Syntheses, structures, and properties of sulfides constructed by SbS₄ teeter-totter polyhedra: Ba₃La₄Ga₂Sb₂S₁₅ and BaLa₃GaSb₂S₁₀

Abstract

Two new pentanary sulfides Ba₃La₄Ga₂Sb₂S₁₅ (1) and BaLa₃GaSb₂S₁₀ (2) have been discovered by high temperature solid state reactions. Ba₃La₄Ga₂Sb₂S₁₅ features a new zero-dimensional structure type, which consists of unique isolated Sb₂S₇ dimers and GaS₄ tetrahedra. In contrast, with less GaS₄ tetrahedra in the formula, BaLa₃GaSb₂S₁₀ forms a one-dimensional structure which is constructed by teeter-totter (SbS₄)_n chains and isolated GaS₄ tetrahedra. On the basis of UV/Vis spectroscopy, their band gaps are determined to be 2.42 and 2.15 eV, respectively, which are much wider than that of analogue La₄FeSb₂S₁₀ (1.0 eV). In addition, theoretical studies illustrate the important role of Ga in expanding the band gaps with respect to Fe atoms. The calculated crystal orbital Hamilton population and electron localization function confirm that the long Sb–S bonds in Ba₃La₄Ga₂Sb₂S₁₅ (2.932(2) Å) and BaLa₃GaSb₂S₁₀ (3.216(5) and 3.219(5) Å) have weak but nonnegligible bonding interactions.

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Introduction

Over the past few decades, Sb-containing chalcogenides have attracted lots of attention because of their diverse structures and various physical properties.^1–24 In terms of their crystallographic structures, the common building units of chalcogenides are MX₄ tetrahedra and MX₆ octahedra (M = cation of group 13 or 14 elements, d¹⁰, or d⁰etc.; X = chalcogen).^25–29 In particular, a Sb³⁺ cation has a lone electron pair configuration of 5s²5p⁰. With stereochemically active lone-pair electrons, the Sb³⁺ cation exhibits flexible structural chemistry with the coordination number ranging from 3 to 6 to form diverse polyhedra. Sb^IIIX_n (n = 3–6) polyhedra can connect with second building units, which further evolve into one-dimensional (1D) chains, two-dimensional (2D) layers or three-dimensional (3D) frameworks via sharing corners, edges or faces.³⁰ Among these Sb^IIIX_n (n = 3–6) polyhedra, the Sb^IIIX₄ teeter-totter polyhedron (Fig. S1‡), which can be considered as a hemioctahedron and differs from the conventional tetrahedron MX₄, has aroused our interest.³¹ More importantly, the Sb^IIIS₄ teeter-totter polyhedron has not yet been seen as an isolated polyhedron, but can be seen as a component of some structures, including (1) isolated finite complex anions, such as in Ba₄LaGe₃SbSe₁₃,³² Ba₄Sb₃S₈Cl,³³ and Ba₈Sb₆S₁₇ ³⁴ (see Fig. S2‡); (2) infinite chains, such as in Pr₄GaSbS₉,⁵ La₄InSbS₉,⁶ La₄FeSb₂S₁₀,⁸ Ba₄SiSb₂Se₁₁,¹³ Na₉Gd₅Sb₈S₂₆,³⁵ SrGeSb₂Se₈,³⁶ and BaSb₂S₄ ³⁷ (see Fig. S3‡); and (3) layers, such as in La₂Ga_0.33SbS₅,¹⁵ RbU₂SbS₈,³⁸ and KU₂SbSe₈ ³⁸ (see Fig. S4‡). Inspired by the above phenomenon, we chose the Sb^IIIS₄ teeter-totter polyhedron as a potential unit to build chalcogenides with novel structures and properties. In addition, with the result that La₄FeSb₂S₁₀ ⁸ exhibits a very narrow band gap of 1.00 eV, replacing Fe atoms with Ga atoms may be an effective way to enlarge band gaps because the latter lacks low energy d–d transitions. Meanwhile, with a relatively high positive charge, alkaline-earth metal cations such as Ba²⁺ can increase the ionic contributions of the lattice energy, which helps to enrich the structure and physical properties of compounds.³⁹

We thoroughly investigated the pentanary Ba/RE/Ga/Sb/S (RE = Rare-earth metal) system where GaS₄ tetrahedra and the unique teeter-totter polyhedron SbS₄ are used as building units. Herein, two new pentanary chalcogenides Ba₃La₄Ga₂Sb₂S₁₅ (1) and BaLa₃GaSb₂S₁₀ (2) were discovered and characterized. The isolated teeter-totter dimer Sb₂S₇ is found for the first time in compound 1. Whereas, compound 2 features a branched chain of (SbS₄)_n which is constructed by the teeter-totter SbS₄ polyhedra. In addition, structural relationships, band gaps and electronic structures are discussed.

Experimental

Synthesis

All related elements were stored in an Ar-filled glovebox (oxygen and moisture levels are all below 0.1 ppm), and all manipulations were carried out in a glovebox. La (with purities of 99.5%), S (99.999%) and CsBr (99.9%) were purchased from Aladdin China (Shanghai) Co. Ltd, Ga shot and Sb (99.999%) were purchased from Sinopharm Chemical Reagent Co. Ltd and Ba rod (99+%) and BaCl₂ (99.5%) were purchased from Alfa Aesar China (Tianjin) Co. Ltd. For compound 1, a total 0.5 g of Ba, La, Ga, Sb and S in a molar ratio of 3 : 4 : 2 : 2 : 15 were mixed in a graphite crucible in a glovebox. For 2, a total 0.5 g of Ba, La, Ga, Sb and S in a molar ratio of 1 : 3 : 1 : 2 : 10 were mixed with 0.2 g of BaCl₂ and 0.4 g of CsBr in a graphite crucible. The graphite crucibles were sealed in evacuated fused-silicon tubes under 10⁻³ Pa atmosphere, then the assemblies were heated to 1253 K for 72 h, and then held for 120 h at this temperature, before being subsequently cooled to 573 K during 120 h before the furnace was turned off. The yellowish-brown and brown crystals were collected after being washed with distilled water. Both crystals are stable in air at room temperature for at least two years.

Single-crystal X-ray crystallography

All crystal data collections were performed at 298 K on a Rigaku Saturn724 diffractometer which is equipped with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). The data were corrected for Lorentz and polarization factors. Absorption corrections were operated by the multiscan method.⁴⁰ Both of the structures were solved by direct methods and refined by the full-matrix least-squares fitting on F² by SHELX-97.⁴¹ Both of the structures were verified using the ADDSYM algorithm with the program PLATON.⁴² The assignments of all atoms were determined on the basis of the interatomic distances and relative displacement parameters. These atoms were refined with anisotropic thermal parameters and a secondary extinction correction. No fractional occupancy was observed. Crystallographic data and structural refinement details are drawn in Table 1, the positional coordinates and isotropic equivalent thermal parameters are drawn in Table S1‡ and some related bond lengths (Å) are given in Table S2.‡

Table 1 Crystal data and structure refinements for Ba₃La₄Ga₂Sb₂S₁₅ (1) and BaLa₃GaSb₂S₁₀ (2)^a

Formula	1	2
a R 1 = ∑‖Fo| − |Fc‖/∑|Fo|, wR2 = [∑w(Fo^2 − Fc^2)^2/∑w(Fo^2)^2]^1/2.
F w	1831.49	1187.9
Crystal system	Orthorhombic	Monoclinic
Crystal color	Yellowish-brown	Brown
Space group	lbam (no. 72)	P21/m (no. 11)
a (Å)	23.628(2)	7.748(4)
b (Å)	7.8863(5)	13.507(7)
c (Å)	13.5905(4)	15.410(8)
α (°)	90	90
β (°)	90	90.09
γ (°)	90	90
V (Å^3)	2532.5(3)	1612.7(15)
Z	4	4
D c (g cm^−3)	4.804	4.893
μ (mm^−1)	16.580	16.373
GOOF on F^2	1.042	1.007
R 1, wR2 (I > 2σ(I))^a	0.0351, 0.0944	0.0405, 0.1105
R 1, wR2 (all data)	0.0412, 0.1351	0.0557, 0.1393
Largest diff peak and hole, e Å^−3	3.558 and −4.595	3.174 and −3.877

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Powder X-ray diffraction

The powder XRD patterns were collected on a Rigaku MiniFlex II diffractometer using Cu Kα radiation at 298 K. Data in the range 2θ = 10–70° were collected with scan steps of 0.02°. The measured and simulated XRD patterns which are calculated from the single crystal data are shown in Fig. S5.‡

Elemental analysis

The elemental analyses have been examined on handpicked single crystals of 1 and 2 with the aid of a field emission scanning electron microscope (FESEM, JSM6700F) equipped with an energy dispersive X-ray spectroscope (EDX, Oxford INCA). The results showed the presence of Ba, La, Ga, Sb and S, and the ratios of elements in compound 1 and 2 are close to the structure determination.

UV/Vis diffuse reflectance spectroscopy

The optical diffuse reflectance spectra of powdered samples were obtained at 298 K using a Perkin-Elmer Lambda 950 UV-Vis spectrophotometer which was equipped with an integrating sphere attachment and BaSO₄ as a reference in the range of 0.25–2.5 μm. The absorption spectra were obtained from the reflection spectra according to the Kubelka–Munk function: α/S = (1 − R)²/2R, in which α is the absorption coefficient, S is the scattering coefficient, and R is the reflectance.⁴³

Theoretical calculations

Band structures and the density of states (DOS) were calculated by the Vienna ab initio simulation package VASP.⁴⁴ The generalized gradient approximation (GGA)⁴⁵ was chosen as the exchange–correlation functional and a plane wave basis with the projector augmented wave (PAW) potentials was used.⁴⁶ The plane-wave cutoff energy of 500 eV, and the threshold of 10⁻⁵ eV were set for the self-consistent-field convergence of the total electronic energy. Pseudo atomic calculations were performed for Ba, 5s²5p⁶6s²; La, 4p⁶5d¹6s²; Ga, 4s²4p¹; Sb, 5s²5p³ and S, 3s²3p⁴. The k integration over the Brillouin zone was performed by the tetrahedron method⁴⁷ using a 3 × 3 × 3 Monkhorst–Pack mesh for compound 1 and 5 × 5 × 16 Monkhorst–Pack mesh for 2. In addition, to analyze the bond strengths of Sb–S, the integrated crystal orbital Hamilton population (COHP) was calculated.^47,48

Results and discussion

Crystal structure

Ba₃La₄Ga₂Sb₂S₁₅ (1). Compound 1 features a new structure type in which the isolated dimer of the teeter-totter Sb₂S₇ polyhedron is stabilized for the first time. The compound crystallizes in the orthorhombic space group Ibam (no. 72) with a = 23.628(2), b = 7.8863(5), c = 13.5905(4) Å and Z = 4. As shown in Fig. 1a, compound 1 contains isolated Sb₂S₇ dimers that are well separated by Ba²⁺ and La³⁺ cations and isolated GaS₄ tetrahedra. The structure can be viewed as a stacking of slabs of Ba–Ga–S ([Ba₃Ga₂S₈] slab, Fig. 2a) and La–Sb–S ([La₄Sb₂S₇] slab, Fig. 3a) along the c direction. More specifically, the Ba–Ga–S slab includes two crystallographically different Ba atoms (4a, 8f), one Ga atom (8f), and two S atoms (16k), and is named as the [Ba₃Ga₂S₈] slab. The slab consists of isolated GaS₄ tetrahedra and Ba²⁺ cations (Fig. 2a). According to Fig. S6,‡ the two crystallographically different Ba ions in compound 1 are all surrounded by ten S atoms with Ba–S lengths of 3.182(3)–3.424(3) Å, which are common in sulfides, such as Ba₂₃Ga₈Sb₂S₃₈ (2.896(2)–3.331(2) Å),⁷ Ba₃(BS₃)_1.5(SbS₃)_0.5 (3.097(2)–3.512(2) Å)¹⁰ and BaAgSbS₃ (3.165(3)–3.369(3) Å).¹⁹ Based on Fig. 2a, two-thirds of the tetrahedral sites are occupied by GaS₄ with the bonds of Ga–S ranging from 2.262(3) to 2.449(3) Å, which are comparable to that in Pr₄GaSbS₉ (2.272(2)–2.312(2) Å),⁵ Ba₂₃Ga₈Sb₂S₃₈ (2.217(2)–2.318(2) Å)⁷ and CsCd₄Ga₅S₁₂ (2.351(4)–2.416(4) Å).²⁵ On the other hand, Fig. 3a shows that the La–Sb–S slab includes two crystallographically different La (8j), one Sb (8j), and three S atoms (8j, 16k and 4c sites). This gives an atomic ratio of 16 La : 8 Sb : 28 S = 4 La₄Sb₂S₇. The [La₄Sb₂S₇] slab contains isolated Sb₂S₇ dimers and La³⁺ cations. The La ion is coordinated by eight S atoms to form a bicapped trigonal prism with the La–S bond lengths between 2.869(3) and 3.054(3) Å (see Fig. S6‡), which are in the normal range, for example as shown in La₄InSbS₉ (2.838(2)–3.040(2) Å),⁶ La₂Ga_0.33SbS₅ (2.893(5)–3.3531(4) Å)¹⁵ and La₇Sb₉S₂₄ (2.787 (3)–3.747(2) Å).²³ In addition, as shown in Fig. 3a, the Sb atoms are coordinated by four S atoms with two short Sb–S bonds of 2.443(3) Å and two long Sb–S bonds in the range of 2.727(4)-2.932(2) Å to form the teeter-totter SbS₄ polyhedra. The uncommon isolated dimer Sb₂S₇ is then formed by two SbS₄ units via corner sharing with the long Sb–S bonds of 2.932(2) Å, which is comparable with the long Sb–S bonds of teeter-totter SbS₄ polyhedra in Nd₄InSbS₉ (2.924(3) Å),⁶ La₄FeSb₂S₁₀ (2.917(2) Å)⁸ and Ba₄Sb₃S₈Cl (3.062(2) Å).³³ Moreover, the significant bonding interaction of the long Sb–S bond (2.932(2) Å) is confirmed by the ICOHP value of −1.068 eV per bond (see Table 2, Fig. 6), which illustrates that the local geometries around Sb atoms are indeed 4-fold coordinated teeter-totter polyhedra as revealed by crystallographic analysis. Although they have similar bond lengths and angles, the Sb₂S₇ dimers in compound 1 are very different from the Sb₂S₆ dimers. The common Sb₂S₆ dimers are constructed by two teeter-totter SbS₄ polyhedra via edge-sharing, then linked into one-dimensional (1D) chains via different second units, e.g. Ga₂S₆, In₂S₆ and SbS₃ in Pr₄GaSbS₉,⁵ Nd₄InSbS₉,⁶ and Ba₄Sb₃S₈Cl ³³ (see Fig. S2‡). Notably, the isolated dimer Sb₂S₇ in 1 is observed for the first time.

Fig. 1 View of the unit cell structures of (a) compound 1, (b) compound 2 and (c) La₄FeSb₂S₁₀.⁸ Legend: black, Ba; red, La; blue, Sb; yellow, S; pink tetrahedron, GaS₄; green tetrahedron, FeS₄.

Fig. 2 (a) The [Ba₃Ga₂S₈] slab in compound 1, (b) [BaLaGaS₄] slabs in compound 2, and (c) [La₂FeS₄] slabs in compound La₄FeSb₂S₁₀.⁸

Fig. 3 (a) View of the [La₄Sb₂S₇] slab in compound 1, and (b) the [La₂Sb₂S₆] slab in 2. The Sb–S bond lengths are in Å.

Table 2 The interatomic distances (Å) and –ICOHP (eV per bond) values of Sb–S in compound 1 and 2

Bond	Interatomic distance	–ICOHP
1
Sb1–S4 × 2	2.443(3)	3.470
Sb1–S2	2.727(4)	1.700
Sb1–S5	2.932(2)	1.068
2
Sb1–S3 × 2	2.454(3)	3.478
Sb1–S12	2.681(5)	1.795
Sb1–S4	2.747(4)	1.357
Sb2–S10 × 2	2.439(3)	3.362
Sb2–S11	2.539(4)	2.506
Sb2–S12	3.219(5)	0.4035
Sb3–S9 × 2	2.435(3)	3.378
Sb3–S12	2.530(4)	2.515
Sb3–S11	3.216(5)	0.4064
Sb4–S1 × 2	2.447(3)	3.505
Sb4–S11	2.675(5)	1.800
Sb4–S2	2.733(4)	1.378

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BaLa₃GaSb₂S₁₀ (2). Compound 2 crystallizes in the monoclinic space group P2₁/m (NO. 11) with a = 7.748(4), b = 13.507(7), c = 15.410(8) Å, β = 90.05° and Z = 4. As show in Fig. 1b, this compound features a 1D structure in which the Ba–La–Ga–S and La–Sb–S slabs are propagating along the b direction. This is similar to La₄FeSb₂S₁₀ which consists of La–Fe–S and La–Sb–S slabs propagating along the c direction (see Fig. 1c).⁸ As shown in Fig. 2b, the Ba–La–Ga–S slab in 2 features a centric 0D structure which contains isolated GaS₄ tetrahedra with Ba1²⁺ and La5³⁺ cations located between them, named as BaLaGaS₄. Compared to the slab of La–Fe–S ([La₂FeS₄] slab, Fig. 2c) in La₄FeSb₂S₁₀, the GaS₄ tetrahedron in 2 replaces the FeS₄ tetrahedron with similar bond lengths and angles of 2.243(3)–2.275(3) Å and 103.8(1)–119.2(1)° vs. 2.298(1)–2.303(1) Å and 106.006(4)–112.31(4)°, respectively.⁸ Meanwhile, to balance the valency, the Ba atoms in 2 substitute the La atoms in La₄FeSb₂S₁₀ with the similar geometry configurations of BaS₁₀ (3.203(3)–3.422(3) Å) vs. LaS₁₀ (3.079(1)–3.336(1) Å) (see Fig. S7‡). On the other hand, the La–Sb–S slab exhibits 1D (SbS₄)_n branched chains along the a axis with La³⁺ cations filling in them, named as La₂Sb₂S₆, which is comparable with the La–Sb–S slab in La₄FeSb₂S₁₀ (see Fig. 3b).⁸ In the [La₂Sb₂S₆] slabs, there are four La atoms (2e), four Sb (2e) and eight S atoms (located at 4f, 2e, 4f, 2e, 4f, 4f, 2e and 2e sites, respectively). Sb1 and Sb4 have the same geometry configuration of SbS₄ teeter-totter polyhedra (2.446(3)–2.747(4) Å). In addition, Sb2 and Sb3 all show three short Sb–S bonds (2.439(3)–2.539(4) vs. 2.435(3)–2.530(4) Å) and one long bond (3.219(5) vs. 3.216(5) Å) to generate SbS₄ teeter-totter polyhedra. The 1D (SbS₄)_n branched chains are then connected by SbS₄ teeter-totter polyhedra via corner sharing.

Remarkably, the long Sb–S bonds of the SbS₄ teeter-totter polyhedra in compound 2 are longer than those in 1 and La₄FeSb₂S₁₀,⁸ 3.216(5)–3.219(5) vs. 2.932(2) vs. 2.917(2) Å. However, the long Sb–S bonds also can be seen in other Sb-containing chalcogenides, such as Pr₄GaSbS₉ (2.446(3)–2.851(2) Å),⁵ Nd₄InSbS₉ (2.431(3)–2.924(3) Å),⁶ Sr₆Sb₆S₁₇ (2.428(2)–3.436(3) Å),¹² La₇Sb₉S₂₄ (2.433(2)–3.446(2) Å),²³ Ba₄Sb₃S₈Cl ³³ (2.375(2)–3.062(2) Å) and Na₉Gd₅Sb₈S₂₆ (2.444(2)–3.274(2) Å).³⁵ According to Table 2, the calculated COHP values of the Sb–S bonds (3.216(5) and 3.219(5) Å) are negative (−0.40637 and −0.40350 eV per bond). This evidences the weak bonding interactions of Sb–S bonds, which demonstrate that the SbS₄ polyhedra are indeed extending in a novel teeter-totter chain motif via corner sharing as shown in Fig. 3b. On the other hand, the angles of S–Sb–S in the SbS₄ units in 1, 2, and La₄FeSb₂S₁₀ ⁸ also show obvious differences. The axial angles of S–Sb–S for 1, 2 and La₄FeSb₂S₁₀ ⁸ are closer and in the range of 152.63(5)–159.4(2)°. However, in compound 2, there are partial axial angles of S–Sb–S which vary from 160.9(2) to 161.1(2)°. The equatorial angles of S–Sb–S are 94.32(2) vs. 99.3(2)–103.1(2) vs. 103.38(5)–104.13(5)° in compound 1, 2 and La₄FeSb₂S₁₀,⁸ respectively.

In addition, guided by the [Ba₃Ga₂S₆], [BaLaGaS₄] and [La₂FeS₄] slabs in compounds 1, 2 and La₄FeSb₂S₁₀,⁸ the extra Ba atoms and GaS₄ tetrahedra in 1 increase unit cell a from 15.066(4), to 15.410(8) and 23.628(8) Å. Meanwhile, extra La in the [La₂Sb₂S₆] slabs of 1 also cannot be ignored in the increase of unit cell a. However, b and c of the unit cell in 1 do not show any obvious change comparable with 2 and La₄FeSb₂S₁₀.⁸

Optical properties

The optical absorption spectra (Fig. 4) show that the band gaps are 2.15 and 2.42 eV for 1 and 2, respectively, which is consistent with their brown colors. These values are larger than those of La₂Ga_0.33SbS₅ (1.76 eV),¹⁵ and Na₉Gd₅Sb₈S₂₆ (1.63 eV),³⁵ however similar to the values of Sm₄GaSbS₉ (2.23 eV),⁵ La₄InSbS₉ (2.07 eV),⁶ and Ba₄LaGe₃SbSe₁₃ (2.0 eV).³² Compared to La₄FeSb₂S₁₀ (1.0 eV),⁸ the band gaps of 1 and 2 are enlarged effectively by the substitution of Fe with Ga. This trend is consistent with that of the corresponding binary compounds, Ga₂S₃vs. FeS. The detailed reasons for the differences among the band gaps of 1, 2 and La₄FeSb₂S₁₀ ⁸ will be discussed below.

Fig. 4 The UV-vis diffuse reflectance of compound 1 and 2.

Theoretical results

The electronic band structures (see Fig. S8a‡) reveal a direct band gap for 1, with the valence band maximum (VBM) and the conduction band minimum (CBM) located at the same X points. While, compound 2 shows the antitype (an indirect band gap) in which the VBM and CBM are located at different k points (see Fig. S8b‡). The calculated band gaps display an increasing trend with the following order: 1 (1.82 eV) < 2 (1.92 eV), which is consistent with the experimental values of 1 (2.15 eV) and 2 (2.42 eV) (see Fig. 4). A comparison of the band gaps of compounds 1, 2 and La₄FeSb₂S₁₀,⁸ shows that there is about a 1.4 eV difference between them. The reason is shown in Fig. 5, which illustrates the total and partial densities of states (PDOS) of compound 1, 2, and La₄FeSb₂S₁₀.⁸ In the PDOS, the contributions of the La-4f states are not shown. As shown in Fig. 5a and b, the VBM of compound 1 and 2 is dominated by the S-3p states with minor contributions from the La-5d, Sb-5p, Ba-5d and Ga-4p states. Moreover, the CBM is mainly occupied by the La-5d states with small amounts of S-3p, Ga-4s, Sb-5p and Ba-5d states. Therefore, the optical gaps of 1 and 2 are mainly determined by the electron excitation from the S-3p to La-5d states. However, the role of Ga and Sb in the overall band structures is important, e.g., Ba₆La₈Ga₄Sb₄S₃₀ (1) has a direct band gap, whereas Ba₃La₉Ga₃Sb₆S₃₀ (2) has an indirect band gap (see Fig. S8‡). The electron transition in an indirect band gap is usually higher in energy, which is consistent with the optical band gap measurement (Fig. 4) where 2 exhibits a wider band gap of 2.42 eV than 2.15 eV of 1. In the case of La₄FeSb₂S₁₀,⁸ the VBM is mainly composed of the S-3p and Fe-3d states mixed with small contributions of the Sb-5p and La-5d states. Strong orbital hybridizations between the S-3p and Fe-3d orbitals are shown near VBM. In addition, the CBM is primarily derived from the Fe-3d and La-5d states with small amounts of the S-3p and Sb-5p states. The optical absorptions close to the near-IR edge are mainly from the S-3p states to the unoccupied Fe-3d states for La₄FeSb₂S₁₀.⁸ According to the analysis above, when Fe atoms are substituted by Ga, the optical absorption responses have changed, from the S-3p to Fe-3d states in La₄FeSb₂S₁₀ ⁸vs. from the S-3p to La-5d states in 1 and 2. In the absence of the unoccupied Fe-3d orbitals which can trap electrons during UV-Vis excitation, the CBM is pulled away from E_f and produces a larger band gap for compound 1 and 2.^49,50 Therefore, 1 and 2 which contain no transition metal showing strong electron correlation possess relatively larger band gaps than La₄FeSb₂S₁₀.⁸

Fig. 5 Total and partial densities of state of (a) compound 1, (b) compound 2 and (c) La₄FeSb₂S₁₀.⁸ Dashed lines show the Fermi energy (E_f).

To further characterize the bonding interactions of the Sb–S bonds in 1 and 2, the COHP and electron localization function (ELF)⁵¹ were calculated (see Table 2, Fig. 6, 7 and S9‡). For compound 1, the bonding states for the shortest Sb1–S4 bond (2.443(3) Å) are located at −2.7 to −17 eV. The slightly longer Sb1–S2 bond (2.727(4) Å) extends from −3 to −17 eV, and the longest Sb1–S5 bond is located at −8 to −17 eV. It is obvious that the bonding interactions become weaker and weaker as the distances of the Sb–S bonds increase. Comparable with 1, a similar situation occurs in compound 2 as is shown in Fig. 7. The shortest Sb–S bonds mainly extend from −2 to −17 eV. Nevertheless, the two longer Sb3–S11 bonds (3.216(5) Å) and Sb2–S12 bonds (3.219(5) Å) are located between −4.2 and −16.6 eV. The values of –COHP for these two longer bonds are 0.4064 and 0.4035 eV per bond, about 12% that of the strongest Sb3–S9 and Sb2–S10 bonds (3.378 and 3.362 eV per bond). Moreover, the results of the COHP calculation are in agreement with that of ELF in Fig. S9‡ (the region with an ELF value of 1.00 means fully localized electrons and the region close to 0.00 is a low electron density area). As shown in Fig. S9a,‡ the longest Sb1–S5 bonds (2.932(2) Å) in 1 show moderate bonding interactions. On the other hand, in 2 (Fig. S9b‡), the two longer Sb2–S12 (3.219(5) Å) and Sb3–S11 (3.216(5) Å) bonds show weak but nonnegligible bonding interactions, located at the region at about the value of 0.25. All of this illustrates that Sb³⁺ cations in 1 and 2 are indeed surrounded by four S atoms to form SbS₄ teeter-totter polyhedra.

Fig. 6 COHP curves of selected bond distances for compound 1. E_f is located at 0 eV.

Fig. 7 COHP curves of selected bond distances for compound 2. E_f is located at 0 eV.

Conclusions

Two new pentanary chalcogenides with Ba/RE/Ga/Sb/S (RE = rare-earth metal), Ba₃La₄Ga₂Sb₂S₁₅ (1) and BaLa₃GaSb₂S₁₀ (2), have been synthesized and characterized. Compound 1 crystallizes with a new structure type in the space group Ibam as a 0D structure containing totally isolated GaS₄ and Sb₂S₇ units; meanwhile, compound 2 crystallizes in the space group P2₁/m and has a 1D structure which is built by (SbS₄)_n chains and isolated GaS₄ units. With a lower spacer GaS₄ tetrahedron concentration, the structure changes from 0D in 1 to 1D in 2: i.e. an isolated Sb₂S₇ dimer vs. (SbS₄)_n chains. As the diffuse reflectance spectra showed, their optical band gaps were 2.12 and 2.42 eV, respectively. Moreover, theory studies demonstrate that replacing Fe atoms with Ga is an effective way to widen band gaps, compared to the analogue La₄FeSb₂S₁₀. In addition, COHP and ELF calculations of the Sb–S bonds reveal that the long Sb–S bonds in 1 (2.932(2) Å) and 2 (3.216(5) and 3.219(5) Å) show weak but indispensable bonding interactions, which play significant roles in constructing the isolated Sb₂S₇ dimer for 1 and (SbS₄)_n chains for 2.

Acknowledgements

This research was supported by the NSF of China (No. 21225104, 21233009, 91422303, 21571020, 21301175, 21171168 and 21671023). We thank Prof. Yong-Fan Zhang at Fuzhou University for helping with the DFT calculations.

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Footnotes

† Dedicated to Professor Mercouri G. Kanatzidis on the occasion of his 60th birthday.

‡ Electronic supplementary information (ESI) available. CCDC 1500607 and 1500608 for BaLa₃GaSb₂S₁₀ and Ba₃La₄Ga₂Sb₂S₁₅. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6qi00346j

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