Molten Alkali Halide Flux Growth of an Extensive Family of

Jun 19, 2019 - aBlue, yellow, red, and green represent space groups P21/m, P21, Pnma, and P212121, ..... A large optical band gap value is consistent ...
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Article Cite This: Inorg. Chem. 2019, 58, 8541−8550

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Molten Alkali Halide Flux Growth of an Extensive Family of Noncentrosymmetric Rare Earth Sulfides: Structure and Magnetic and Optical (SHG) Properties Mohammad Usman,† Mark D. Smith,† Gregory Morrison,† Vladislav V. Klepov,† Weiguo Zhang,‡ P. Shiv Halasyamani,‡ and Hans-Conrad zur Loye*,† †

Department of Chemistry and Biochemistry, University of South Carolina, Columbia, South Carolina 29208, United States Department of Chemistry, University of Houston, Houston, Texas 77204, United States

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‡

S Supporting Information *

ABSTRACT: Twenty new alkali rare earth thiosilicates and thiogermanates with the general formula ALnTS4 (A = alkali metal, Ln = lanthanide, and T = Si, Ge) were grown as X-ray diffraction-quality single crystals from molten alkali chloride fluxes. These include KNdSiS4, KPrSiS4, RbLnSiS4 (Ln = Ce, Pr, Nd, Gd, Tb, Dy, and Ho), RbLaGeS4, CsLnSiS4 (Ln = La, Pr, and Nd), and CsLnGeS4 (La, Ce, Pr, Nd, Eu, Gd, and Tb). Herein, we discuss the use of a molten chloride flux growth approach for the preparation of the title compounds and their structure determination via single-crystal X-ray diffraction. In addition, we comment on the magnetic properties of RbNdSiS4, CsNdSiS4, CsNdGeS4, and CsGdGeS4, which were found to be paramagnetic for T = 2−300 K and exhibited negative Weiss temperatures with no obvious antiferromagnetic transition down to 2 K. The optical properties of CsLaGeS4 and CsNdTS4(T = Si, Ge) were measured by UV−vis spectroscopy. Second harmonic generation measurements performed on CsLaGeS4 confirmed the crystallization of the compound in the noncentrosymmetric orthorhombic space group, P212121; CsLaGeS4 was found to be SHG-active with nearly half the intensity of α-SiO2 upon irradiation with a Nd:YAG 1064 nm laser, and a semiconductor exhibiting a band gap of 3.60 eV based on UV− vis diffuse reflectance measurements.

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INTRODUCTION Potential optical applications of metal chalcogenides in the farinfrared transparency region stimulated extensive research in the late 1980s with numerous reports on a wide variety of organic and inorganic chalcogenide compounds.1−5 This research typically focused on the exploratory synthesis of rare-earth-containing chalcogenide compounds and resulted in the fabrication of new materials with intriguing magnetic and optical properties.6−9 These materials not only possessed interesting physical properties but also featured new structure types. For example, K6Yb3(PS4)5 features an interwoven pair of open frameworks,10 while Eu2GeS4 undergoes an interesting phase transition from polar space group P21 to centrosymmetric space group P21/c at Tc ≈ 335 K, which also made it a potential ferroelectric material.11 The most common main group chalcogenide building block in these compounds is the tetrahedral TQ4 building unit (T = Si, Ge; Q = S, Se), and in the series of structures featuring the TQ4 building unit, the quaternary compounds with the general formula ALnTQ4 (A = alkali metal; Ln = rare earth; T = Si, Ge; Q = S, Se) constitute one of the most prevalent structure types that crystallize in numerous structural variations.12,13 Specifically, these compounds have the propensity to crystallize © 2019 American Chemical Society

in a variety of space groups ranging from the noncentrosymmetric space groups, P21 and P212121, to the centrosymmetric space groups, P21/c and Pnma.14,15 Almost all of the ALnTQ4 phases reported to date were prepared from molten alkali polychalcogenide fluxes, A2Qn (A = alkali metal, Q = S, Se), which function simultaneously as the high-temperature flux and the alkali metal source.12,16,17 Although this synthesis procedure is very effective, the solidified polychalcogenide flux matrix is difficult to dissolve in common solvents, making it challenging to obtain large quantities of a product for physical property measurements. A second approach to preparing ALnTQ4 compounds, which involves the preparation of polycrystalline samples of the targeted composition via the stoichiometric combination of its constituent elements followed by its recrystallization from a suitable flux, is also used frequently. One of the earliest “1114” rare earth chalcogenides prepared by this approach is CaYbInQ4, which was synthesized by holding a stoichiometric mixture of CaQ (Q = S, Se), Yb, In, and Q (S, Se) at 1000 °C for 6 days followed by crystallization of the resulting CaYbInQ4 Received: March 25, 2019 Published: June 19, 2019 8541

DOI: 10.1021/acs.inorgchem.9b00849 Inorg. Chem. 2019, 58, 8541−8550

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a

CsPrSiS4 430.15 301(2) green plates P212121 6.4370(2) 6.7258(2) 17.7904(5) 90 770.22(4) 4 3.710 12.110 768 0.240 × 0.080 × 0.010 32.498 1.048 R1 = 0.0148, wR2 = 0.0359 0.572/−0.734

CsLaSiS4 428.16

300(2) yellow plates Pnma 17.8614(11) 6.7182(4) 6.4776(4) 90 777.29(8) 8 3.659 11.228 760 0.20 × 0.15 × 0.02 36.36 1.276 R1 = 0.0249, wR2 = 0.0506 1.391/−1.158

The Rint value for orthorhombic is 0.533.

formula formula weight (g/mol) temp (K) crystal habit space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z density (g/cm3) μ (mm−1) F(000) crystal size (mm × mm × mm) Θmax (deg) GoF final R indices (all data) largest diff. peak and hole (e−/Å3) 301(2) blue plates P212121 6.4179(2) 6.7135(2) 17.7418(5) 90 764.43(4) 4 3.767 12.620 772 0.16 × 0.03 × 0.02 32.876 1.077 R1 = 0.0183, wR2 = 0.0331 1.029/−0.839

CsNdSiS4 433.48 100(2) yellow plates P212121 6.6095(3) 6.7971(3) 17.7436(7) 90 797.14(6) 4 3.938 14.496 832 0.20 × 0.18 × 0.02 40.312 1.024 R1 = 0.0233, wR2 = 0.0526 1.929/−2.205

CsLaGeS4 472.65 302(2) yellow plates P212121 6.5591(2) 6.8020(2) 17.8421(6) 90 796.03(4) 4 3.954 14.868 836 0.240 × 0.080 × 0.010 32.500 1.087 R1 = 0.0133, wR2 = 0.0386 0.630/−0.847

CsCeGeS4 473.86 302(2) green plates P212121 6.5407(12) 6.7929(13) 17.812(3) 90 791.4(3) 4 3.984 15.359 840 0.240 × 0.080 × 0.010 36.22 1.152 R1 = 0.0163, wR2 = 0.0353 1.418/−0.865

CsPrGeS4 474.65

Table 1. Crystallographic and Refinement Data for CsLnTS4 (Ln = Lanthanide and T = Si, Ge)a

301(2) blue plates P212121 6.5195(3) 6.7800(3) 17.7842(7) 90 786.10(6) 4 4.039 15.870 844 0.20 × 0.04 × 0.04 36.211 1.065 R1 = 0.0209, wR2 = 0.0429 2.015/−0.938

CsNdGeS4 477.98 301(2) red plates P212121 6.4669(2) 6.7484(2) 17.7148(6) 90 773.10(4) 4 4.173 17.534 856 0.04 × 0.02 × 0.01 32.536 1.277 R1 = 0.0643, wR2 = 0.1707 5.846/−3.099

CsEuGeS4 485.70

301(2) yellow plates P212121 6.4244(8) 6.7247(9) 17.689(2) 90 764.21(17) 4 4.282 18.784 0.06 × 0.04 × 0.01 31.229 1.028 R1 = 0.0285, wR2 = 0.0399 1.154/−0.922

0.05 × 0.02 × 0.01 32.698 1.024 R1 = 0.0306, wR2 = 0.0498 1.855/−1.717

CsTbGeS4 492.66

301(2) yellow plates P21/n 6.4657(2) 6.7354(2) 17.6830(5) 90.5947(14) 770.04(4) 4 4.235 18.071

CsGdGeS4a 490.99

Inorganic Chemistry Article

DOI: 10.1021/acs.inorgchem.9b00849 Inorg. Chem. 2019, 58, 8541−8550

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Inorganic Chemistry Table 2. Crystallographic and Refinement Data for RbLnTS4 (Ln = Lanthanide and T = Si, Ge) formula formula weight (g/mol) temp (K) crystal habit

RbCeSiS4 381.92

RbPrSiS4 382.71

RbNdSiS4 386.04

RbGdSiS4 399.05

RbTbSiS4 400.72

302(2) yellow plates

302(2) green rods

302(2) colorless rods

space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z density (g/cm3) μ (mm−1) F(000) crystal size (mm × mm × mm) Θmax (deg) GoF final R indices (all data)

Pnma 17.168(2) 6.6489(9) 6.4714(8) 90 738.72(17) 4 3.434 13.822 692 0.240 × 0.080 × 0.010 29.994 1.033 R1 = 0.0179, wR2 = 0.0452

P212121 6.4611(2) 6.6485(2) 17.1155(6) 90 735.22(4) 4 3.457 14.384 696 0.240 × 0.080 × 0.010 32.497 1.118 R1 = 0.0185, wR2 = 0.0476

largest diff. peak and hole (e−/Å3)

1.008/−0.773

1.603/−1.088

301(2) light blue plates P212121 6.4416(2) 6.6473(2) 17.0676(4) 90 730.82(4) 4 3.509 14.908 700 0.14 × 0.10 × 0.05 35.153 1.080 R1 = 0.016, wR2 = 0.0282 0.678/−0.590

RbHoSiS4 406.73

RbLaGeS4 425.21

302(2) 302(2) colorless blocks colorless rods

302(2) yellow plates

302(2) yellow plates

P212121 6.3755(2) 6.6240(2) 16.9820(4) 90 717.17(4) 4 3.696 17.200 716 0.240 × 0.080 × 0.010 36.32 1.185 R1 = 0.0157, wR2 = 0.0362

P212121 6.3492(2) 6.6142(2) 16.9643(5) 90 712.41(4) 4 3.736 17.932 720 0.240 × 0.080 × 0.010 32.498 1.116 R1 = 0.0225, wR2 = 0.0598

P212121 6.3355(2) 6.6058(2) 16.9465(5) 90 709.23(4) 4 3.786 18.577 724 0.240 × 0.080 × 0.010 36.37 1.098 R1 = 0.0184, wR2 = 0.0480

P212121 6.3218(2) 6.5996(2) 16.9307(5) 90 706.37(4) 4 3.825 19.275 728 0.240 × 0.080 × 0.010 36.31 1.127 R1 = 0.0185, wR2 = 0.0506

P212121 6.6238(4) 6.7435(4) 17.2595(10) 90 770.94(8) 4 3.663 16.607 760 0.240 × 0.080 × 0.010 32.495 1.159 R1 = 0.0342, wR2 = 0.0884

1.360/−1.563

1.486/−2.080

1.539/−1.498

1.971/−1.479

2.506/−2.261

powder from a CaCl2/KCl eutectic flux.18 CsSmGeS4 is an exception because it was serendipitously grown out of a CsCl flux in a reaction intended to grow single crystals of preprepared AgSm3GeS7.2 Given the time-consuming nature of the polychalcogenide flux and two-step recrystallization growth methods, developing a new approach that yields single crystals of these phases in less time would be quite desirable. For many years now, we have demonstrated that alkali halide fluxes can serve as excellent media in which a rather extensive list of elemental or binary reagents can be dissolved and from which single crystals can be grown. Furthermore, such flux growth reactions result in a good yield of crystals that can be readily isolated by dissolving the flux and can be used for property measurements.19−27 In addition to the exploration of new oxides, we are now pursuing the use of alkali halide fluxes for growing single crystals of new complex metal sulfides with interesting physical properties. So far, we have successfully synthesized a family of uranium-containing thiophosphates, featuring complex topologies, from alkali iodide melts and reported on their magnetic and optical properties.28 Recently, we carried out the flux synthesis of SrZn2OS2, a phasematchable polar oxysulfide with a large second harmonic generation response grown from a KCl-KF eutectic flux.29 We have applied this approach to the preparation of new lanthanide chalcogenide phases and herein we describe our initial results: (i) 20 new alkali rare earth thiosilicates and thiogermanates with the general formula ALnTS4; (ii) KNdSiS4, KPrSiS4, RbLnSiS4 (Ln = Ce, Pr, Nd, Gd, Tb, Dy, and Ho), RbLaGeS4, CsLnSiS4 (Ln = La, Pr, and Nd), and CsLnGeS4 (La, Ce, Pr, Nd, Eu, Gd, and Tb), that were obtained as high-quality single crystals. Herein we report on the single-crystal structures of the title compounds and their magnetic and optical properties.

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RbDySiS4 404.30

gadolinium pieces (∼20 mm, 99.9%, Alfa Aesar), terbium ingots (99%, Alfa Aesar), KCl (99.6%, Mallinckrodt), RbCl (99.8%, Alfa Aesar), CsCl (99%, Alfa Aesar), silicon powder (99.5%, Alfa Aesar), germanium powder (99.999%, Cerac), and sulfur (Fisher Chemical) were used for the synthesis of all title compounds. La2S3, Si powder, and Ge powder were used as received. KCl, RbCl, and CsCl were dried overnight in an oven set at 260 °C. All Ln2S3 (Ln = Nd, Gd, and Tb) reagents were synthesized by charging stoichiometric amounts of the rare earth metal ingots, which were finely chopped using a sharp metal cutter, and sulfur powder in a carbon-coated fused silica tube. The tube was evacuated via a vacuum manifold to a pressure of ∼10−4 Torr and flame-sealed using an oxygen/methane torch. The tube containing the charge was placed in a programmable furnace, heated at a ramp rate of 50 °C/h to 500 °C, kept at this temperature for 72 h, ramped at 10 °C/h to 600 °C, held at this temperature for 24 h, and finally ramped at 50 °C/h to 700 °C and held at this temperature for 24 h, whereupon the furnace was turned off. Powder X-ray diffraction data of the resulting polycrystalline product revealed the respective sesquisulfide phase as the major product together with persulfide side products as indicated by strong diffraction peaks. In all cases, this product was loaded back into a carbon-coated fused silica tube and sealed under vacuum. It was then held overnight to 800 °C in a tube furnace with one end protruding out of the furnace to maintain roughly room temperature in this portion of the tube throughout the course of the reaction. This resulted in the decomposition of the impurity phases and condensation of the excess sulfur in the cold end while leaving pure Ln2S3 powder at the hot end. Powder X-ray diffraction confirmed the synthesis of phase-pure Ln2S3 (Nd, Gd, and Tb). A similar process was used for the reaction between Eu pieces and sulfur, albeit the final sulfide product contained a mixture of EuS and EuS2. This EuS/EuS2 mixture was used for the synthesis of CsEuGeS4. Ln2S3 (Ce, Pr, Dy, and Ho) was synthesized by methods reported elsewhere.30,31 Single crystals of all ALnTS4 phases of suitable size and quality for single-crystal X-ray diffraction were grown by layering a mixture of 1/2 mmol of Ln2S3, 1 mmol of Si powder, or 1 mmol of Ge and 21/2 mmols of sulfur beneath a layer of 1 g of oven-dried ACl in an 8-in.long, finely carbon-coated fused silica tube. The loaded silica tube was evacuated via a vacuum manifold to a pressure of ≲10−4 Torr, flamesealed using an oxygen/methane torch, and placed in a programmable furnace. A three-step reaction temperature profile was used that

EXPERIMENTAL SECTION

Materials and Methods. La2S3(99.9%, Strem), neodymium ingots (99.1%, Alfa Aesar), europium pieces (99.9%, Alfa Aesar), 8543

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Inorganic Chemistry Table 3. Crystallographic and Refinement Data for KPrSiS4 and KNdSiS4 formula formula weight (g/mol) temp (K) crystal habit space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z density (g/cm3) μ (mm−1) F(000) crystal size (mm × mm × mm) Θmax (deg) GoF final R indices (all data) largest diff. peak and hole (e−/Å3)

KPrSiS4 336.34 301(2) green plates P21 6.4941(2) 6.5991(2) 8.6583(3) 107.7620(10) 353.37(2) 2 3.161 8.709 312 0.400 × 0.060 × 0.010 36.54 1.085 R1 = 0.0101, wR2 = 0.0257 0.519/−0.468

KNdSiS4 339.67 301(2) light-blue needles P21 6.4711(6) 6.5977(6) 8.6451(8) 107.857(4) 351.32(6) 2 3.211 9.216 314 0.140 × 0.080 × 0.020 33.630 1.080 R1 = 0.0313, wR2 = 0.0678 2.693/−1.322

Scheme 1. Space Groups for ALnSiS4 Compoundsa

a

Blue, yellow, red, and green represent space groups P21/m, P21, Pnma, and P212121, respectively. X = not reported.

consisted of a ramp at 300 °C/h to 500 °C, a dwell at this temperature for 12 h, a ramp at 60 °C/h to 700 °C, a dwell at this temperature for 12 h, and finally a ramp at 60 °C/h to 900 °C, a dwell at this temperature for 24 or 48 h, and then fast cooling to room temperature by shutting the furnace off. Single-Crystal X-ray Diffraction (SCXRD). X-ray intensity data from a pale-yellow plate of CsLaGeS4 and crystals of all other title compounds were collected at 100(2) and 301(2) K, respectively, using a Bruker D8 QUEST diffractometer equipped with a PHOTON-100 CMOS area detector and an Incoatec microfocus source (Mo Kα radiation, λ = 0.71073 Å).32 The crystals were mounted on a microloop with immersion oil. The raw area detector data frames were reduced and corrected for absorption effects using the SAINT+ and SADABS programs.33 Final unit cell parameters were determined by the least-squares refinement of a large array of reflections taken from each data set. Initial structural models were obtained with SHELXS using direct methods.34 Subsequent difference Fourier calculations and full-matrix least-squares refinement against F2 were performed with SHELXL-2018 using the ShelXle interface.35 Excluding a few phases discussed below, all title compounds crystallize in noncentrosymmetric, orthorhombic space group P212121, which was uniquely consistent with the pattern of systematic absences in the intensity data of the compounds. The asymmetric unit contains three metal atoms and four sulfur atoms. All atoms are located at positions of general crystallographic symmetry (Wyckoff symbol 4a). Near convergence, the absolute structure (Flack) parameter was consistent with the crystal being a two-component inversion twin. An inversion twin matrix was included in the final refinement cycles for all compositions crystallizing in space group P212121. CsLaSiS4 and RbCeSiS4 crystallize in centrosymmetric, orthorhombic space group Pnma. The asymmetric unit consists of one cesium atom, one lanthanum atom, one silicon atom, and three independent sulfur atoms. All atoms are located at positions of general crystallography symmetry (Wyckoff symbol 4c). In contrast, KNdSiS4 and KPrSiS4

crystallize in noncentrosymmetric, polar orthorhombic space group P21 consistent with the pattern of systematic absences in the intensity data and confirmed by structure solution. A check with ADDSYM of the final model showed no missed symmetry elements.36 The compounds adopt the KYbSiS4 structure type.37 The asymmetric unit consists of one potassium atom, one Nd or Pr atom, one silicon atom, and four independent sulfur atoms, all located on positions of general crystallographic symmetry (Wyckoff symbol 2a). All atoms were refined with anisotropic displacement parameters. CsGdGeS4 crystallizes in the P21/c monoclinic space group, with the asymmetric unit containing one cesium atom, one gadolinium atom, one germanium atom, and four sulfur atoms, all located at positions of general crystallographic symmetry (Wyckoff symbol 4e). For all compositions, all non-hydrogen atoms were refined with anisotropic displacement parameters. Relevant crystallographic and refinement data for all title compounds are provided in Tables 1, 2, and 3. Scheme 1 lists the space groups of all ALnSiS4 phases (A = K, Rb, and Cs). Interatomic Ln−S and T−S distances for a selection of ALnTS4 compounds are listed in Table 4. Powder X-ray Diffraction (PXRD). Powder X-ray diffraction patterns were collected on polycrystalline samples of RbNdSiS4, CsNdSiS4, CsLaGeS4, CsNdGeS4, and CsGdGeS4 by utilizing a Bruker D2 PHASER powder X-ray diffractometer using Cu Kα radiation, λ = 1.5418 Å. The step scan covered the 2θ angular range of 5−65° in steps of 0.02°. Powder patterns for all of the aforementioned phases are provided in the Supporting Information. Minor unmatched reflections were observed in a few phases that are not magnetically and optically active on the basis of theoretically consistent magnetic and optical measurements and calculations. Energy-Dispersive Spectroscopy (EDS). EDS was performed on single crystals of selected ALnTS4 using a TESCAN Vega-3 SBU scanning electron microscope (SEM) with a Thermo EDS attachment operated in an ultralow vacuum mode. Crystals were mounted on an SEM stub with carbon tape and analyzed using 20 kV accelerating 8544

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voltage and an accumulation time of 20 s. EDS verified the presence of the respective alkali, group 14 and rare earth metals and sulfur in an approximate 1:1:1:4 ratio. All SEM images and EDS data are provided in the Supporting Information. UV−Vis Diffuse Reflectance Spectroscopy. UV−vis diffuse reflectance spectroscopy data for compounds CsNdTS4 (T = Si, Ge) and CsLaGeS4 were obtained using a PerkinElmer Lambda 35 UV− vis scanning spectrophotometer equipped with an integrating sphere in the range of 300−900 nm. The reflectance data were converted to absorbance data using the Kubelka−Munk function.38 Magnetic Susceptibility. Magnetic property measurements for RbNdSiS4, CsNdSiS4, CsNdGeS4, and CsGdGeS4 were performed using a Quantum Design magnetic property measurement system (QD MPMS 3 SQUID Magnetometer). The magnetic susceptibility was measured under zero-field-cooled (zfc) and field-cooled (fc) conditions from 2 to 300 K at an applied magnetic field of 0.1 T. Magnetization as a function of applied field was measured from −5 to 5 T at 2 K. Data were corrected for the sample shape and radial offset effects as described previously.39 Second Harmonic Generation (SHG). Powder SHG measurements were performed on a modified Kurtz nonlinear optical (NLO) system using a pulsed Nd:YAG laser (Quantel Ultra 50) with a wavelength of 1064 nm. Comparisons with known SHG materials were made using ground crystalline α-SiO2. A detailed description of the equipment and methodology has been published elsewhere. No index-matching material was used in any of the experiments.40,41

2.814(6) 2.992(5) 2.886(6) 2.922(6) 2.215(6) 2.179(5) 2.202(6) 2.217(6) 2.8515(9) 3.0685(8) 2.9014(9) 2.9513(9) 2.1244(12) 2.0908(11) 2.1217(12) 2.1241(12)

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RESULTS AND DISCUSSION Synthesis. The serendipitous synthesis of single crystals of CsLaSiS4 from a mixture of La2S3 and US2 in a CsCl/KCl

Figure 1. Local coordination environments of Ln for all compositions crystallizing in space group P212121 or P21 (a), local coordination environments of La and Ce in CsLaSiS4 and RbCeSiS4 crystallizing in space group Pnma (b), and T (Si,Ge) in all reported phases (c).

eutectic flux contained in a sealed silica tube, while attempting to crystallize new actinide chalcogenides,9 demonstrated the suitability of the halide fluxes for the crystal growth of complex sulfides and prompted us to pursue the crystal growth of additional ALnTS4 phases. Interestingly, the Si was incorporated into the phase via chemical etching of the fused silica tube by the mixed chloride flux. The facile crystal growth of CsLaSiS4 strongly suggested that a variety of sulfide reagents could likely be dissolved in halide fluxes and that additional new sulfide phases could be obtained via this approach. Indeed, we succeeded in preparing CsLaGeS4 simply by the addition of Ge to the reaction mixture of La2S3 and sulfur in the CsCl/KCl eutectic flux. Additional phases were prepared by this route. In these reactions, the identity of the alkali cation in ALnTS4 (Ln = rare earth; T = Si, Ge) can be controlled simply by choosing the appropriate alkali halide flux. Although all phases formed without any notable difficulties, their yield was sometimes low, which was an impediment to performing physical property measurements.

T−S(1) T−S(2) T−S(3) T−S(4)

2.8340(18) 2.8340(18) 2.8667(19) 3.0848(19) 2.117(3) 2.120(3) 2.091(2) 2.121(3) Ln−S(3)

Ln−S(4)

2.939(2) Ln−S(2)

2.1183(7) 2.0877(10) 2.1218(12)

2.8787(10)

2.8551(6) 2.8551(6) 2.9007(7) 2.9390(7) 2.1217(9) 2.0901(8) 2.1249(9) 2.1211(9)

2.9239(9) 3.1189(9) 2.9710(10) 2.9979(10) 2.2120(10) 2.1793(9) 2.2181(10) 2.2144(10)

2.8559(11) 3.0556(11) 2.9143(12) 2.9477(12) 2.2097(12) 2.1773(10) 2.2110(12) 2.2140(12)

2.8049(10) 2.9871(10) 2.2115(10) 2.2167(11) 2.1774(11) 2.2047(11)

2.7810(17) 2.9698(16) 2.8526(19) 2.8963(19) 2.2097(18) 2.1756(16) 2.2026(19) 2.2113(19)

2.8129(16) 2.826(5) 2.8537(8) 2.9228(9)

2.9045(9) 2.9060(9) 2.8689(9) 3.1041(9) 2.9178(9) 2.9684(9) 2.0924(11) 2.1252(12) 2.1246(12) 2.1258(13) 2.8494(6)

2.8702(10)

2.8727(11) 2.9072(11) 2.8223(11)

2.8187(19) 2.8352(19) 2.8355(102.8476(11) 2.841(6) 2.851(6) 2.8930(12) 2.8939(12) 2.886(2) 2.897(2) Ln−S(1)

2.9245(6) 2.9245(6) 2.9488(6) 2.9488(6) 2.8710(8)

2.8848(9) 2.8898(9) 2.9459(10) 2.9583(10) 2.8675(8) 2.8857(7) 2.8894(7)

CsEuGeS4 RbCeSiS4 KNdSiS4

Table 4. Interatomic Distances (Å) for Select ALnTS4

CsNdSiS4 CsLaGeS4 CsPrSiS4 RbNdSiS4

CsNdGeS4

CsGdGeS4

CsTbGeS4

Inorganic Chemistry

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Inorganic Chemistry

Figure 2. Polyhedral representation of CsLaSiS4 and RbCeSiS4 (Pnma phases). The Ln trigonal prisms, silicon tetrahedra, alkali ions, and sulfur atoms are depicted in orange, blue, pink, and yellow, respectively (a). Perspective view of a [LaSiS4]− layer down the a axis.

Figure 3. Polyhedral representation of the CsNdGeS4 (P212121 phase). Nd and Ge are shown as orange and purple polyhedra, and Cs ions are shown as pink spheres (a). Illustration of one of the [NdGeS4]− slabs making up the crystal structure (b).

that all attempts to acquire phase-pure samples failed if the T source (T = Si, Ge) was SiO2 or GeO2 because this led to the formation of roughly equal amounts of the targeted ALnTS4 phase and undesired Ln2OS2 (Ln = lanthanide). Utilizing elemental silicon or germanium as the T source, on the other hand, enabled the preparation of phase-pure samples. This is not surprising because the oxygen from the SiO2 or GeO2 was unable to leave the sealed fused silica tube. Furthermore,

Revisiting some of the early literature on ALnTQ4 phases revealed that their syntheses were performed under reaction conditions involving multidwelling steps.16 The synthesis of the title ALnTS4 phases was carried out by subjecting the reaction to a series of different dwelling temperatures and dwelling times, which resulted in the preparation of several cesium-containing early lanthanide thiogermanates in nearly quantitative yield as large single crystals. It is worth mentioning 8546

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Figure 4. UV−vis optical spectra for CsNdTS4 (T = Si, Ge) (a) and CsLaGeS4 converted from the diffuse reflectance data by the Kubelka−Munk function (b).

dissolve the ionic alkali chloride fluxes, and second, although it contains trace amounts of water, it is not wet enough to degrade the reported ALnTS4 compounds. All reported ALnTS4 compounds survive storage in capped glass vials over several months. Crystal Structure. Although the title compounds crystallize in different space groups (Pnma, P21, P21/n, and P212121), they all exhibit the overall ALnTQ4 layered structure type and are homeotypic. In all title ALnTS4 compounds with the exception of CsLaSiS4 and RbCeSiS4, which features Lncentered (Ln = La, Ce) trigonal prisms, the rare earth cation coordinates with seven sulfur atoms to form a Ln-centered polyhedron that can be best described as a monocapped trigonal prism. The group 14 cation is coordinated to four sulfur atoms forming a T-centered TS4 tetrahedron. The TS4 tetrahedra and LnS7 polyhedra edge share to form infinite twodimensional [LnTS4]− slabs in the ac plane of the unit cell. The basic structure of all title compounds is composed of layers featuring the [LnTS4]− slabs with the monovalent alkali cations residing in the interlayer space and providing charge balance to the overall structure. Detailed structural analysis of CsSmGeS4, homeotypic with all title compounds including the origin of its crystallization in noncentrosymmetric space group P212121, has been explained thoroughly by Bucher et al.2 Briefly, the noncentrosymmetric crystallization results from the CsS8 polyhedra that are interconnected about a 21 screw axis, forming a helix running down the c axis. Figure 1 illustrates the local coordination environments of the Ln3+ ion in CsLaSiS4 and RbCeSiS4 and Ln3+ and T4+ ions in all other ALnTS4 compositions. The general crystal structure of CsLaSiS4 and RbCeSiS4 featuring Ln in a trigonal prismatic coordination environment is shown in Figure 2, and the general ALnTS4 crystal structure of all other compositions is depicted in Figure 3. UV−Vis Diffuse Reflectance Spectroscopy. The optical properties of compounds CsNdSiS4, CsLaGeS4 and CsNdGeS4 were examined on ground samples and are illustrated in Figure 4. The UV−vis absorption spectra for CsNdTS4 (T = Si, Ge) was found to consist of many weak absorption bands consistent with the f−f electronic transitions of the Nd3+

Figure 5. Temperature-dependent magnetic properties of RbNdSiS4, CsNdSiS4, CsNdGeS4, and CsGdGeS4 measured at H = 0.1 T.

Table 5. Magnetic Properties for RbNdSiS4, CsNdSiS4, CsNdGeS4, and CsGdGeS4 compound

observed μeff (μB/Ln3+) from Curie−Weiss fit (200−300 K)

RbNdSiS4 CsNdSiS4 CsNdGeS4 CsGdGeS4

3.72(2) 3.61(2) 3.55(2) 7.92(3)

calculated μeff (μB/Ln3+) 3.63 3.63 3.63 7.94

and and and and

3.69a 3.69a 3.69a 7.95a

ΘW (K) −58.8 −39.4 −33.8 −19.5

a

Values taken from ref 40 pertaining to magnetism on sevencoordinate rare-earth-containing oxides.

elemental Si was used as the silicon source in alkali chloride fluxes because SiO2, while reactive, is not soluble in chloride melts but does dissolve in a pure fluoride or fluoride−chloride melts.42 Fluoride fluxes, however, are difficult to use in silica tube reactions because of chemical etching and breakage of the silica tube by fluoride ions. For postreaction workup, methanol proved to be an effective solvent for two main reasons. First, it is polar and hence able to 8547

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Figure 6. Magnetic-field-dependent magnetic properties of RbNdSiS4, CsNdSiS4, and CsNdGeS4 (a) and CsGdGeS4 (b).

cation. In the spectrum of CsNdSiS4, a weak shoulder located next to the broad absorption band at 800 nm corresponds to the multiplet 4F3/2, while the broad absorption band at 800 nm corresponds to 4F5/2. In addition, the absorption band at 690 nm, a weak band at 640 nm and the most intense peak close to 590 nm correspond to the multiplets 4F7/2, 2H11/2 and 4G5/2, respectively. These data are consistent with what has been reported on the optical properties of Nd3+ ions by Jiang et al.43 For CsLaGeS4, the absorption spectrum exhibits a steep upturn close to 3.70 eV. Extrapolation of the linear segment of the absorption plot to the x-axis provides an approximate band gap of Eg = 3.60 eV. A large optical band gap value is consistent with the white color of CsLaGeS4 powdered sample used for UV−vis diffuse reflectance measurements. No shoulders peaks are observed below 3.60 eV suggesting the absence of potential optical impurities. Magnetic Properties. Figure 5 shows the inverse magnetic susceptibilities for RbNdSiS4, CsNdSiS4, CsNdGeS4, and CsGdGeS4. All magnetic data are summarized in Table 5. All four compounds follow Curie−Weiss behavior above 50 K. Fitting the magnetic susceptibilities to the Curie−Weiss law results in effective moments of 3.71(3)μB/Nd3+, 3.61(2)μB/ Nd3+, 3.55(3)μB/Nd3+, and 7.92(3)μB/Gd3+ for RbNdSiS4, CsNdSiS4, CsNdGeS4, and CsGdGeS4 respectively. These values are in reasonable agreement with the calculated magnetic moments of 3.62μB/Nd3+ and 7.94μB/Gd3+ for free RE3+ ions and other compounds containing seven-coordinate rare earth ions.44 No magnetic ordering was observed down to 2 K, and large negative Weiss temperatures observed in all compounds suggest antiferromagnetic interactions between the f electrons of adjacent rare earth ions. No differences were observed between the FC and ZFC data for all compositions; therefore, only the ZFC data are provided. The magnetic field dependences (M vs H) measured at 2 K for all compositions are shown in Figure 6. The sigmoidal shape of the M vs H plots for all compositions are indicative of the saturation of the magnetic moments with increasing magnetic field. Second Harmonic Generation (SHG). Materials that crystallize in a noncentrosymmetric crystal class, excluding 432, may exhibit SHG behavior. The SHG efficiency is typically

correlated with the material’s dipole moments, where large dipole moments can lead to a large SHG response.45−47 CsLaGeS4 was observed to be SHG-active with an efficiency of ∼0.5(α-SiO2). Although the SHG response is small, it confirms the crystallization of the compound in a noncentrosymmetric space group.

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CONCLUSIONS We have successfully demonstrated that alkali halide fluxes can be used to synthesize complex sulfides as high-quality single crystals that can be used for a variety of physical property measurements. Numerous new compositions were synthesized using the alkali chloride flux growth method, and characterized using single crystal X-ray diffraction, powder X-ray diffraction, energy-dispersive spectroscopy, UV−vis diffuse reflectance spectroscopy, magnetic susceptibility measurements, and second harmonic generation measurements. All compounds crystallize in the layered ALnTQ4 structure type with the alkali ions occupying the interlayer space. RbNdSiS4, CsNdSiS4, CsNdGeS4, and CsGdGeS4 exhibited large negative Weiss temperatures without any evidence of a magnetic transition in the susceptibility data down to 2 K. CsLaGeS4 was found to be SHG-active, corroborating its crystallization in noncentrosymmetric space group P212121.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00849. SEM images and EDS results for CsLaGeS4, CsNdGeS4, KNdSiS4, RbNdSiS4, CsNdSiS4, and CsGdGeS4; powder X-ray diffraction patterns for RbNdSiS4, CsNdSiS4, CsLaGeS4, CsNdGeS4, and CsGdGeS4; and CCDC numbers for all title compounds (PDF) Accession Codes

CCDC 1892416−1892431 and 1892433−1892436 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/ data_request/cif, or by e-mailing [email protected]. 8548

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uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; (fax) +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Gregory Morrison: 0000-0001-9674-9224 Vladislav V. Klepov: 0000-0002-2039-2457 P. Shiv Halasyamani: 0000-0003-1787-1040 Hans-Conrad zur Loye: 0000-0001-7351-9098 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the United States Department of Energy, Office of Science, Basic Energy Sciences for funding this work through award DESC0018739. The SHG work was performed at the University of Houston. W.Z. and P.S.H. thank the Welch Foundation (grant E-1457) for support.

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