■ INTRODUCTION

Metal chalcogenides have become the foundation of many modern technologies due to their structural and compositional diversity, which results in a plethora of desired physical properties. 1 Chalcometallates, one class of chalcogenides, result from the combination of the chalcogens with certain main group elements that yield complex anionic framework structures. The zur Loye group, utilizing the molten flux synthetic method, has explored the synthesis of single crystals of a variety of chalcometallate materials, including thiophosphates, thiogermanates, and thiosilicates, to explore their magnetic and optical properties. 2,3 A major obstacle to the synthesis of these compounds is the lack of commercially available rare earth sulfide-starting materials, which necessitates their preparation in the laboratory prior to exploring the syntheses of chalcometallate compounds. Recently, we have demonstrated the use of the boron chalcogen mixture (BCM) method to synthesize single crystals of LnBS 3 (Ln = La, Ce, Pr, Nd) and measured their nonlinear optical properties. 4 The BCM method allows for the in situ sulfurization of lanthanide oxide starting reagents, thereby avoiding the need for lanthanide sulfide reagents. 5 As this method has been previously used to synthesize a number of lanthanide and transition metal sulfide compounds, we again chose to utilize the BCM method to explore a family of chalcometallates, RE 3 Mg 0.5 SiS 7 (RE = rare earth) compounds to investigate their magnetic and optical properties. [6][7][8] The RE 3 M x TQ 7 (M = metal, T = Si, Ge) compounds represent an extensive family of quaternary rare-earth chalcogenides that due to the unique structural flexibility of the La 3 Mn 0.5 GeS 7 structure type can accommodate a variety of elements that can impart optical and magnetic properties to the material. [9][10][11][12][13][14][15][16][17][18][19] The structural flexibility of this family results from the metal atom's ability to have full occupancy and to take on a trigonal planar coordination environment, an example being La 3 AgSiS 7 , or be half occupied and take on an octahedral coordination environment, such as in RE 3 Cd 0.5 GeS 7 . 20,21 The compositional flexibility stems from the wide variety of combinations of rare earths, metals, tetrels, and chalcogenides and consists of almost 2000 reported phases. 22 A survey of this family of compounds was recently published by Zhou et al., which highlighted the lack of certain elemental compositions within this phase space. 20 One series with minimal reported phases is the rare earth magnesium thiosilicates, RE 3 Mg 0.5 SiS 7 , the La 3 Mg 0.5 SiS 7 and Y 0.5 Mg 0.5 SiS 7 analogues being the only ones reported. 23,24 For this reason, we decided to investigate this series in order to explore the optical and magnetic properties of rare earth metals in various structural environments. Furthermore, Mg 2+ is diamagnetic and optically inactive, enabling us to study solely the behavior of the rare earths in these compounds. Many of these compounds, including those presented in this paper, crystallize in the noncentrosymmetric space group P6 3 , which makes these structures potentially SHG active and, thus, able to exhibit nonlinear optical properties (NLO). To date, the synthesis of the known RE 3 M x TQ 7 (M = metal, T = Si, Ge) compounds has primarily been carried out by solid-state syntheses, although a few other synthetic routes have been reported, such as molten flux synthesis and arc melting of precursors. 25,26 We chose to utilize the combined BCM and molten flux methods to grow crystals of the title compounds for structural characterization and, in some cases, also performed solid-state synthesis using the BCM method to create bulk samples. Herein, we present the synthesis of highquality single crystals of RE 3 Mg 0.5 SiS 7 (RE = La-Nd, Sm-Er) and the determination of their crystal structures. Furthermore, the magnetic properties of RE RE 3 Mg 0.5 SiS 7 (RE = Nd-Er) were synthesized by the addition of 50 mg of RE 2 O 3 , 10 mg of boron, 45 mg of sulfur, 5 mg of SiO 2 , and 2 mg of MgO powders into a fused silica tube (10 mm × 12 mm inner and outer diameters, about 15 cm length) along with 250 mg of NaI flux (75 mg in the case of Nd). The fused silica tube was evacuated to 10 -4 torr and flame-sealed using a methane/oxygen torch. The sealed fused silica tube was placed into a programmable furnace set to heat to 760 °C in 20 h to dwell at this temperature for 20 h and to cool to 560 °C in 20 h, at which point the furnace was shut off and allowed to return to room temperature. RE 3 Mg 0.5 SiS 7 (RE = La, Pr) were synthesized using a similar procedure, except for the use of 75 mg of a CaCl 2 /NaCl eutectic instead of NaI, and the reagent mixture was ground using a mortar and pestle prior to the addition of the flux. The tube was heated to 950 °C in 20 h, dwelled at this temperature for 20 h, and cooled to 750 °C in 20 h, at which point the furnace was shut off and allowed to return to room temperature. RE 3 Mg 0.5 SiS 7 (RE = Ce) was obtained using the same synthetic procedures as the RE = La and Pr analogues but with the use of 75 mg of NaCl flux instead of the CaCl 2 /NaCl eutectic.

To prepare samples for property measurements of RE 3 Mg 0.5 SiS 7 (RE = La-Sm), solid-state syntheses were performed using the BCM method, and the same materials were used for the flux reactions, however, with the absence of the flux. The masses used were also consistent with the masses mentioned above. For these reactions, the reagents were intimately ground. The ground mixture was added to a fused silica tube that was placed into a programmable furnace set to ramp to 950°C and dwelled for 12 h, at which point the furnace was shut off and allowed to return to room temperature.

Methanol was initially used for the dissolution of the flux, however, impurities in the product remained. For that reason, water was used for the workup after establishing that the target compositions are sufficiently air and water stable. This successfully removed the impurity phases from the later rare earths (RE = Gd-Er). The powder X-ray diffraction patterns of all materials used for property measurements can be seen in Figures S1-S4.

Caution. Boron sulfides are moisture-sensitive and produce H 2 S gas in contact with moisture and water. All reaction work needs to be performed in fume hoods with the proper safety procedures in place.

Single-Crystal X-ray Diffraction (SXRD). X-ray intensity data of yellowish-brown plate crystals of all title compositions were collected at 300 K using a Bruker D8 QUEST diffractometer equipped with a PHOTON-II area detector and an Incoatec microfocus source (Mo Kα radiation, λ = 0.71073 Å). Single 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. 27 Final unit cell parameters were determined by the least-squares refinement of a large array of reflections from each data set. 28 An initial structural model was obtained with SHELXT. Subsequent difference Fourier calculations and full-matrix least-squares refinement against F 2 were performed with SHELXL-2018 using the ShelXle interface. 29 All compounds crystalize in the noncentrosymmetric, hexagonal space group P6 3 . The asymmetric unit contains three sulfur atoms, one lanthanide atom, one magnesium atom, and one silicon atom. All atoms are all located in positions of general crystallographic symmetry. All atoms were refined with anisotropic displacement parameters. The Mg occupancy refined in all cases to ∼0.5 within standard deviation (0.497(5) for Nd 3 Mg 0.5 SiS 7 ) and was therefore fixed at 0.5. The crystallographic data and diffraction results are listed in Table 1.

Powder X-ray Diffraction (PXRD). Powder X-ray diffraction (PXRD) data were collected using a powder sample of RE 3 Mg 0.5 SiS 7 obtained by the solid-state synthesis (Figures S1-S4). Data were collected on a Bruker D2 PHASER diffractometer using Cu Kα radiation over a 2θ range 10-65°with a step size of 0.02°.

Magnetic Susceptibility. Magnetic property measurements of Ce 3 Mg 0.5 SiS 7 , Sm 3 Mg 0.5 SiS 7 , and Dy 3 Mg 0.5 SiS 7 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 in an applied magnetic field of 0.1 T. Magnetization as a function of the 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. 30 UV-Vis Diffuse Reflectance Spectroscopy. UV-vis diffuse reflectance data, 300-900 nm, for La 3 Mg 0.5 SiS 7 were obtained using a PerkinElmer Lambda 35 UV-vis scanning spectrophotometer equipped with an integrating sphere. The reflectance data were converted to absorbance using the Kubelka-Munk function. 31 Second-Harmonic Generation (SHG). The Kurtz-Perry method was adopted to assess the SHG performance of the sample. Polycrystalline powder of the sample was placed in the tubes of quartz and then irradiated with a pulsed infrared beam (1064 nm) produced by a Q-switched Nd:YAG laser. The generated secondharmonic signals (532 nm) were collected by the detector. KDP (KH 2 PO 4 ) served as a standard during the SHG test procedure. 32,33 ■ RESULTS AND DISCUSSION Synthesis. A review of RE 3 M x TQ 7 compounds reveals that a significant number of compositions that are structurally and compositionally related to RE 3 M x TQ 7 , and that are of interest for better understanding of the crystal chemistry of thiosilicates, have not been reported. We targeted the synthesis of some of these compositions to investigate their crystal chemistry and, at the same time, to extend the use of our BCM method to more diverse structural families. Therefore, we used the BCM method to synthesize the RE 3 Mg 0.5 SiS 7 family both as powders and as single crystals. Table 2 lists the known compositions of the Ln 3 Mg 0.5 SiS 7 structure type, while Table 3 lists the known compositions of the Ln 3 Mg 0.5 GeS 7 structure type; clearly, more thiogermanates than thiosilicates have been reported.

We performed the synthesis of RE 3 Mg 0.5 SiS 7 RE = La, Ce, Pr, Nd, Sm-Er series, targeting both single crystals and powders. Using NaI (Nd, Sm-Er) and CaCl 2 /NaCl (La, Ce, Pr) as fluxes and using boron powder to remove the oxygen from our RE 2 O 3 reagents, we successfully obtained high-quality single crystals for all of the target compositions. Although the

RE = Sm-Er target phases were crystallized as the major product, Mg 2 (SiS 4 ) was identified as a minor impurity phase in these reactions and, even after numerous attempts to optimize the flux crystal growth reaction conditions, could not be eliminated. For RE = La-Nd, the target phases would not form when using the same synthetic procedure used to obtain the RE = Sm-Er compounds due to the formation of RE 3 (SiS 4 ) 2 I as the major reaction product. 34 It is important to note that while Sm 3 (SiS 4 ) 2 I has been reported, the chlorine analogue has not. 35 This may be the reason why the NaI flux led to the formation of Sm 3 (SiS 4 ) 2 I, while the chloride flux did not, vide infra, form Sm 3 (SiS 4 ) 2 Cl. To eliminate the RE 3 (SiS 4 ) 2 I phases entirely from the product, the NaI flux was changed to the NaCl/CaCl 2 eutectic. This led to the successful synthesis of the target phases, La 3 Mg 0.5 SiS 7 and Pr 3 Mg 0.5 SiS 7 , which formed as minor products along with a new rare earth calciumcontaining thiosilicate as the major product. We will report on the latter materials in the near future. To eliminate the formation of this calcium-containing phase for the synthesis of the cerium and neodymium phases, a NaCl flux was used. The target phase, Ce 3 Mg 0.5 SiS 7 , was successfully synthesized using this method. The target phase, Nd 3 Mg 0.5 SiS 7 , was synthesized by decreasing the flux amount from 200 to 75 mg. Attempts at the synthesis of RE = Tm-Lu using the synthetic routes described above failed. In all cases, only MgRE 2 S 4 and Mg 2 (SiS 4 ) were isolated. This is perhaps not unexpected as no other RE-Mg-Si-S compositions belonging to this structure type have been reported, as illustrated in Table 2.

Structure Description. The RE 3 Mg 0.5 SiS 7 structure is a member of the RE 3 M 0.5 TQ 7 family of compounds that crystallizes in the hexagonal crystal system, adopting the noncentrosymmetric space group P6 3 . All of the title compositions are isostructural and crystallize in this space group. This three-dimensional structure consists of RES 8 bicapped trigonal prisms that edge and corner share to create a ring-like assembly. Isolated SiS 4 tetrahedra throughout the structure are located between each ring-like assembly of RES 8 bi-capped trigonal prisms. Face-sharing MgS 6 octahedra are located in the middle of the ring-like assembly. Figure 1 illustrates the local coordination environments of the RE 3+ , Si 4+ , and Mg 2+ cations, respectively, along with the general structure of RE 3 Mg 0.5 SiS 7 . Figure 2 contains a plot of the unit cell volume of RE 3 Mg 0.5 SiS 7 as a function of the rare-earth ionic radius. The decrease in volume when going from La to Er is expected due to the lanthanide contraction in these compounds.

Magnetic Properties. The magnetic susceptibility data for Ce 3 Mg 0.5 SiS 7 , Dy 3 Mg 0.5 SiS 7 , and Sm 3 Mg 0.5 SiS 7 are shown in Figure 3. The data in these plots were analyzed and the magnetic moment was extracted from a fit to the Curie-Weiss Law. The moments and Weiss constants are summarized in Table 4 and were found to be in excellent agreement with the calculated moments. The negative Weiss temperatures suggest antiferromagnetic interactions, but as seen in Figure 3, no antiferromagnetic transitions are observed above 2 K, the lowest temperature measured.

The Samarium analogue, as expected, exhibits Van Vleck paramagnetism in the 2-300 K temperature range measured. 36 Due to the lack of Curie-Weiss behavior, the effective moment of Sm 3+ at room temperature was determined to be 1.45 μ B /Sm 3+ . This is in agreement with other Sm chalcogenides, such as the reported BaSm 2 S 4 , which has an experimentally reported moment of 1.47 μB /Sm 3+ . 37 Optical Properties. UV-vis diffuse reflectance data were collected on the La 3 Mg 0.5 SiS 7 sample. A Tauc plot indicates that La 3 Mg 0.5 SiS 7 is a direct band gap semiconductor with a band gap of 2.77 eV (Figure S5). In addition, La 3 Mg 0.5 SiS 7 was examined for SHG behavior. Materials that crystallize in a noncentrosymmetric crystal class may exhibit SHG behavior, 36 and a multitude of reported RE 3 M x TQ 7 compositions that crystallize in the noncentrosymmetric space group P6 3 have had their SHG properties analyzed. 38,39 La 3 LiTS 7 (T = Ge, Sn) exhibits a strong NLO effect and high laser damage threshold (LDT), likely caused by a mixed contribution of LnS 8 and TS 4 groups. 40 We decided to investigate the SHG properties of La 3 Mg 0.5 SiS 7 , which had been reported as SHG inactive. 24 Measurements on our La 3 Mg 0.5 SiS 7 sample indicated that it was in fact SHG active with an efficiency of ∼0.16 times the standard KDP. Figure 4 shows the SHG intensity vs time of La 3 Mg 0.5 SiS 7 and the KDP standard. Due to the coloration of the crystals of the other compositions that interferes with SHG measurements, no other samples were tested for SHG activity.