DOI: 10.1021/cg9000335
Solvothermal Syntheses of Two Novel Layered Quaternary Silver-Antimony(III) Sulfides with Different Strategies
2009, Vol. 9 3821–3824
Hua-Gang Yao,† Min Ji,† Shou-Hua Ji,# Ren-Chun Zhang,† Yong-Lin An,*,† and Gui-ling Ning† †
Department of Chemistry and #Department of Materials, Dalian University of Technology, Dalian 116024, China Received January 13, 2009; Revised Manuscript Received June 10, 2009
ABSTRACT: Two new silver antimony sulfides, KAg2SbS3 (1) and K2Ag3Sb3S7 (2), have been synthesized solvothermally in the presence of thiophenol as a mineralizer. Compound 1 contains a hexagonal net formed by condensation of Ag2SbS3 six-membered rings. These sheets are linked through Ag-S bonds to obtain double layers. The structure of 2 consists of infinite [Sb2S4]2- chains and [Ag6Sb2S10]¥8- columns linked to form layers. The [Sb2S4]¥2- chains are generated by vertex-shared SbS33- pyramids, while [Ag6Sb2S10]¥8- columns are composed of edge-shared of SbS3 pyramid, AgS3 triangles, and a AgS4 tetrahedron, and contain a Ag3SbS3 semicube. The optical band gaps of the two compounds are 2.1 and 2.2 eV, respectively. Chalcogenides have been the focus of much attention due to their structural diversity and interesting chemical and physical properties such as nonlinear optics,1 optical storage,2 and thermal electrics.3 The majority of them involve ternary systems associated with alkali metals and organic counterions.4-10 Of particular interest are the thioantimonates(III) which exhibit a large variety of structure types even for identical compositions of the SbxSy anions. This is a consequence of the stereochemical effect of the inert lone pair of electrons associated with Sb(III), together with the ability of Sb(III) to adopt a flexible coordination behavior. The structural features can be greatly enhanced by the incorporation of transition metal ions into anionic frameworks. Using organic molecules and transition metal complexes (TMC) as structure directing molecules, a large number of quaternary chalcogenides of thioantimonates(III) containing transition metal were synthesized under solvo(hydro)thermal conditions.11-22 In our continuing interest to access new quaternary thioantimonates chalcogenides, we have found that one class of materials with a very rich chemistry is quaternary alkali metal silver antimony sulfides because of alkali metal ions exhibiting fastion conductivity in purely inorganic chalcogenides23 and various coordinations of Ag by sulfur atoms (e.g., 2, 3, 4), diverse linkages of silver-sulfur polyhedra through vertex or edges and even complex argentophilic interactions. Such examples Cs3Ag2Sb3S8, Cs2AgSbS4,24 MAg2SbS4 and M2AgSbS4 (M = K, Rb)25 with alkali metals as counterions, which were prepared by oxidation of elemental silver under supercritical ammonia condition, contain SbS43- building blocks. And these compounds are substantially different from those synthesized in polyamine solvents consisting of SbS33 units, in which the polyamines play a templating role and may exist hydrogen bonding interactions with the host.20-22 In an attempt to investigate the replacement of organic amine with alkali metal cations and obtain new structure types of the silver thioantimonates(III), we used thiophenol to serve as a mineralizer which can avoid the forming of silver sulfide under alkaline conditions. In this publication, we report the solvothermal synthesis, crystal structure, and optical and thermal properties of KAg2SbS3 (1) and K2Ag3Sb3S7 (2), and open a new route for synthesis of an M-Ag-Sb(III)-S system. Compound 1 was synthesized by a reaction of K2CO3, AgNO3, Sb2S3, and S in mixed solvent of thiophenol/pyridine and ethylenediamine at 150 C for 6 days.26 Our research showed that *To whom correspondence should be addressed. r 2009 American Chemical Society
thiophenol is essential for the synthesis because thiophenol appears to act as a mineralizer, not simply as a solvent, which can dissolve silver sulfide under amine alkaline conditions; otherwise, KAg2SbS3 will not be obtained. It was also wellknown that thiophenol could reduce sulfur powder to form sulfur ions,27,28 and thus excess thiophenol can avoid oxidation of Sb(III) to Sb(V) by elemental sulfur. Compound 2 was prepared by a reaction of K2CO3, AgSPh,29 and Sb2S3 in mixed solvent of glycol/pyridine and ethylenediamine at 150 C for 7 days.26 Using AgNO3 instead of AgSPh cannot generate compound 2 because of the forming of silver sulfide. The pure phases of 1 and 2 were confirmed by comparing powder X-ray diffraction patterns of the bulk sample with the calculated pattern from the single-crystal structure.30 Compound 1, KAg2SbS3, crystallizes in a triclinic space group P1 with four formula units in the unit cell, and contains a novel layered anionic framework separated by K cations. Each layer is a complex structure consisting of silver ions linked in a complicated fashion by a series of trigonal SbS33- groups (Figure 1). Thus, Ag(2) and Ag(4) are approximately tetrahedrally coordinated by four sulfur atoms, whereas Ag(1) and Ag(3) are nearly trigonal pyramidally coordinated by three sulfur atoms. The two silver atoms (Ag(2) and Ag(4)) that are bound into close proximity to symmetry-equivalent silver atoms by two sulfides, which act as bridging ligands. Of the two tetrahedrally coordinate silver atoms, all the Ag-S bond distances vary from 2.5083(15) to 2.7208(15) A˚, and S-Ag-S angles are between 99.19(4) and 124.70(5), comparable to values in the reported in the literature for silver sulfur with slightly distorted tetrahedral coordination.20,21,31 Each antimony is approximately trigonal pyramidally coordinated by sulfur. As shown in Figure 1, there are two kinds of antimony existing in SbS33- groups. The Sb(1) is coordinated by one μ3 sulfur atom and two μ4 sulfur atoms, while the Sb(2) is coordinated by three μ3 sulfur atoms. The Sb-S distances are all quite similar and range from 2.4090(14) to 2.4436(14) A˚. These distances are typical for Sb(III)-S bond lengths, which are comparable to those previously observed thioantimonates(III),11-23 but are considerably longer than the common Sb(V)-S in K3SbS4 3 4.5H2O,32 Na3SbS4 3 9H2O,33 K2AgSbS4, and RbAg2SbS4.25 The Sb(1)-Ag(1) and Sb(2)-Ag(3) separations (3.0236(6), 2.9404(6)) are slightly longer than those found in silverantimony alloys,34 and the orientation of lone pair from the antimony atom of SbS33- groups always points toward the silver centers. These distances cannot indicate the existing of Sb-Ag bonding interactions. Published on Web 07/09/2009
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Figure 1. View of KAg2SbS3 down the [010] direction showing a [Ag2SbS3]- double layer formed by linkage of two distinct hexagonal net. Ag, pink; Sb, green; S, yellow.
Figure 3. (a) [Ag6Sb2S10] ¥8- column of compound 2. (b) View along the [100] direction showing the interconnection of [Sb2S4]2chains and [Ag6Sb2S10]8- columns of compound 2. Ag, pink; Sb, green; S, yellow.
Figure 2. View along the [100] direction showing crenelated layers of compound 1 with cations between the layers. Ag, pink; Sb, green; S, yellow; K, purple.
Vertex-shared of the SbS3, AgS3, and AgS4 units yields a sixmembered ring, in which metal and sulfur atoms alternate. The condensation of these rings forms a [Ag2SbS3]- hexagonal net in the ac crystallographic plane. Two parallel hexagonal nets are linked by Ag-S bonds to form a double layer of stoichiometry [Ag2SbS3]-, in which the two hexagonal nets are displaced with respect to one another. Within these layers, two of the sulfur atoms are each tetrahedral and are bound to three silver atoms and an antimony atom. The other four sulfur atoms are each bound to two silver atoms and an antimony atom and are pyramidal, with a lone pair occupying the apical position. The double layers are crenelated viewing along the [100] direction, ensuring that the lone pairs of the sulfur atoms point out of the layers toward the potassium ions located between the layers (Figure 2). The shortest interlayer separation is approximately 6.2 A˚. In contrast to the reported double layers [C2N2H9]2[Ag5Sb3S8],20,21 [C6N4H20]2[Ag5Sb3S8],22 and [C6N4H20]0.5[Cu3Sb2S5]16 constructed only by heterorings, the structure of the [Ag2SbS3]double layer is a new structure type. In the [Ag5Sb3S8]2- anion, the overall stoichiometry contains one-eighth AgSb2S3 and seveneighths Ag2SbS3 rings. The [Cu3Sb2S5]- may be considered as the condensation of Cu2SbS3 and CuSb2S3 rings in a 4:1 ratio. However, compound 1, containing only Ag2SbS3 heterorings, can be regarded as the y = ¥ member of the family of materials, [AgSb2S3]xþ [Ag2SbS3]y-.20 Although the same stoichiometry of anions is found in [C6N2H18]0.5[Cu2SbS3], [C4N3H15]0.5[Cu2SbS3], [C8N4H22]0.5[Cu2SbS3],16 [C2N2H10]0.5[Cu2SbS3], [C3N2H12]0.5[Cu2SbS3], [C4N2H14]0.5[Cu2SbS3],17 [C2N2H8]0.5 [Cu2SbSe3],18 [C2N2H8]0.5[Cu2SbS3],19 [C2N2H9][Ag2SbS3],20 they have different structures with KAg2SbS3 (1). These compounds are isostructural, and their anionic layers contain 6-membered M2SbQ3 and 10-membered M3Sb2Q5 rings (M = Cu, Ag; Q = S, Se), which are different from compound 1 constructed by sixmembered rings to finally form a double layer.
Compound 2 crystallizes in a orthorhombic space group Cmc21 with four formula units in the unit cell, and has a layered anion, [Ag3Sb3S7]2-, built up of infinite [Sb2S4]2- chain and [Ag6Sb2S10]8- columns. The [Sb2S4]¥2- chains are generated by vertex-shared SbS33- pyramids which have their terminal sulfur atoms bridged to silver atoms. The Sb-S distances are between 2.3984(19) and 2.490(2) A˚, and S-Sb-S angles range from 86.10(5) and 95.08(8). These are comparable to the previously reported compound Cs3Ag2Sb3S8,24 MSbS2 (M = Na, K, Cs)35 existing analogous [SbS2]¥- chain. Edge-shared of one SbS3 pyramid, two AgS3 triangles, and a seriously distorted AgS4 tetrahedron produce a Ag3SbS5 group with a distorted Ag3SbS3 semicube. The similar Sb3S63- semicube is a common secondary building unit found in antimony sulfides.36,37 These Ag3SbS5 units are linked through three μ5 sulfur to form [Ag6Sb2S10]¥8column (Figure 3a). In the central column, two Ag atoms are bound by three sulfur atoms to form AgS3 triangles with the Ag-S bond distances ranging from 2.526(2) to 2.545(2) A˚ and S-Ag-S angles between 105.79(11) and 130.79(7). The other tetrahedrally coordinated Ag atom has a contact to a sulfur atom at a long distance of 2.913(6) A˚ resulting serious distortion of the tetrahedron. The condensation of central [Ag6Sb2S10]¥8- columns and [Sb2S4]¥2- chains generate layered [Ag3Sb3S7]2- anion in the bc crystallographic plane (Figure 3b). It is noteworthy that the same layered anion is found in [C4N2H14][Ag3Sb3S7],21 which was prepared under solvothermal conditions in the presence of ethylenediamine as the structure director and crystallizes in an orthorhombic space group Pnma. In [C4N2H14][Ag3Sb3S7], tetrahedrally coordinate Ag has contact to a S atom at a long distance of 2.944(4) A˚, and another environment of three S atoms to form AgS3 triangles with the Ag-S bond distances ranging from 2.390(3) to 2.647(2) A˚ and S-Ag-S angles between 90.43(8) and 133.73(4). These geometrical parameters are in good agreement with values observed in 2, indicating that they exhibit a similarly strong distortion of the AgS4 tetrahedron. A remarkable difference between the two compounds is respective cations. In compound 2, K cations are located between the adjacent anionic layer (Figure 4), and each K has contacts to nine sulfur atoms that average 3.402(9) A˚ and are arranged in a tricapped trigonal prismatic coordination environment. These K-S bond distances
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Figure 4. View along the [001] direction of a pair of layered [Ag3Sb3S7]2- anion with cations between the layers. Ag, pink; Sb, green; S, yellow; K, purple.
are longer than those in related compounds.24,25 The amine cations are located in pairs between the layers, however, and the two NH3 groups of the amines are oriented toward the networks forming a sandwich-like arrangement in [C4N2H14][Ag3Sb3S7]. Thus, between the anionic layers and the structure directing ammonium ions exist hydrogen bonding interactions. The thermal behavior of 1 and 2 was investigated by DSCTGA under a flow of nitrogen (40 mL/min) from 50 to 550 C at a heating rate of 10 C/min. The results of 1 and 2 show no weight loss and endothermic melting transition centered approximately at 299 and 228 C, respectively. The X-ray powder patterns confirmed that the residues of the two compounds were amorphous. The UV-vis reflectance spectroscopy measurements on 1 and 2 confirmed that they are wide-bandgap semiconductors. The optical absorption spectrum of 1 exhibits a steep absorption edge, revealing an optical bandgap of 2.1 eV. The spectrum of 2 also shows a similar absorption edge with a corresponding bandgap very close to that of 1 at 2.2 eV. This intense absorption is probably due to charge-transfer transitions from a primarily sulfur-based filled valence band to a mainly silver-based empty conduction band. In conclusion, we have shown that thiophenol is a very effective mineralizer for the synthesis of KAg2SbS3. Compared to the previously reported mineralizer HSCH2CH(SH)CH2OH,38,39 thiophenol is more stable under higher temperatures. Thus, it also can be used to synthesize selenides. This offers a new possible protocol for the construction of diverse new quaternary chalcogenides containing transition metal silver by the use of thiophenol acting as a mineralizer. We are actively moving this synthesis strategy toward other related transition metals and expect to obtain more new chalcogenide materials. Acknowledgment. We thank the National Natural Science Foundation of China for financially supporting this work (20671015). Supporting Information Available: CSD 420015 and 420016 contain the supplementary crystallographic data for 1 and 2, respectively. X-ray diffraction pattern, DSC curves, and UV-vis reflectance spectrum. This material is available free of charge via the Internet at http://pubs.acs.org.
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