Synthesis and characterization of a new quaternary selenide Sr(3)GeSb(2)Se(8)

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Synthesis and characterization of a new quaternary

selenide Sr

3

GeSb

2

Se

8 Ching-Yi Yu

a

, Ming-Fang Wang

a

, Ming-Yang Chung

a

, Shyue-Ming Jang

b

,

Jih-Chen Huang

b

, Chi-Shen Lee

a,

*

a

Department of Applied Chemistry and Institute of Molecular Science, National Chiao Tung University, 1001 Ta-Hsueh Road, Hsinchu 30010, Taiwan

b

Industrial Technology Research Institute, N300, UCL/ITRI Building 22, 321, Kuang Fu Road, Section 2, Hsinchu 300, Taiwan Received 1 August 2007; received in revised form 26 November 2007; accepted 10 December 2007

Available online 23 December 2007

Abstract

A new quaternary selenide Sr3GeSb2Se8has been synthesized at 750C from pure elements in a stoichiometric ratio under vacuum. The product as-synthesized is characterized by single-crystal X-ray diffraction, diffuse reflectance spectroscopy, and thermoanalysis. The title com-pound crystallizes in the orthorhombic space groupPnma with a¼ 12.633(4) A˚ , b ¼ 4.301(1) A˚, c ¼ 28.693(7) A˚, V ¼ 1558.8(8) A˚3,Z¼ 4, R1/ wR2¼ 0.0582/0.1221, and GOF ¼ 1.077. The structure of Sr3GeSb2Se8features a new structural type that consists of one-dimensional chains of vertex-sharing tetrahedra1

N½MSe3 (M ¼ Ge/Sb) and fused double chains of edge-shared square pyramids1N½M2Se5, which are held together by

Keywords: Crystal structure; Germanium; Antimony; Selenium; Solid-state synthesis; Semiconductor

1. Introduction

Multinary chalcogenides exhibit varied structural types that have attracted intensive research for their prospective nonlin-ear optics [1], ferroelectrics [2], and thermoelectric applica-tions [3e8]. Much attention has been devoted to the synthesis of metal chalcogenides, such as CsBi4Te6 [9] and

AgPbmSbTe2þm[10,11], that might produce compounds with

a narrow band gap for a large figure of merit (ZT¼ sS2/k, whereZ¼ figure of merit; T ¼ temperature; S ¼ Seebeck coef-ficient; s¼ electrical conductivity; k ¼ thermal conductivity). For the reported chalcogenides, almost all main-group metals cooperate with the elements of group 16 (S, Se, Te) and alkali, alkaline-earth metals or rare-earth elements to form multinary chalcogenides[12]. Among these compounds, only a few ter-nary selenides SreGeeX and SreSbeX (X¼ S, Se, Te) have been structurally characterized, such as Sr2Ge2Se5 [13],

Sr2Ge2X4(X¼ S, Se) [14,15], Sr3Sb4S9 [16], and Sr6Sb6S17

[17]. Only one multinary chalcogenide Ba4LaGe3SbSe13

[18], containing Ge, Sb and Se, appears to have been reported. Our exploratory research has focused on chalcogenides of the form AeeM1eM2eX (alkaline-earth element Ae; M1¼ Ge,

Sn; M2¼ Sb, Bi; X ¼ S, Se, Te) system. On extending the

pro-ject to the SreGeeSbeSe system, we synthesized a new qua-ternary chalcogenide Sr3GeSb2Se8 having one-dimensional

chains of corner-sharing tetrahedra and fused square-pyramidal double chains. Here we report the synthesis, structure and characterization of Sr3GeSb2Se8.

2. Experiments 2.1. Synthesis

Sr3GeSb2Se8was synthesized by a solid-state method. The

initial reagents were strontium (Sr, 99.0%, Alfa Aesar), germanium (Ge, 99.999%, Alfa Aesar), antimony (Sb, 99.5%, Alfa Aesar), and selenium (Se, 99.999%, Alfa Aesar).

* Corresponding author. Tel.:þ886 3 5131332; fax: þ886 3 5723764. E-mail address:[email protected](C.-S. Lee).

Solid State Sciences 10 (2008) 1145e1149

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In a typical reaction, these pure elements in stoichiometric proportions were mixed in an Ar-filled glove box (total mass w0.5 g), placed in a silica tube, sealed under dynamic vac-uum, and heated slowly to 750C within 72 h. This tempera-ture was maintained for 3 days, followed by slow cooling to 550C over 7 days, and finally to about 23C on simply termi-nating the power. Initial reactions were intended to synthesize ‘Sr3Ge2Sb2X9’ (X¼ S, Se, Te). Based on the powder X-ray

dif-fraction experiment, the S and Te reactions yield mixtures of Sr2GeS4, SrS for ‘Sr3Ge2Sb2X9’ and SrTe, Sb2Te3, Te for

‘Sr3Ge2Sb2Te9’ reactions. From the ‘Sr3Ge2Sb2Se9’ reaction,

the products contain a dark brown product with irregularly shaped crystals. As confirmed by single-crystal diffraction, we succeeded in preparing Sr3GeSb2Se8in quantitative yields

using the elements in a stoichiometric ratio and the same heating profile as described above. Analysis of Sr3Ge2Sb2Se9

crystals with energy-dispersive spectra (SEM/EDX, Hitachi H-7500 Scanning Electron Microscope) showed the presence of Sr, Ge, Sb and Se. Sr3GeSb2Se8is sensitive to air and

mois-ture such that it gradually decomposes to form a red amorphous powder in air in 7 days.

2.2. Single-crystal X-ray diffraction (XRD)

A single crystal of compound Sr3GeSb2Se8 (0.1 0.1

0.1 mm3) was mounted on a glass fiber with epoxy glue; inten-sity data were collected on a diffractometer (Bruker APEX CCD equipped with graphite-monochromated Mo Ka radia-tion, l¼ 0.71073 A˚ ) at 25(2)_{C. The distance from the crystal}

to the detector was 5.0 cm. Data were collected in scans 0.3 in u within groups of 600 frames each at f settings 0and 60. The duration of exposure was 60 s/frame. The values of 2q varied between 3.50and 56.58. Diffraction signals obtained from all frames of reciprocal-space images were used to deter-mine the unit-cell parameters. The data were integrated using the Siemens SAINT program and were corrected for Lorentz and polarization effects [19]. Absorption corrections were based on fitting a function to the empirical transmission surface as sampled by multiple equivalent measurements of numerous reflections. The structural model was obtained with the direct method and refined with full-matrix least-square refinement based onF2using the SHELXTL package[20]. 2.3. Structure determination

A dark brown crystal of Sr3GeSb2Se8revealed an

ortho-rhombic unit cell (a¼ 12.633(4) A˚ , b ¼ 4.301(1) A˚, c ¼ 28.693(7) A˚ , V ¼ 1558.8(8) A˚3) and systematic absences indi-cated space group Pnma (no. 62). Using direct methods, a structural model was built with six unique sites for metal atoms (Sr, Ge and Sb) and eight unique sites for Se atoms. The crystal structure was initially refined in a model of Sr3GeSb2Se8 with fully occupied sites of Sr (Sr(1)eSr(3)),

Ge (M(1)), Sb (M(2), M(3)) and Se (Se(1)eSe(8)). The isotro-pic displacement parameters (Uiso) of M(1)eM(3) sites

exhibited unreasonable values, which indicated that M(1)e M(3) sites had partial or mixed occupancy by Ge or Sb

cations. A subsequent refinement indicated that the M(1) site is occupied 65%/35% by Ge/Sb, whereas the M(2) and M(3) sites contain 6%/94% and 24%/76% of Ge/Sb, respectively. This result yielded a charge-balanced formula Sr3GeSb2Se8.

Final structural refinements yielded R1/wR2 to be 0.0582/

0.1221. All atomic positions were refined with anisotropic dis-placement parameters. Tables 1e3 summarize the crystallo-graphic data, atomic coordinates and interatomic distances of Sr3GeSb2Se8. Further details of the crystal-structure

inves-tigation can be obtained from the Fachinformationszentrum Karlsruhe, Eggenstein-Leopoldshafen, Germany (fax: þ49 7247 808 666; e-mail:[email protected]) on quoting the depository number CSD-418942.

2.4. Characterization

X-ray powder diffraction data of the products were mea-sured on a BraggeBrentano-type powder diffractometer (Bruker D8 Advance, operated at 40 kV and 40 mA, Cu Ka, l¼ 1.5418 A˚ ). For phase identification, XRD data were collected in a 2q range from 5 to 60 with a step interval of 0.02. Diffuse reflectance measurements were performed near 25C with a UVevisible spectrophotometer (Jasco V-570); an integrating sphere was used to measure the diffuse reflectance spectra over the range 200e2000 nm. Samples as ground powder were pressed onto a thin glass slide holder; a BaSO4plate served as reference. Thermogravimetric

anal-yses (TGA) were performed on a thermogravimetric analyzer (PerkineElmer pyres). The sample was heated to 920C un-der a constant flow of N2. Measurements of electrical

resis-tivity were performed with the standard four-probe method on cold pressed bars (1 1 5 mm3_{). The sample was}

an-nealed at 500C for 6 h before the measurement. For several samples, the electrical conductivity was so poor that the electrical resistance exceeded the limit (w103MU) of the instrument.

Table 1

Crystallographic data for Sr3GeSb2Se8

Empirical formula Sr3GeSb2Se8

Refined formula Sr3Ge0.95(5)Sb2.05(5)Se8

Crystal size/mm3 0.1 0.1 0.1

Formula mass/g mol1 1210.63

Temperature/K 293(2)

Wavelength/A˚ 0.71073

Crystal system Orthorhombic

Space group Pnma (no. 62)

a/A˚ 12.633(4) b/A˚ 4.301(2) c/A˚ 28.683(7) V/A˚3 _1558.8(8) Z 4 Calc. density/g cm3 5.158 Absorption coefficient/mm1 34.153

Transmission range 0.5046e1

Independent reflections 2188 [R(int)¼ 0.0497]

GOF onF2 1.076

R1/wR2[I > 2s(I )] 0.0518/0.1241

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3. Results and discussion 3.1. Structure description

Sr3GeSb2Se8crystallizes in a new structural type with

or-thorhombic space groupPnma, with 14 independent positions, three for Sr, three for mixed occupancy of Sb and Ge, and eight for Se atoms. The crystal structure of Sr3GeSb2Se8

viewed along [010] is shown inFig. 1. The major structural feature of this compound is the presence of alternative stacks of corner-shared tetrahedra and chains of edge-shared square pyramids, which are separated by Sr2þ ions. The Sr atoms are connected to seven or eight selenium atoms in a polyhedron close to a monocapped (Sr(2)) or bicapped (Sr(1), Sr(3)) trigo-nal prism with SreSe distances ranging between 3.1421(17) and 3.645(3) A˚ . The coordination environment of Sr2þatoms is common in similar chalcogenides containing the Sr2þ cat-ion, such as Sr2Ge2Se4 [14], Sr3Sb4S9 [16], and Sr6Sb6S17

[17]. The M(1)eM(3) sites form a one-dimensional structure composed of corner-sharing tetrahedral chains 1

N½MSe3

(M(1)) and chains of edge-sharing double square pyramids

1

N½M2Se5 (M(2), M(3)), which are shown in Fig. 2. For an 1

N½MSe3 unit, each central metal atom forms severely

dis-torted tetrahedral units (bond angles between 93.89(9) and 119.26(12)) with four MeSe bonds of length 2.429(3), 2.427(2) and 2.492(2) (2) A˚ , which share corners to form an infinite chain parallel to the b-axis (Fig. 2a). These Se(3) atoms are shared between two MSe4 tetrahedra. The MeSe

distance is greater than for regular GeeSe (w2.35 A˚ ) but smaller than for SbeSe (w2.6e2.8 A˚ ) distances, indicating that the M(1) site has a mixed occupancy. The1N½MSe3 chain

is rare in antimony chalcogenides, but similar corner-sharing GeSe4 one-dimensional chains are found in A2GeX3

(A¼ Na, K, Rb, Cs; X ¼ S, Se) [21e24]. Regarding the

1

N½M2Se5 unit, the M(2) and M(3) sites form distorted

square-pyramidal units with four selenium atoms that share edges to form a ribbon shape with fused square-pyramidal units parallel to the b-axis (Fig. 2b). The distortion of the square-pyramidal unit is caused mainly by three short and two long MeSe bonds in each polyhedron. The M(2) atoms comprise three short bonds to Se between 2.611(2) and 2.774(1) A˚ and two longer ones at 3.020(2) A˚, whereas M(3) exhibits three short MeSe bonds at 2.642(2)e2.864(2) A˚

Table 2

Atomic positional coordinates, isotropic displacement parameters (103A˚2_),

and site occupancies for Sr3GeSb2Se8

Atoms Position x y z U (eq) Occ.

Sr(1) 4c 0.1716(1) 0.2500 0.6853(1) 19(1) Sr(2) 4c 0.3056(1) 0.2500 0.8433(1) 20(1) Sr(3) 4c 0.4699(1) 0.2500 0.5877(1) 28(1) M(1) 4c 0.0716(2) 0.2500 0.2178(1) 41(1) Ge 0.65(2) Sb 0.35(2) M(2) 4c 0.4547(1) 0.2500 0.0672(1) 30(1) Ge 0.06(2) Sb 0.94(2) M(3) 4c 0.2656(1) 0.2500 0.4719(1) 31(1) Ge 0.24(2) Sb 0.76(2) Se(1) 4c 0.1612(1) 0.2500 0.1429(1) 21(1) Se(2) 4c 0.2351(1) 0.2500 0.2622(1) 21(1) Se(3) 4c 0.0278(2) 0.2500 0.7787(1) 88(1) Se(4) 4c 0.0015(1) 0.2500 0.3441(1) 18(1) Se(5) 4c 0.3520(1) 0.2500 0.3881(1) 19(1) Se(6) 4c 0.1827(1) 0.2500 0.5755(1) 27(1) Se(7) 4c 0.1050(2) 0.2500 0.0052(1) 36(1) Se(8) 4c 0.3816(1) 0.2500 0.9492(1) 36(1) Table 3

Selected interatomic distances (A˚ ) for Sr3GeSb2Se8

Sr(1)eSe(1) 2 3.250(2) M(1)eSe(1) 2.429(3) Sr(1)eSe(2) 2 3.298(2) M(1)eSe(2) 2.427(2) Sr(1)eSe(3) 3.236(3) M(1)eSe(3) 2 2.492(2) Sr(1)eSe(4) 2 3.182(2) Sr(1)eSe(6) 3.154(2) M(2)eSe(4) 2.611(2) Sr(2)eSe(2) 2 3.208(2) M(2)eSe(6) 2 2.774(1) Sr(2)eSe(4) 2 3.250(2) M(2)eSe(8) 2 3.020(2) Sr(2)eSe(5) 2 3.200(2) Sr(2)eSe(8) 3.187(2) M(3)eSe(5) 2.642(2) M(3)eSe(7) 2 2.864(2) Sr(3)eSe(1) 2 3.142(2) M(3)eSe(8) 2 2.917(2) Sr(3)eSe(5) 2 3.188(2) Sr(3)eSe(6) 3.645(3) Sr(3)eSe(7) 2 3.337(2) Sr(3)eSe(7) 3.166(3)

Fig. 1. (a) Polyhedral representation of Sr3GeSb2Se8viewed along the

crystal-lographic b-axis [010], showing one-dimensional chains of 1

N½MSe3 (gray

polyhedra) and1

N½M2Se5 (dark gray polyhedra). Big black and small gray

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and two longer ones at 2.917(2) A˚ . The interatomic distances of M(2)-Se and M(3)-Se compare satisfactorily with the Sbe Se distances in Sb2Se3. The chains of fused double square

pyr-amids form a pair that are related to each other via ana-glide plane parallel to the a-axis. Similar edge-sharing double chains were found in some multinary chalcogenides, such as K2Pr2xSb4þxSe12 [25] and La7Sb9S24 [26]. Combining the

Sr2þ, GeeSe and SbeSe anionic units, we express the charge-balanced formula as [Sr2þ]3[Ge4þ][Sb3þ]2[Se2]8, which is

consistent with the refined formula Sr3Ge0.95Sb2.05Se8.

3.2. Characterization

To determine the optical band gap, the UVevis diffuse re-flectance spectrum of ground crystals of Sr3GeSb2Se8 was

measured between 200 and 2000 nm (6.2e0.62 eV); cf.

Fig. 3. Sr3GeSb2Se8is expected to be a semiconductor because

the charge of cations and anions is balanced. Those measure-ments of optical diffuse reflectance reveal that the band gap is near 0.75 eV. Thermogravimetric (TGA) measurements ob-tained on heating polycrystalline samples reveal two stages of mass loss until the mass remains stable after 900C (Fig. 4). The total mass loss in these two stages drops by ap-proximately 55%. These results were reproduced by heating the as-synthesized powder in a quartz ampoule under N2

flow and subsequently heating to 500, 700, and 900C. The first decomposition step with mass loss (w5%) occurs from 400 to 500C (Fig. 4). X-ray powder diffraction of the mate-rial obtained at this stage showed a broad peak at 2q w 26 (Fig. 5). The second step starts at w700C with weight loss of w 50%, corresponding to the decomposition of Sr3GeSb2Se8. The PXRD pattern of the residue just before Fig. 2. (a and b)1

N½MSe3 and1N½M2Se5 chains along the [010] direction.

Solid and dashed lines indicate MeSe distances <2.87 and <3.02 A˚ , respectively.

Fig. 3. TGA curve of Sr3GeSb2Se8(N2flow, heating rate¼ 5C/min).

Fig. 4. Diffuse reflectance spectrum of Sr3GeSb2Se8.

Fig. 5. Powder X-ray diffraction patterns of the residues at 25 (pure Sr3GeSb2Se8), 500, 700, 900C.

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the final decomposition step at 700C indicates that additional products of GeSe, GeSe2, SrSe and the title compound are

present. The PXRD pattern of the residue at 900C contains a mixture of crystalline SrSe and an amorphous phase. Poly-crystalline product was found in the cool part of silica ampoule and the PXRD pattern can be indexed as a combination of Sb2Se3, GeSe, and GeSe2.

4. Conclusions

We have synthesized a new quaternary selenide Sr3GeSb2Se8

that exhibits unique one-dimensional chains of vertex-sharing tetrahedra 1N½MSe3 (M ¼ Ge/Sb) and fused double chains of

edge-sharing square pyramids 1N½M2Se5. Sr3GeSb2Se8 is an

electron-precise compound with a measured band gap 0.75 eV. Acknowledgements

We thank Prof. Eric W.G. Diau for measuring the diffuse reflectance spectrum. The National Science Council (NSC94-2113-M-009-012, 94-2120-M-009-014) supported this research.

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