-ordered model were unstable. In oxides containing both Ga andGe atoms, the identification of the distribution of these cationsby XRD analysis is of major concern because of their similarX-ray scattering factors.14–16In this study, we reexamined the Ga/Ge cation disorderedstate of La3Ga3Ge2S3O10 and newly synthesized a series ofLn3Ga3Ge2S3O10 (Ln = La, Ce, Pr, Nd) by both molten chlorideflux methods and conventional solid-state reactions. Neutronpowder diffraction (NPD), in addition to SCXRD, was employedto characterize the crystal structure, especially the Ga/Gecation ordered pattern.Experimental sectionReagentsLa2S3 (Kojundo Chemical Laboratory, 3 N), La2O3 (RareMetallic, 4 N), CeO2 (Rare Metallic, 4 N), Pr6O11 (Rare Metallic,3 N), Nd2O3 (Rare Metallic, 3 N), Ga2O3 (Rare Metallic, 3 N),GeO2 (Rare Metallic, 4 N), Ge (Kojundo Chemical Laboratory, 4N), S (Kojundo Chemical Laboratory, 4 N), BaCl2 (RareMetallic, 3 N), and NaCl (Rare Metallic, 4 N) powders wereused as received. BaCl2 and NaCl were heated overnight at370 °C prior to use. La2O3 was preheated at 1000 °C in airprior to use. All raw materials were stored in an argon-filledglovebox (moisture and oxygen levels less than 0.1 ppm), andall manipulations before starting the reaction were carried outin a glovebox or under vacuum.Crystal growth and elemental analysisSingle crystals of Ln3Ga3Ge2S3O10 (Ln = La, Ce, Pr, Nd) wereobtained by the flux growth method using a BaCl2–NaCl eutec-tic molten salt. For Ln = La, La2S3 (0.5 mmol), Ga2O3(0.5 mmol), and GeO2 (0.5 mmol) were combined according tothe literature.8 For Ln = Ce and Pr, CeO2 (0.6 mmol)/Pr6O11(0.1 mmol), Ga2O3 (0.3 mmol), Ge (0.4 mmol), and S(0.6 mmol) were combined. For Ln = Nd, Nd2O3 (0.3 mmol),Ga2O3 (0.3 mmol), Ge (0.3 mmol), S (0.6 mmol) and GeO2(0.1 mmol) were combined. Each set of starting materials wasloaded in an alumina crucible with BaCl2 (2.7 mmol) and NaCl(4.0 mmol). The crucibles were flame-sealed in fused silicatubes under a vacuum of 1 Pa, heated in a muffle furnace to850 °C at 5 °C min−1, held at this temperature for 24 h, cooledto 550 °C at 0.08 °C min−1, and finally cooled naturally toroom temperature. The products were then washed with soni-cated water and extracted from the flux. Transparent rod-shaped crystals of the title compounds were collected byvacuum filtration with approximately yields of 65% for La,10% for Ce, 30% for Pr, and 60% for Nd based on lanthanide(Fig. 2). Elemental analysis of single crystals ofFig. 1 Side and top views of the crystal structure of Ln3Ga3Ge2S3O10 (Ln = La, Ce, Pr, Nd), which possesses one-dimensional chains of LnS2O6square antiprisms, and isolated dimers and triangular tubes composed of M1O4 tetrahedra and M2S2O2 tetrahedra, respectively.Paper Dalton Transactions10434 | Dalton Trans., 2025, 54, 10433–10440 This journal is © The Royal Society of Chemistry 2025Open Access Article. Published on 12 June 2025. Downloaded on 7/2/2025 1:47:57 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d5dt01100kLn3Ga3Ge2S3O10 was performed using a scanning electronmicroscope (SEM, HITACHI, S-43000) equipped with an energydispersive X-ray (EDX) spectrometer. The accelerating voltagewas set to 15 keV. EDX analysis indicated a Ln : Ga : Ge : Satomic ratio of approximately 3 : 3 : 2 : 3, which was in goodagreement with the chemical composition determined bysingle-crystal structure analysis.Solid state reactionPolycrystalline powder samples of Ln3Ga3Ge2S3O10 (Ln = La–Nd) were synthesized by conventional solid-state reactions.The Ln = La, Pr, and Nd phases were synthesized from a stoi-chiometric mixture of La2O3 (0.3 mmol), Ga2O3 (0.3 mmol),GeO2 (0.1 mmol), Ge (0.3 mmol), and S (0.6 mmol) for Ln =La, Pr6O11 (0.1 mmol), Ga2O3 (0.3 mmol), Ge (0.4 mmol) and S(0.6 mmol) for Ln = Pr, Nd2O3 (0.3 mmol), Ga2O3 (0.3 mmol),GeO2 (0.1 mmol), Ge (0.3 mmol), and S (0.6 mmol) for Ln =Nd. The Ln = Ce phase was synthesized using a mixture ofCeO2 (0.6 mmol), Ga2O3 (0.3 mmol), Ge (0.4 mmol), and S(0.6 mmol), which contains 0.1 mmol excess of oxygen com-pared with the stoichiometry of Ce3Ga3Ge2S3O10. Each set ofstarting materials was ground with an agate mortar and pestle,pressed into a pellet, sealed in a silica tube under a vacuum of1 Pa, and heated in a muffle furnace at 1000 °C for 24 h. Forneutron powder diffraction studies, scaled-up syntheses(approximately 10 times the amount) of polycrystalline powdersamples of La3Ga3Ge2S3O10 and Nd3Ga3Ge2S3O10 were per-formed using a similar synthesis procedure.8,12Single crystal structure determinationX-ray intensity data of single crystals of Ln3Ga3Ge2S3O10 (Ln =La–Nd) were collected using a Rigaku XtaLAB mini II diffract-ometer (Mo Kα radiation). Data collection covered more than96% of the reciprocal space to 2θmax ∼ 60° with Rint = 4.17%for La, 3.42% for Ce, 3.19% for Pr, and 3.31% for Nd afterabsorption correction. The crystal structure was solved using adual-space algorithm method (SHELXT)17 and refined using afull-matrix least-squares method with SHELXL18 using anOlex219 graphical user interface.Powder XRD and UV-Vis-NIRPowder XRD patterns were collected on a Rigaku MiniFlex-600diffractometer (Cu Kα radiation) from the 2θ range of 5–70°with a step of 0.02° at room temperature. The UV-vis–NIRreflectance spectra were collected using a Shimadzu UV- 2600UV-Vis-NIR spectrometer (used in the diffuse reflectancemode) equipped with an integrating sphere in the range of220–1200 nm. Deuterium and halogen lamps were used assources of UV and visible-NIR light, respectively. The recordedreflectance spectra were converted into the absorption data viathe Kubelka–Munk function.Powder neutron diffractionTime-of-flight neutron powder diffraction (NPD) measure-ments were conducted at room temperature using aniMATERIA20 installed at J-PARC MLF BL20 in Japan.La3Ga3Ge2S3O10 (2.44 g) and Nd3Ga3Ge2S3O10 (2.57 g) wereindependently loaded into a vanadium can with 5.8 mm innerdiameter, and the diffraction data were collected using a back-scattering detector bank. The NPD data were analyzed byRietveld refinement using the Z-code program.21Results and discussionStructure determination using single-crystal X-ray diffractiondataTypical dimensions of single crystals of Ln3Ga3Ge2S3O10 (Ln =La–Nd) were ranged from 0.1 × 0.1 × 0.2 to 0.5 × 0.5 × 0.8 mm3,indicating that these crystals grew preferentially along thec-axis. Single crystals of La3Ga3Ge2S3O10 and Nd3Ga3Ge2S3O10were colorless and pale purple, respectively, as previouslyreported (Fig. 2). Ce3Ga3Ge2S3O10 and Pr3Ga3Ge2S3O10 werecolorless and lime-green, respectively. The colors ofPr3Ga3Ge2S3O10 and Nd3Ga3Ge2S3O10 are characteristic of f–ftransitions in Pr3+ and Nd3+. These single crystals are stable inair and water insoluble. Single-crystal structure analysis ofLn3Ga3Ge2S3O10 (Ln = La–Nd) was performed at room tempera-ture. As reported previously, the Ln = La phase adopts the hex-agonal space group P6̄2c (no. 190) with lattice parameters of a= 10.1701(4) Å and c = 7.5198(3) Å.8 Additionally, the structurerefinement based on the cation ordered model that Ga and Geatom occupied 6g and 4f sites, respectively, smoothly con-verged in contrast to our previous report. The Ln = Ce–Ndanalogs were also found to adopt the same space group withlattice parameters, which were proportional to the ionic radiusof Ln3+ ions (Fig. 3). The structure refinements indicated theGa/Ge ordering, as reported previously for Nd3Ga3Ge2S3O10.12The details of the final refined structure for all the phases arelisted in Table 1. The atomic coordinates and isotropicthermal displacement parameters are listed in Table 2 and theanisotropic displacement parameters are listed in Table S1.†Selected interatomic distances and angles are listed inTable S2.† The metal–ligand bond distances in GeO4 andGaS2O2 tetrahedra in all phases are very close to the sum oftheir ionic radii (rGe4+ = 0.39 Å, rGa3+ = 0.47 Å, rO2− = 1.4 Å, rS2− =1.84 Å).22 In contrast, in the LnS2O6 tetrahedron, the Ln–O/Ln–S bond distances are broad compared to the sum of their ionicradii, but their average bond distances are in good agreementFig. 2 Photographs of single crystals of Ln3Ga3Ge2S3O10 (Ln = La, Ce,Pr, Nd) on a 1 mm-grid glass plate.Dalton Transactions PaperThis journal is © The Royal Society of Chemistry 2025 Dalton Trans., 2025, 54, 10433–10440 | 10435Open Access Article. Published on 12 June 2025. Downloaded on 7/2/2025 1:47:57 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d5dt01100kwith the sum of their ionic radii. Bond valence sum (BVS) cal-culations23 were carried out for all the atoms, as shown inTable 3. The BVS values of Ln (= La, Ce, Pr, Nd), Ga, and Geatoms were consistent with the nominal oxidation numberexpected from the chemical composition.X-ray powder diffractionFig. 4 shows the room-temperature X-ray powder diffractionpatterns of polycrystalline Ln3Ga3Ge2S3O10 (Ln = La–Nd)samples synthesized by conventional high-temperature solid-state reactions. These compounds were obtained in nearly asingle phase. All of these XRD patterns could be readilyindexed to the hexagonal cell in the space group P6̄2c, exceptfor a few minor peaks that could be assigned to GeO2. Similarto the single-crystal structure analysis, the lattice constants ofthe polycrystalline samples also varied with the ionic radius ofLn3+ (Fig. S1†).Structure determination using neutron powder diffractiondataTo investigate the Ga and Ge atom distribution inLa3Ga3Ge2S3O10, neutron powder diffraction experiments wereperformed on La3Ga3Ge2S3O10 at room temperature, togetherwith Nd3Ga3Ge2S3O10, for comparison. Fig. 5 shows the resultsof Rietveld refinements of these two oxysulfides. For bothLa3Ga3Ge2S3O10 and Nd3Ga3Ge2S3O10, the profile fitting to theGa/Ge ordered model smoothly converged well with reliablefactors, Rwp = 5.40, Rp = 4.24, RB = 2.58 for La and Rwp = 8.24,Rp = 5.79, RB = 5.97 for Nd (Fig. 5). To examine the possibilityof Ga/Ge atoms being disordered, structure refinement wasperformed under the assumption that Ga and Ge atoms aredisordered at the 6g and 4f sites. The results showed that 6gand 4f sites were more than 99% occupied by Ga and Geatoms, respectively, within statistical errors. Thus, we concludethat the Ga and Ge atoms in the polycrystalline samples areordered like the single crystals. Tables S3 and Table 3 summar-ize the final obtained crystallographic data including isotropicdisplacement parameters for all atoms. The occupancy factorsof Ga on 6g and Ge on 4f were fixed at unity in the finalrefinement.Ga/Ge cation ordering and size effect of Ln3+ on the latticesThe present structure refinements against the SCXRD andNPD data of La3Ga3Ge2S3O10 revealed the Ga/Ge cation order,which differed from the Ga/Ge disorder characterized in theprevious study.8 One possible reason for the cation disorderingcan be ascribed to a technical problem in the structure refine-ments. We reviewed the previous SCXRD data from the begin-ning and found that the cation ordered state was also stabil-Fig. 3 The a-axis and c-axis lengths plotted as a function of the ionicradius of Ln3+.Table 1 Results of structure refinement of Ln3Ga3Ge2S3O10 (Ln = La, Ce, Pr, Nd) using single-crystal XRD dataFormula La3Ga3Ge2S3O10 Ce3Ga3Ge2S3O10 Pr3Ga3Ge2S3O10 Nd3Ga3Ge2S3O10Formula weight 1027.25 1030.88 1033.25 1043.24T (K) 297 297 293 294Crystal system Hexagonal Hexagonal Hexagonal HexagonalSpace group P6̄2c P6̄2c P6̄2c P6̄2ca (Å) 10.1701(4) 10.0993(4) 10.0921(7) 10.0682(5)c (Å) 7.5198(3) 7.4738(4) 7.4251(5) 7.3802(5)α (°) 90 90 90 90β (°) 90 90 90 90γ (°) 120 120 120 120V (Å3) 673.58(6) 660.17(6) 654.93(9) 647.89(8)Z 2 2 2 2ρcalc (g cm−3) 5.065 5.186 5.240 5.348μ (mm−1) 20.096 21.201 22.042 23.023F000 912 918 924 930θ (°) 3.534–30.53 2.329–30.64 3.5840–30.38 2.336–30.607Rint (%) 4.17 3.42 3.19 3.31No. of reflections (collected/unique) 9330/734 9529/723 2246/467 2524/658Goodness of fit on F2 1.061 1.083 0.977 1.041R1, wR2 [I > 2σ(I)] 0.0109, 0.0251 0.0109/0.0208 0.0128/0.0234 0.0192/0.0378R1, wR2 (all data) 0.0114, 0.0252 0.0126/0.0210 0.0136/0.0236 0.0216/0.0386Diff peak, hole (e Å−3) 0.433, −0.387 0.454, −0.494 0.373, −0.330 0.712, −0.644Flack parameter −0.002(13) 0.001(12) −0.03(3) −0.05(3)Paper Dalton Transactions10436 | Dalton Trans., 2025, 54, 10433–10440 This journal is © The Royal Society of Chemistry 2025Open Access Article. Published on 12 June 2025. Downloaded on 7/2/2025 1:47:57 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d5dt01100kized. Perhaps, due to similar X-ray scattering factors of Ga andGe atoms, the previous refinements fell into a local minimumthat stabilizes the cation disordered state.As shown in Fig. 3, the lattice parameters change linearlywith the ionic radius of the Ln3+. On the other hand, the dis-tortion index24 (D) around the metal centers, which was calcu-lated using the metal–anion bond distances determined fromthe SCXRD data, exhibits unusual behaviors against the size ofLn3+ (Table S4†). The D values for the Ge-centered tetrahedrain Ln = La are 0.00911, which are remarkably larger than thosefor the corresponding tetrahedra in the other Ln ions by15–31%, while the D values for the Ln- and Ga-centered poly-hedra remained similar regardless of the Ln species. This canbe rationalized by considering the volume of LnO6S2 squareantiprisms, which form three-member rings via common Satoms in the ab plane and surround the Ga/Ge-centered tetra-hedra. The GeO4 tetrahedron may be too small to fit into aframework consisting of LaS2O6, which has the largest volumecompared to other Ln-centered polyhedra. We attempted tosynthesize other members with Ln3+ smaller than Nd3+, but itwas not successful.Optical properties and bandgapFig. 6a shows the UV-Vis–NIR diffuse reflectance ofLn3Ga3Ge2S3O10 (Ln = La–Nd). The spectra of Ln = Pr and Ndexhibited a UV cutoff edge close to 250 nm, as seen forLa3Ga3Ge2S3O10, which can be attributed to an optical tran-sition from the valence band maximum composed of Ln-5dand Ga-4s, 4p orbitals to the conduction band minimum com-posed of O-2p and S-3p orbitals.8,12 A series of complex opticalbands in a broad range of wavelengths result from the f–f tran-sitions characteristic of the localized 4f orbitals of Ln ions. Incontrast, the Ln = Ce phase exhibited stepwise absorptionbelow 430 nm, followed by a cutoff edge at 340 nm. Thischaracteristic absorption is due to the presence of the Ce-4fground state between the VBM and CBM: optical transitionsfrom the Ce-4f ground state to the lowest and second lowestCe-4d states occur. Fig. 6b shows the absorption spectra con-verted from the diffuse reflectance spectra using the Kubelka–Table 2 Crystallographic and refinement data obtained from single-crystal structure analysis of Ln3Ga3Ge2S3O10 (Ln = La, Ce, Pr, Nd)Atom Site x y z Occupancy Uiso/ Å2La3Ga3Ge2S3O10La 6h 0.35751(3) 0.38034(3) 1/4 1 0.00771(8)Ge 4f 2/3 1/3 0.51367(7) 1 0.00446(13)aGa 6g 0 0.18935(5) 0 1 0.00652(12)aS 6h 0.01248(14) 0.32093(14) 1/4 1 0.0106(2)O1 6g 0.1757(3) 0.1757(2) 0 1 0.0089(7)O2 12i 0.5052(3) 0.3439(3) 0.4667(3) 1 0.0111(5)O3 2c 2/3 1/3 3/4 1 0.033(2)Ce3Ga3Ge2S3O10Ce 6h 0.35730(3) 0.38047(3) 1/4 1 0.00888(7)Ge 4f 2/3 1/3 0.51309(7) 1 0.00542(13)Ga 6g 0 0.19034(6) 0 1 0.00761(11)S 6h 0.01396(15) 0.32373(15) 1/4 1 0.0120(3)O1 6g 0.1770(3) 0.1770(3) 0 1 0.0111(7)O2 12i 0.5038(3) 0.3439(3) 0.4663(3) 1 0.0115(5)O3 2c 2/3 1/3 3/4 1 0.032(2)Pr3Ga3Ge2S3O10Pr 6h 0.35754(5) 0.38139(6) 1/4 1 0.00797(13)Ge 4f 2/3 1/3 0.51145(11) 1 0.0039(2)Ga 6g 0 0.19057(10) 0 1 0.0070(2)S 6h 0.01632(2) 0.3260(2) 1/4 1 0.0118(5)O1 6g 0.1764(6) 0.1764(6) 0 1 0.0135(15)O2 12i 0.5033(4) 0.3433(4) 0.4658(5) 1 0.0106(9)O3 2c 2/3 1/3 1/4 1 0.036(4)Nd3Ga3Ge2S3O10Nd 6h 0.35688(5) 0.38212(6) 1/4 1 0.00923(14)Ge 4f 2/3 1/3 0.51014(13) 1 0.0056(2)Ga 6g 0 0.19121(11) 0 1 0.0076(2)S 6h 0.0183(3) 0.3287(3) 1/4 1 0.0121(5)O1 6g 0.1778(6) 0.1778(6) 0 1 0.0123(14)O2 12i 0.5029(5) 0.3435(5) 0.4643(6) 1 0.0114(9)O3 2c 2/3 1/3 1/4 1 0.036(4)Table 3 The values of bond-valence-sum calculations for Ln, Ge, andGa sites obtained from the single crystal structure analysisFormula Ln at 6h Ga at 6g Ge at 4fLa3Ga3Ge2S3O10 2.81 3.03 3.91Ce3Ga3Ge2S3O10 2.83 2.95 4.03Pr3Ga3Ge2S3O10 2.92 2.98 4.05Nd3Ga3Ge2S3O10 2.81 2.98 4.04Dalton Transactions PaperThis journal is © The Royal Society of Chemistry 2025 Dalton Trans., 2025, 54, 10433–10440 | 10437Open Access Article. Published on 12 June 2025. Downloaded on 7/2/2025 1:47:57 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d5dt01100kMunk function. The optical band gaps of Ln = La, Ce, Pr, andNd estimated by the extrapolation method were 4.70, 3.51,4.60, and 4.64 eV, respectively. Similar band gap values for Ln= La, Pr, and Nd are consistent with the similar Ln 5d energylevels.25SHG measurementsThe powder SHG intensities of Ln3Ga3Ge2S3O10 (Ln = Ce, Pr)were measured using the Kurtz–Perry method at λ = 1064 nm,with polycrystalline KDP as a reference compound. The SHGsignals are plotted as a function of particle size in Fig. 7. TheSHG intensities of both phases increased with increasing par-ticle size in the size region smaller than 150 μm, and werealmost saturated in the larger particle size region. These beha-viors indicate that they are type-I phase-matchable likeFig. 4 Room-temperature X-ray powder diffraction patterns of Ln3Ga3Ge2S3O10 (Ln = La, Ce, Pr, Nd) polycrystalline samples obtained via high-temperature solid-state reactions. All phases can be assigned to a hexagonal cell in the space group P6̄2c.Fig. 5 Neutron powder diffraction patterns collected from (a)La3Ga3Ge2S3O10 and (b) Nd3Ga3Ge2S3O10 at room temperature. Rietveldrefinements were performed on the basis of the structural modelsdetermined by single crystal structure analysis.Fig. 6 (a) UV-Vis–NIR diffuse reflectance and (b) absorption spectra ofLn3Ga3Ge2S3O10 (Ln = La, Ce, Pr, Nd). The bandgaps were estimated tobe 4.70, 3.51, 4.60, and 4.64 eV, respectively.Paper Dalton Transactions10438 | Dalton Trans., 2025, 54, 10433–10440 This journal is © The Royal Society of Chemistry 2025Open Access Article. Published on 12 June 2025. Downloaded on 7/2/2025 1:47:57 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d5dt01100kLa3Ga3Ge2S3O10 and Nd3Ga3Ge2S3O10.8,12 The SHG intensitiesof Ln = Ce and Pr are comparable to that of KDP at the largestparticle sizes measured, but somewhat smaller than those ofthe La and Nd analogs (∼2 × KDP). These high SHG intensitiesare consistent with a previous theoretical conclusion that theGa/Ge-centered tetrahedra forming non-centrosymmetric sub-lattices are mainly responsible for the SHG response ofLa3Ga3Ge2S3O10.8,26,27ConclusionsAlthough a Ga/Ge disordered state was previously suggestedfor single crystals of La3Ga3Ge2S3O10, re-investigation bysingle-crystal XRD and neutron diffraction experiments per-formed in this study showed that La3Ga3Ge2S3O10 has a Ga/Geordered state. Similar cation-ordered arrangements wereobserved for other lanthanide analogues. UV-Vis-NIR absorp-tion spectra revealed band gaps larger than 4.60 eV forLa3Ga3Ge2S3O10, Pr3Ga3Ge2S3O10, and Nd3Ga3Ge2S3O10, exceptfor Ce3Ga3Ge2S3O10 with a small band gap value of 3.51 eVbecause of Ce-4f1 electronic configuration.Conflicts of interestThe authors declare no competing financial interests.Data availabilityThe data supporting this article have been included as part ofESI. X-ray crystallographic files for the structure have been de-posited in Cambridge Crystallographic Data Centre (CCDC)with no. of 2449159, 2449160, 2449238, and 2449239.†AcknowledgementsThis study was supported by World Premier InternationalResearch Center Initiative (WPI), the Japan Society for thePromotion of Science (JSPS) KAKENHI (grant no. 25K01507,25K01657, 25H01652), Bilateral Program (No.JPJSBP120237714), and Core-to-Core Program(JPJSCCA20200004). Y. T. acknowledges the grant from theMurata Science Foundation. 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This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d5dt01100k Button 1: