Two Mercury Antimony Chalcogenides Cs2HgSb4S8 and Cs2Hg2Sb2Se6 with Cesium Cations as Counterions.

Two novel layer structure compounds, Cs2HgSb4S8 and Cs2Hg2Sb2Se6, were synthesized in organic solvent under solvothermal conditions. The Cs2HgSb4S8 is formed of [HgSb4S8]2- ribbons and S atoms by corner sharing. The Cs2Hg2Sb2Se6 is made up of [SbHg2Se6]5- ribbon and disorder trigonal-pyramidal SbSe3 by sharing μ3-Se. These compounds are characterized by single-crystal X-ray diffraction, powder X-ray diffraction, solid-state optical absorption spectra, and so on.

Introduction

Since Bedard et al.[1] reported the solvothermally synthesized open-framework structure of tin(IV) sulfides in 1989, a significant method has been developed in the synthesis of chalcogenides. Meanwhile, chalcogenides also have been attracting extensive research interest due to their excellent performance in the fields of semiconductor, ion exchange, and nonlinear optics.[2−9] Very recently, a number of antimony chalcogenides have been continuously prepared because the stereochemical effect of lone electron pairs and the various coordinations of stibium by chalcogen atoms can give rise to large structural and compositional diversities.[10,11] In other words, the stibium atom is coordinated with chalcogen atoms to form pyramidal [SbIIIQ3]3– or tetrahedral [SbVQ4]4– (Q = S, Se) primary units. The pyramidal [SbQ3]3– unit has a powerful tendency of a variety of condensations to form polynuclear anions which can link by sharing corners or edges to come into being rings or chains structure. However, the tetrahedral [SbVQ4]4– mostly binds cations directly to form low-dimensional structure because building chalcogen atoms need to stay charge balanced.[12−14]

In recent years, researchers have focused on the incorporation of transition metal (TM) ions into antimony chalcogenides to form novel chalcogenide materials.[15] The transition metal ions contain Co2+, Ni2+, Fe2+, and Mn2+, and they are coordinated with organic amine to form [TM(amine)]2+ complex cations acting as counterions, resulting in low-dimensional structural antimony chalcogenides.[16] A large number of A-Hg-Sb-Q (A= [TM(amine)n]2+) compounds are known in which the transition metal ions are integrated into organic amine frameworks as charge balancing ions, such as [Ni(en)3]0.5HgSbS3 (en = ethylenediamine),[17] [Co(en)3]Hg2Sb2S6, [Ni(1,2-dap)3]2HgSb3S7Cl (1,2-dap = 1,2-diaminopropane), [Ni(1,2-dap)3]HgSb2S5, [Mn(dien)2]HgSb2S5 (dien = diethylenetriamine), [TM(tren)]HgSb2S5 (TM = Mn, Fe, Co, tren = tris(2-aminoethyl)amine), [TM(dien)2]Hg3Sb4S10 (TM = Mn, Co, Ni),[18] [Co(dien)2]HgSb2S5, [Ni(dien)2]HgSb2S5,[19] [Mn(phen)]2HgSb2S6 (phen = 1,10-phenanthroline), [TM(tren)]HgSb2Se5 (TM = Mn, Fe, Co),[20] [Ni(1,2-pda)]HgSb2Se5, [Mn(dien)2]HgSb2Se5, [Ni(en)3]Hg2Sb2Se6, [Ni(en)(teta)]Hg2Sb2Se6,[21] and [TM(1,2-dap)3]HgSb2Se5 (TM = Co, Fe).[22] On the contrary, only a few compounds of antimony chalcogenides with alkali metal ions or alkaline earth metal ions as counterions are known. Meanwhile, transition metal ions Ag+, Cu2+, and Hg2+ compared with Co2+, Ni2+, Fe2+, and Mn2+ are easily formed frameworks of group 15 elements. For example, CsAgSb4S7,[23] KCu2SbS3,[24] K2MSbS3 (SH) (M = Zn, Cd),[25] K3Ag9Sb4S12, Rb3Ag9Sb4S12, Cs3Ag9Sb4S12,[26] BaAgSbS3, BaAgSbS3·H2O,[27] Rb2Cu2Sb2S5, Cs2Cu2Sb2S5,[10] BaCuSbS3, and BaCuSbSe3[28] were synthesized by the high-temperature solid-state approach, reactive flux methods, and hydro(solvo)thermal methods. However, A-Hg-Sb-Q (A= alkali metal cations) antimony chalcogenides were synthesized relatively little. As far as we know, there are only a few compounds of KHgSbS3,[29] RbHgSbTe3,[30] MHgSbSe3 (M = K, Rb, Cs),[31] and [tetaH2]0.25Rb0.5HgSbSe3.[17] Although most of the mercury(II)–antimony chalcogenides were synthesized by using organic amines or transition metal complexes as counterions, alkali metal ions or alkaline earth metal ions as counterions are still uncommon. In this article, we obtained two new mercury(II)–antimony chalcogenides Cs2HgSb4S8 (1) and Cs2Hg2Sb2Se6 (2).

Results and Discussion

We have investigated solvent factor in the synthesis of these compounds. For compound 1, we used 1,4-diaminobutane as reaction solvent. Other conditions remained unchanged, and a large number of small crystals were obtained. When we used 1,2-diaminopropane instead of 1,4-diaminobutane as the solvent to synthesize compound 2, a very small amount of the crystals was obtained. Their structures were determined by single-crystal X-ray diffraction.

Structural Descriptions

Cs2HgSb4S8 (1)

Compound 1 has a novel layered structure, in which the two-dimensional (2D) layered [HgSb4S8]2 network is separated by counterions Cs + cations and stack along  the a axis (Figure ). The layer can be described as being composed of 12-membered rings and 16-membered rings. The [SbHgS5] chain is composed of trigonal–pyramidal Sb(4)S3 units and tetrahedron HgS4 units by corner sharing (Figure a). In trigonal–pyramidal Sb(4)S3 units, the Sb(4) atom is coordinated by three S atoms, and the Sb–S distances range from 2.358(6) to 2.461(6) Å with S–Sb(4)–S angles ranging from 93.0(2) to 96.8(2)° (Table S1). In tetrahedron HgS4 units, the Hg atom is integrated into four atoms, and the Hg–S distances range from 2.415(6) to 2.659(6) Å with angles ranging from 95.0(2) to 133.1(2)°. The Hg–S bond lengths are similar to the HgS4 tetrahedron of [Ni(en)3]0.5HgSbS3 (Hg–S: 2.456(2)–2.6756(18) Å) (Table S1).[17] In compound 1 appears Sb3S3 six-membered rings, which are composed of three SbS3 units by corner sharing (Figure b), and it is similar to some thioantimonates.[32,33] The [SbHgS5] chain and Sb3S3 six-membered rings form the [HgSb4S8]2– ribbon by sharing S(1) and S(5) self-assembly (Figure c). The Sb(3)–S(5) and Sb(2)–S(1) bond lengths are 0.2399(6) Å and 0.2471(6) Å, respectively. Such [HgSb4S8]2– ribbon further fused via sharing the μ2-S(4) atoms to form a 2-D anionic layer along the ab plane (Figure d). It is worth mentioning that the compound 1 possesses a new type of Hg–Sb–S structure. Up to now, this structure has been not reported. In addition, Mn2Sb4S8(N2H4)2[34] and compound 1 have similar Sb4S8 units, but we found that their structures are very different. The Mn2Sb4S8(N2H4)2 consists of hydrazine-bridged neighboring 2-D Mn2Sb4S8 layers to form 3-D networks.[34] Although the Mn2Sb4S8(N2H4)2 and compound 1 are all composed of six-membered Sb3S3 rings, the formation of the two types of rings is completely different. There are two different Sb coordination geometries for SbS4 and SbS5 in Mn2Sb4S8(N2H4)2, while compound 1 only contains trigonal pyramidal SbS3.

Figure 1

2-D layer anions [HgSb4S8]2 are separated by the cations Cs+ and stack along the a axis.

Figure 2

(a) [SbHgS5] chain. (b) The Sb3S3 six-membered rings. (c) The [HgSb4S8]2– ribbon. (d) 2-D anionic layer stack along the ab plane.

Cs2Hg2Sb2Se6 (2)

The compound 2 has a layered structure. It consists of the 2-D [Hg2Sb2Se6]2– anion layer, and Cs+ cations stack along the c axis (Figure ). The Hg atom is similar to compound 1, coordinated with four Se atoms to form tetrahedron HgSe4 units. In tetrahedron HgSe4 units, Hg–Se distances range from 2.585(7) to 2.753(10) Å with Se–Hg–Se angles ranging from 106.7(3) to 120.6(2)°. The [Hg2Se6] chain consists of tetrahedron HgSe4 units through sharing Se(1) and Se(4) (Figure a). The Sb(3) atom is coordinated by four Se atoms, forming distorted tetrahedron Sb(3)Se4 units (Figure b). Sb(3)–Se distances range from 2.902(12) to 3.007(12) Å with Se–Sb(3)–Se angles ranging from 91.2(3) to 174.3(4)° (Table S2). The ribbon structure is composed of a [Hg2Se6] chain and tetrahedron SbSe4 units by corner sharing (Figure c). An anionic layer is made up of a [SbHg2Se6]5– ribbon and two distorted trigonal pyramidal SbSe3 units by sharing μ3-Se atom self-assembly along the bc plane (Figure e). There are two different types of Sb atoms in the crystal structure. The main difference is attributable to the coordination environment of Se atoms. In Sb(3)Se4 units, the Sb(3)–Se bond lengths are longer than those of other selenioantimonates,[35] which are similar to the distorted ψ-SbSe4 trigonal bipyramid of [Mn(dien)2]2Sb4Se9 (Sb2–Se6:2.8087(11) and 3.0002(11) Å).[14] In disorded trigonal–pyramidal SbSe3 units (Figure d), Sb1 and Sb2 atoms are disordered; the Sb atom is coordinated by three Se atoms; and Sb–Se distances range from 2.762(15) to 2.876(15) Å with Se–Sb–Se angles ranging from 95.2(5)–126(2)°. The CsHgSbSe3[31] and compound 2 have the same stoichiometry of Cs:Hg:Sb:S (1:1:1:3), while their structure makes a difference. The CsHgSbSe3 includes terminal Se atoms, while 2 has none. On the other hand, their structures all include [Hg2Se6] chains, but the [Hg2Se6] chains of CsHgSbSe3 are connected with trigonal pyramidal SbSe3 units to form two-dimensional (2D) anionic layers of [HgSbSe3]−. The [Hg2Se6] chains of 2 are connected with tetrahedron SbSe4 units and trigonal pyramidal SbSe3 units, forming 2D anionic layers of [Sb2Hg2Se6]2–. As far as we know, these compounds, which contain [enH2]0.5HgSbS3, [tetaH2]0.5HgSbS3, [1,2-pdaH]HgSbS3, [tetaH2]0.25Rb0.5HgSbSe3, [Ni(en)3]0.5HgSbS3,[17] [Co(en)3]Hg2Sb2S6,[18] [Ni(en)3]Hg2Sb2Se6, [Ni(en)(teta)]Hg2Sb2Se6,[21] KHgSbS3,[29] RbHgSbTe3,[30] and MHgSbSe3 (M = K, Rb, Cs),[31] and compound 2 have the same stoichiometry of Hg:Sb:Q (Q = S, Se, Te) (1:1:3). It is different that these compounds are all composed of ring structure. In contrast, compound 2 appears to have a ribbon structure, and the most interesting is SbSe4 in compound 2. We found that RbHgSbTe3, RbHgSbSe3, KHgSbSe3, and compound 2 have similar [Hg2Se6] chains, whereas their counterions are also very different.

Figure 3

2-D [Hg2Sb2Se6]2– anion layer and Cs+ cations stack along the c axis.

Figure 4

(a) [Hg2Se6] chain. (b) Sb(3)Se4 units. (c) [SbHg2Se6]5– ribbon. (d) The disorder trigonal pyramidal SbSe3 units. (e) 2-D anionic layer stack along the bc plane.

Optical Properties

The solid-state optical absorption spectra indicate that the optical band gaps of Cs2HgSb4S8 and Cs2Hg2Sb2Se6 are 2.13 and 1.77 eV, respectively (Figure ). The band gaps of 1 show a red shift of the optical absorption edge compared with those of [enH2]0.5HgSbS3 (2.49 eV), [tetaH2]0.5HgSbS3 (teta = triethylenetetramine) (2.40 eV), [1,2-pdaH]HgSbS3 (2.55 eV), and [Ni(en)3]0.5HgSbS3 (2.68 eV).[17] The band gaps of 2 show a blue shift of the optical absorption edge compared with those of CsHgSbSe3 (1.65 eV),[31] RbHgSbSe3 (1.75 eV),[30] and [tetaH2]0.25Rb0.5HgSbSe3 (1.42 eV).[17]

Figure 5

(a) Solid-state optical absorption spectra of Cs2HgSb4S8. (b) Solid-state optical absorption spectra of Cs2Hg2Sb2Se6.

Thermogravimetric and Differential Scanning Calorimeter Analyses

The thermogravimetric (TG) curve of compounds 1 and 2 shows the weight loss of 30% and 24%, which corresponds to the collapse of the structure of the compound. Differeential scanning calorimeter (DSC) curves of compounds 1 and 2 display the endothermic peak at 343 and 354 °C, respectively (Figure S2).

Conclusions

In summary, we have prepared two-layer mercury antimony chalcogenides Cs2HgSb4S8 and Cs2Hg2Sb2Se6 by the solvothermal method. Two compouds have a similar ribbon structure, but the structure is different. Two compounds contain alkali metal ions (Cs+) as counterions, and transition metal ions (Hg2+) were incorporated into antimony chalcogenides. The solid-state optical absorption spectra indicate that two compounds are semiconductors.

Experimental Section

Materials and Methods

All reagents and chemicals are analytical and without further purification. The powder X-ray diffraction is done using a Rigaku XRD-6000 diffractometer using Cu–Kα radiation at 40 kV and 40 mA (Figure S1). The solid-state optical absorption spectra are on a Shimadzu UV-2550 double monochromatic sepctrophotometer over the range of 220–800 nm at the room temperature with Ba2SO4 power as reflectance. The absorption spectrum data were calculated from the Kubelka–Munk function: α/S = (1 – R)2/2R, where α is the absorption coefficient, S the scattering coefficient, and R the reflectance.[36] Thermogravimetric and differeential scanning calorimeter analyses were carried out with an STA 449 F5 Jupiter at 30 to 600 °C at 5 °C/min under a nitrogen atmosphere at 35 mL/min.

Syntheses of Cs2HgSb4S8 (1)

Cs2CO3 (33 mg, 0.1 mmol), HgI2 (45 mg, 0.1 mmol), Sb2S3 (34 mg, 0.1 mmol), and S (13 mg, 0.4 mmol) were dispersed in 522 mg of 1,2-diaminopropane (1,2-dap), and 127 mg of H2O was placed in a thick Pyrex tube (ca. 20 cm long). The sealed tube was heated at 140 °C for 7 days to obtain yellow block crystals of 1 (19% yield based mercury). The crystals were washed with ethanol and dried and stored under vacuum.

Syntheses of Cs2Hg2Sb2Se6 (2)

Cs2CO3 (33 mg, 0.1 mmol), HgI2 (45 mg, 0.1 mmol), Sb2S3 (34 mg, 0.1 mmol), and Se (36 mg, 0.4 mmol) were dispersed in 500 mg of 1,4-diaminobutane (1,4-dab), and 120 mg of H2O was placed in a thick Pyrex tube (ca. 20 cm long). The sealed tube was heated at 160 °C for 7 days to obtain red polyhedron crystals of 2 (25% yield based mercury). The crystals were washed with ethanol, dried, and stored under vacuum.

Single-Crystal X-ray Determination

Single crystals of 1 and 2 with dimensions of 0.29 × 0.25 × 0.20 mm and 0.26 × 0.21 × 0.18 mm were collected with intensity data on a Bruker APEX-II CCD diffractometer equipped with the graphite monochromated Mo-Kα radiation (λ = 0.71073 Å) at 296 K. The structures of 1 and 2 were solved by direct methods and refined by full-matrix least-square methods on F2 by the SHELXL 97 package.[37] Crystallographic data and structural refinement parameters for the two compounds are summarized in Table . Some relevant bond distances and bond angles are listed in the Supporting Information.

Table 1

Crystal Data and Structure Refinement Parameters for 1 and 2

chemical formula	Cs2HgSb4S8	Cs2Hg2Sb2Se6
formula weight	1209.89	1384.26
temperature/K	296(2)	296(2)
wavelength/nm	0.071073	0.071073
crystal system	triclinic	triclinic
space group	P-1	P-1
a/Å	7.3196(19)	8.029(5)
b/Å	10.6224(6)	8.869(5)
c/Å	11.7739(14)	12.091(7)
α (deg)	99.51(2)	68.49(1)
β (deg)	92.06(2)	84.22(1)
γ (deg)	92.10(2)	74.13(1)
V/Å3	901.4(3)	770.5(7)
Z	2	2
Dc/(Mg·m–3)	4.457	2.865
μ (mm–1)	19.279	42.117
θ range (deg)	1.95–25.00	1.81–25.00
GOF on F2	1.144	1.233
(I > 2σ(I))	R1 = 0.0622	0.1042
	wR2 = 0.1435	0.2140
R indices (all data)	R1 = 0.1345	0.1928
	wR2 = 0.1533	0.2323

R1 = Σ∥Fo| – |Fc∥/Σ|Fo|, wR2 = Σ[(w(Fo2 – Fc2)2)/Σ[w(Fo2)2)]1/2.