A Challenge toward Novel Quaternary Sulfides SrLnCuS3 (Ln = La, Nd, Tm): Unraveling Synthetic Pathways, Structures and Properties

We report on the novel heterometallic quaternary sulfides SrLnCuS3 (Ln = La, Nd, Tm), obtained as both single crystals and powdered samples. The structures of both the single crystal and powdered samples of SrLaCuS3 and SrNdCuS3 belong to the orthorhombic space group Pnma but are of different structural types, while both samples of SrTmCuS3 crystallize in the orthorhombic space group Cmcm with the structural type KZrCuS3. Three-dimensional crystal structures of SrLaCuS3 and SrNdCuS3 are formed from the (Sr/Ln)S7 capped trigonal prisms and CuS4 tetrahedra. In SrLaCuS3, alternating 2D layers are stacked, while the main backbone of the structure of SrNdCuS3 is a polymeric 3D framework [(Sr/Ln)S7]n, strengthened by 1D polymeric chains (CuS4)n with 1D channels, filled by the other Sr2+/Ln3+ cations, which, in turn, form 1D dimeric ribbons. A 3D crystal structure of SrTmCuS3 is constructed from the SrS6 trigonal prisms, TmS6 octahedra and CuS4 tetrahedra. The latter two polyhedra are packed together into 2D layers, which are separated by 1D chains (SrS6)n and 1D free channels. In both crystal structures of SrLaCuS3 obtained in this work, the crystallographic positions of strontium and lanthanum were partially mixed, while only in the structure of SrNdCuS3, solved from the powder X-ray diffraction data, were the crystallographic positions of strontium and neodymium partially mixed. Band gaps of SrLnCuS3 (Ln = La, Nd, Tm) were found to be 1.86, 1.94 and 2.57 eV, respectively. Both SrNdCuS3 and SrTmCuS3 were found to be paramagnetic at 20–300 K, with the experimental magnetic characteristics being in good agreement with the corresponding calculated parameters.

Of the ABCX 3 compounds, the quaternary chalcogenides ALnCuX 3 are of particular interest, since transition and rare earth elements exhibit rich crystal chemistry and specific spectroscopic properties. According to quantum mechanical calculations, the band gap of SrLnCuS 3 is 1.00-1.51 eV [26], indicating that these compounds are promising materials for solar cells with an efficiency of about 20% [39][40][41][42]. However, the lack of experimental data on the band gap of SrLnCuS 3 limits their practical utilization.
Polycrystalline samples of SrLnCuS 3 were obtained by the melting of a stoichiometric ratio of sulfides SrS, Ln 2 S 3 and Cu 2 S, followed by annealing at 970-1170 K for 2-3 (!) months [22,[34][35][36][43][44][45]. Single-crystal samples were synthesized from elemental Sr, Cu and Ln (Ln = Nd, Y, Sc) using a halide flux at 1070 K for 8 days [46,47]. Recently, we have reported on the synthesis of EuLnCuS 3 by the sulfidation of a mixture of oxides obtained by thermolysis of co-crystallized copper nitrate and rare-earth nitrates for 25 h [48]. Thus, using this approach would, likely, significantly reduce the synthesis time towards SrLnCuS 3 . Notably, strontium oxide is more active than europium oxide with respect to the material of the reactor used for synthesis due to the presence of an alkaline earth element. Therefore, it is suggested that the initial synthesis temperatures to produce SrLnCuS 3 should be lower than those of EuLnCuS 3 .
With all this in mind, in this work we have focused on the synthesis of SrLnCuS 3 (Ln = La, Nd, Tm), as well as studying their crystal structures, magnetic and optical properties.

Results and Discussion
The heterometallic quaternary sulfides SrLnCuS 3 (Ln = La, Nd, Tm) were obtained both in the form of single crystals as well as powdered samples. The former synthetic procedure requires heating a stoichiometric ratio of the elements Sr, Cu, Ln and S in the presence of CsBr as a flux for 8 days at 1070 K, while the latter synthetic approach significantly decreases the reaction time in comparison to both a single-crystal method and the sulfide-melting method [22,35,[43][44][45]47].
To date, for the quaternary sulfides, only four crystal structures of SrLnCuS 3 (Ln = Nd, Y, Sc), solved from the single-crystal X-ray diffraction data, have been known so far (Table 1) [46,47], while twelve crystal structures of a series of SrLnCuS 3 (Ln = La, Ce, Pr, Nd, Sm, Gd, Ho, Er, Tm, Yb, Lu, Sc), solved from the powder X-ray diffraction data, have been reported (Table 2) [22,36,44,45]. In the present work, we report for the first time the crystal structures of SrLaCuS 3 and SrTmCuS 3 (Table 1), solved from the single-crystal X-ray diffraction data, as well as the crystal structure of SrLnCuS 3 (Ln = La, Nd, Tm), solved from the powder X-ray diffraction data (Table 2). Notably, the same synthetic approach for the formation of single crystals yields one structural type, Ba 2 MnS 3 , for SrLaCuS 3 , and two structural types, BaLaCuS 3 and Eu 2 CuS 3 , for SrNdCuS 3 of the orthorhombic space group Pnma, while for SrTmCuS 3 , the structural type KZrCuS 3 of the orthorhombic space group Cmcm was established (Table 1). However, different synthetic approaches toward powdered samples yield two structural types, BaLaCuS 3 and Ba 2 MnS 3 , for SrLaCuS 3 , one structural type, BaLaCuS 3 , for SrNdCuS 3 of the same orthorhombic space group Pnma, and one structural type, KZrCuS 3 , of the orthorhombic space group Cmcm for SrTmCuS 3 (Table 2). Interestingly, while the b axis in the structures of SrLaCuS 3 and SrNdCuS 3 is very similar and varies from 4.0072(3) Å to 4.11053(6) Å, the a and c axes differ significantly ranging from 8.1682(6) Å to 16.0394(11) Å (Tables 1 and 2). Furthermore, the cell volume is almost the same in the structures of SrLaCuS 3 , regardless of the structural type, and of 533.85(7)-535.97(1) Å 3 , but differs in the structures of SrNdCuS 3 . Particularly, the cell volume is 518.63(2)-519.14(1) Å 3 in the structural type BaLaCuS 3 , and is remarkably larger in the structural type Eu 2 CuS 3 (545.96(3) Å 3 ). The cell volume in the structures of SrLaCuS 3 is smaller and of 510.50(6)-511.34(3) Å 3 . It should also be noted that in both crystal structures of SrLaCuS 3 obtained in this work, the crystallographic positions of strontium and lanthanum were partially mixed by about 50% and 45%, respectively, while only in the structure of SrNdCuS 3 , solved from the powder X-ray diffraction data, the crystallographic positions of strontium and neodymium were partially mixed by about 11% (Table 3). In both structures of SrTmCuS 3 , each atom fully occupies its own crystallographical position (Table 3).
A 3D crystal structure of both SrLaCuS 3 and SrNdCuS 3 was constructed from the (Sr1/Ln1)S 7 -and (Sr2/Ln2)S 7 -capped trigonal prisms as well as CuS 4 tetrahedra ( Figure 1). However, different structural types of these compounds are clearly reflected in the packing of coordination polyhedra. Particularly, in the structure of SrLaCuS 3 , the (Sr1/La1)S 7 -and (Sr2/La2)S 7 -capped trigonal prisms each form alternating 2D layers, of which the (Sr1/La1)S 7based layers are further strengthened by 1D polymeric chains (CuS 4 ) n (Figure 1). The main backbone of the structure of SrNdCuS 3 is a polymeric 3D framework [(Sr1/Ln1)S 7 ] n , further strengthened by 1D polymeric chains (CuS 4 ) n , with 1D channels along the b axis, filled by the Sr2 2+ /Ln2 3+ cations, which, in turn, form 1D dimeric ribbons along the b axis ( Figure 1). A 3D crystal structure of SrTmCuS 3 differs significantly from the La-and Ndbased derivatives and is constructed from the SrS 6 trigonal prisms and TmS 6 octahedra as well as CuS 4 tetrahedra (Figure 1). The latter two polyhedra are packed together into 2D layers, which are separated by 1D chains (SrS 6 ) n and 1D free channels along the a axis ( Figure 1). In the discussed structures of SrLaCuS 3 , the Sr/La-S bond lengths are 2.910(1)-3.09999(17) Å, while the Sr/Nd-S bonds in the structures of SrNdCuS 3 vary in a broader range from 2.843(1) Å to 3.136(2) Å ( Table 4). The Sr-S bond lengths in the structures of SrTmCuS 3 are similar to those in the La-and Nd-based derivatives, and of 2.966(3)-3.093(2) Å, while the Tm-S bonds are shorter and of 2.703(2)-2.719(2) Å ( Table 4). The Cu-S distances within the coordination tetrahedron in all the reported structures vary from 2.325(2) Å to 2.388(3) Å ( Table 4).
The IR and Raman spectra of SrLnCuS 3 (Ln = La, Nd, Tm) each contain bands exclusively up to about 350 cm −1 (Figure 2), thus the discussed compounds are both IR and Raman transparent, at least in the region of 350-4000 cm −1 . The most intense bands in the IR and Raman spectra were observed at about 220-230 and 65-80 cm −1 , respectively.
For SrLnCuS 3 (Ln = La, Nd, Tm), the experimental band gaps were obtained from the Kubelka-Munk function, and are of 1.86, 1.94 and 2.57 eV, respectively ( Figure 3). Notably, the band gap values for SrLaCuS 3 and SrNdCuS 3 are similar, which might tentatively be explained by the same orthorhombic space group Pnma in the crystal structures of these sulfides, while the crystal structure of the discussed SrTmCuS 3 exhibits the orthorhombic space group Cmcm.
For SrLnCuS3 (Ln = La, Nd, Tm), the experimental band gaps were obtained from the Kubelka-Munk function, and are of 1.86, 1.94 and 2.57 eV, respectively (Figure 3). Notably, the band gap values for SrLaCuS3 and SrNdCuS3 are similar, which might tentatively be explained by the same orthorhombic space group Pnma in the crystal structures of these sulfides, while the crystal structure of the discussed SrTmCuS3 exhibits the orthorhombic space group Cmcm.
Symmetry codes for SrTmCuS 3 : (i) −x, −y, −z; (ii) −x, −y, −1/2 + z; (iii) 1 + x, y, z; (iv) 1 + x, −y, −z; The field-dependent magnetic moment of both SrNdCuS 3 and SrTmCuS 3 at 296 K is linear, which is characteristic for a paramagnetic compound (Figure 4). From this dependence, the effective magnetic moment was calculated as 3.611 and 7.378 µ B for the Nd-and Tm-based sulfides, respectively ( Table 5). The temperature-dependent reciprocal magnetic susceptibility at 20-300 K is well described by the Curie-Weiss law and is the same in both the zero-field cooled (ZFC) and nonzero-field cooled (FC) modes ( Figure 4). As such, the C, µ and θ values were calculated for both compounds at 20-300 K (Table 5).
Experimental magnetic characteristics for SrNdCuS 3 and SrTmCuS 3 are in good agreement with the corresponding calculated parameters, obtained in the model of free ions Nd 3+ and Tm 3+ , respectively (Table 5). Table 4. Bond lengths (Å) in the crystal structures of SrLnCuS 3 (Ln = La, Nd, Tm). For symmetry codes see Figure 1.

Methods
The single-crystal X-ray diffraction data for SrLaCuS 3 and SrTmCuS 3 were collected at room temperature with a Bruker-Nonius κ-CCD diffractometer (Mo-Kα radiation, graphite monochromator) equipped with a CCD detector. The collected intensity data and the numerical correction of the absorption for the measured crystals were processed using the DENZO [54] and HABITUS [55] programs, respectively. The crystal structures were solved and refined using the SHELX-2013 software package [56,57].
The powder X-ray diffraction data for SrLnCuS 3 (Ln = La, Nd, Tm; Figure 5) were collected at room temperature with a ДPOH 7 (Burevestnik, Saint Petersburg, Russia) powder diffractometer (Cu-Kα radiation, graphite monochromator). The step size of 2θ was 0.02 • , and the counting time was 35 s per step. The lattice parameters were determined using the ITO program [58] and the crystal structures were refined by the derivative difference minimization (DDM) method [59] in the anisotropic approximation for all atoms. The effects of preferred orientation, anisotropic broadening of peak and sample surface roughness and displacement were taken into account during refinement. The data for the isostructural sulfides Ba 2 MnS 3 [60], BaLaCuS 3 [61] and KZrCuS 3 [9] were used as initial structural models for SrLaCuS 3 , SrNdCuS 3 and SrTmCuS 3 , respectively. The anomalous disbalance of the thermal parameters of the Sr and Ln sites after the preliminary refinement of the structures indicated a possible mixed filling of their sites due to similar ionic radii. Indeed, refinement of the occupancy of the Sr and Ln sites for the structures of SrLaCuS 3 and SrNdCuS 3 improved agreement between the calculated and experimental data as well as balanced the thermal parameters.  Scanning electron microscopy (SEM) was performed on a JEOLJSM-6510 LV (JEOL Ltd., Tokyo, Japan) equipped with an energy dispersive spectrometer.
The Fourier-transform infrared (FTIR) absorption spectra in the range of 60-675 cm −1 were recorded on a VERTEX 80v FT-IR spectrometer (Bruker OJSC, Ettlingen, Germany). The attenuated total reflectance infrared (ATR-IR) absorption spectra in the range of 400-4000 cm −1 were recorded on a Cary 630 FTIR spectrometer (Agilent Technologies Inc., Santa Clara, CA, USA) equipped with an ATR attachment and a DTGS detector. The Raman spectra were collected in backscattering geometry using a triple monochromator Horiba JobinYvon T64000 Raman spectrometer (Horiba Ltd., Tokyo, Japan) operating in subtractive mode. The spectral resolution for the recorded Stokes-side Raman spectra was better than 3 cm −1 (this resolution was achieved by using gratings with 1800 grooves mm −1 and 100 µm slits). The single-mode radiation at 532 nm from the Spectra-Physics Excelsior laser was used as an excitation light source, the power on the sample being 5 mW.
The diffuse reflectance spectra were recorded on a UV-2600 spectrophotometer (Shimadzu OJSC, Tokyo, Japan) equipped with an ISR-2600Plus attachment with the photomultiplier PMT of the R-928 type and InGaAs detectors. BaSO 4 (99.8%) was used as a standard.
The temperature-dependent (20-300 K) magnetic susceptibilities of SrLnCuS 3 (Ln = Nd, Tm) were studied on a Quantum Design MPMS3 SQUID magnetometer in a 200 Oe (15.92 kA m −1 ) magnetic field. The measurements were performed in the zero-field cooled (ZFC) and nonzero-field cooled (FC) modes. The room temperature magnetic properties of SrLnCuS 3 (Ln = Nd, Tm) were studied on a vibrating sample magnetometer with a Puzey electromagnet [62]. The magnetic field was varied in the range -15 ÷ 15 kOe (-1.2 ÷ 1.2 MA m −1 ).

Synthesis
Crystalline samples of SrLnCuS 3 (Ln = La, Nd, Tm) in the form of single crystals were obtained from a stoichiometric ratio of the elemental strontium, copper, lanthanide and sulfur in the presence of CsBr as a flux. The reaction mixture was heated in an evacuated quartz ampoule for 8 days at 1070 • C. A thin layer of graphite was deposited on the inner wall of the quartz ampoule by a pyrolytic method to avoid side reactions with quartz glass, leading to the formation of oxosilicates. The reaction product was purified from flux residues with demineralized water. The resulting yellow needle-like crystals were suitable for a single-crystal X-ray diffraction analysis.
Powdered samples of SrLnCuS 3 (Ln = La, Nd, Tm) were prepared by reductive sulfidation of the oxide mixtures in a flow of H 2 S and CS 2 , obtained by decomposition of ammonium thiocyanate (argon was used as a carrier gas) according to a synthetic procedure reported recently [36]. According to X-ray phase analysis, the resulting samples comprised a mixture of oxides Ln 2 O 3 , SrLnCuO 4 , (Sr,Ln) 2 CuO 4 , CuSrO 2 , SrLn 2 O 4 , and Sr x Ln 2-x CuO 4 , which were subjected to reductive sulfidation in a flow of H 2 S and CS 2 , yielding the titular quaternary sulfides. Sulfidation was carried out with grinding of the resulting product in three steps: heating at 870 K for 6 h, followed by heating at 1070 K for 4 h, followed by heating at 1170 K for 20 h. This stepwise increasing of the reaction temperature is of importance to avoid the decomposition of reactor material, which, in turn, leads to the formation of silicate impurities. Thus, sulfidation is initiated by isothermal heating at 870 K until the formation of sulfides and oxysulfides, which are not so aggressive with respect to the quartz material. The sulfidation product was cooled in an argon flow. The resulting products SrLnCuS 3 were examined by SEM-EDX, and the obtained data are in agreement with the powder X-ray diffraction data and are collected in Table 6.

Conclusions
In summary, we report on a novel heterometallic quaternary sulfides SrLnCuS 3 (Ln = La, Nd, Tm), which were synthesized both in the form of single crystals as well as powdered samples. The former synthetic procedure allows the production of pure samples but requires heating for 8 days, while the latter synthetic approach significantly decreases the reaction time to less than 2 days. However, synthesis of the powdered samples also yields some impurities, though of minor quantities. The structures of both the single crystal and powdered samples of SrLaCuS 3 and SrNdCuS 3 belong to the orthorhombic space group Pnma but of different structural types, namely Ba 2 MnS 3 , BaLaCuS 3 and Eu 2 CuS 3 . SrTmCuS 3 crystallizes in the orthorhombic space group Cmcm with the structural type KZrCuS 3 for both the single crystal and powdered samples.
Three-dimensional crystal structures of the herein-obtained SrLaCuS 3 and SrNdCuS 3 are formed from the (Sr/Ln)S 7 -capped trigonal prisms as well as CuS 4 tetrahedra, however, yielding different packing of coordination polyhedra. Particularly in SrLaCuS 3 , alternating 2D layers are stacked, while the main backbone of the structure of SrNdCuS 3 is a polymeric 3D framework [(Sr/Ln)S 7 ] n , strengthened by 1D polymeric chains (CuS 4 ) n , with 1D channels, filled by the other Sr 2+ /Ln 3+ cations, which, in turn, form 1D dimeric ribbons. A 3D crystal structure of SrTmCuS 3 is constructed from the SrS 6 trigonal prisms, TmS 6 octahedra and CuS 4 tetrahedra. The latter two polyhedra are packed together into 2D layers, which are separated by 1D chains (SrS 6 ) n and 1D free channels. Different crystal packings in the reported structures are, most likely, explained by both the formation of different structural types, as well as different Sr-and Ln-based coordination polyhedra. Furthermore, in both crystal structures of SrLaCuS 3 obtained in this work, the crystallographic positions of strontium and lanthanum are partially mixed, while only in the structure of SrNdCuS 3 , solved from the powder X-ray diffraction data, are the crystallographic positions of strontium and neodymium partially mixed.
The optical properties of SrLnCuS 3 (Ln = La, Nd, Tm) were revealed by diffuse reflectance spectroscopy, and the band gaps were found to be 1.86, 1.94 and 2.57 eV, respectively. Similar band gap values for SrLaCuS 3 and SrNdCuS 3 might tentatively be explained by the same orthorhombic space group Pnma.
Finally, SrNdCuS 3 and SrTmCuS 3 are paramagnetic at 20-300 K. Experimental magnetic characteristics for these sulfides are in good agreement with the corresponding calculated parameters, obtained in the model of free ions Nd 3+ and Tm 3+ , respectively.