Synthesis, Structure, and Properties of a New Family of Mixed

Energy Science, Kyung Hee UniVersity, Seoul 130-701, South Korea, and Department ... North Carolina State UniVersity, Raleigh, North Carolina 27695-82...
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Chem. Mater. 2006, 18, 1219-1225

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Synthesis, Structure, and Properties of a New Family of Mixed-Framework Chalcohalide Semiconductors: CdSbS2X (X ) Cl, Br), CdBiS2X (X ) Cl, Br), and CdBiSe2X (X ) Br, I) Lei Wang,† Yi-Chung Hung,‡ Shiou-Jyh Hwu,*,† Hyun-Joo Koo,§ and M.-H. Whangbo| Department of Chemistry, Clemson UniVersity, Clemson, South Carolina 29634-0973, Department of Chemistry, Rice UniVersity, Houston, Texas 77251-1892, Department of Chemistry and Institute of Basic Energy Science, Kyung Hee UniVersity, Seoul 130-701, South Korea, and Department of Chemistry, North Carolina State UniVersity, Raleigh, North Carolina 27695-8204 ReceiVed October 6, 2005. ReVised Manuscript ReceiVed December 3, 2005

A new family of quaternary mixed-framework chalcohalide semiconductors has been synthesized by conventional solid-state reactions in an intermediate temperature region of 400-430 °C. These include CdSbS2Cl (1), CdSbS2Br (2), CdBiS2Cl (3), CdBiS2Br (4), CdBiSe2Br (5), and CdBiSe2I (6). Singlecrystal structure analyses of this compound series reveal two types of crystal structures depending upon the combination of chalcohalide anions, and they crystallize in an orthorhombic Pnma (type I) and monoclinic C2/m (type II) space group. Type I is adopted by two sulfochlorides, 1 and 3, and one selenobromide, 5. Type II is adopted by two sulfobromides, 2 and 4, and one selenoiodide, 6. Both structure types have slabs built of corner and edge shared Cd-centered CdQ6-xXx (Q ) S, Se; X ) Cl, Br, I; x ) 0, 2, 4) octahedra that resemble the (110) plane of a distorted NaCl-type structure. In the type I structure, this slab contains only CdQ4X2 octahedra, while type II shows alternating CdQ6 and CdQ2X4 octahedra. In both structure types, each M3+ (M ) Sb, Bi) cation of CdMQ2X forms a distorted square pyramid, MQ5, as found in M2Q3. The MQ5 unit is weakly coordinated to three X atoms to form a distorted bicapped trigonal prism, MQ5X3. In forming the extended network structure, the CdQ6-xXx and MQ5X3 units are solely linked through chalcogenide anions. The Fourier transform infrared spectroscopy studies suggest that these chalcohalides are transparent in the mid-IR region (1400-4000 cm-1). The UV-vis spectroscopy results in a band gap ranging from 1.3 to 2.2 eV, showing a red shift with respect to the corresponding binary chalcogenides CdQ. The results of tight-binding electronic band structure calculations suggest that the origin of this red shift is due to the lone-pair effects from Sb and Bi.

Introduction Mixed-framework compounds, despite their convoluted chemistry, are attractive in advanced materials research because of their integrated structural and physical properties. Inorganic/organic hybrid compounds, for instance, are wellknown for their unique structural and electronic properties that are otherwise unattainable from their individual components.1-3 Some of the recent examples include organically templated micro- and macroporous inorganic solids,1 * To whom correspondence should be addressed. Telephone: (864)656-5031. Fax: (864)656-6613. E-mail: [email protected]. † Clemson University. ‡ Rice University. § Kyung Hee University. | North Carolina State University.

(1) (a) Rowsell, J. L. C.; Yaghi, O. M. Microporous Mesoporous Mater. 2004, 73, 3-14 and references therein. (b) Fe´rey, G. Chem. Mater. 2001, 13, 3084-3098. (2) Some earlier examples: (a) Mitzi, D. B.; Wang, S.; Field, C. A.; Chess, C. A.; Guloy, A. M Science 1995, 267, 1473-1476. (b) Mitzi, D. B.; Field, C. A.; Harrison, W. T. A.; Guloy, A. M. Nature 1994, 369, 467-469 and references therein. (3) For example: (a) Huang, X.; Li, J.; Zhang, Y.; Mascarenhas, A. J. Am. Chem. Soc. 2003, 125, 7049-7055. (b) Zheng, N.; Bu, X.; Wang, B.; Feng, P. Science 2002, 198, 2366-2369 (c) Li, H.; Kim, J.; Groy, T. L.; O’Keeffe, M. O.; Yaghi, O. M. J. Am. Chem. Soc. 2001, 123, 4867-4868. (d) Klimov, V. I.; Mikhailovsky, A. A.; Xu, S.; Malko, A.; Hollingsworth, J. A.; Leatherdale, C. A.; Eisler, H. J.; Bawendi, M. G. Science 2000, 290, 314-317.

layered perovskites made of organic-based metal halides,2 and crystalline solids containing periodic arrays of semiconducting II-VI mixed-metal chalcogenide supertetrahedra with organic molecules filling created voids.3 Employing molten-salt methods, we have synthesized a novel class of low-dimensional solids via salt inclusion.4-7 These new compounds exhibit hybrid (composite) structures of covalent and ionic lattices made of transition metal oxides and alkali and alkaline-earth metal halides.4 These novel solids exhibit salt-templated porous structures,5 noncentrosymmetric lattices,6 and frameworks containing magnetic nanostructures.7 Studies show that, like the inorganic/organic hybrid materials, the inclusion of salt has a significant impact (4) (a) Hwu, S.-J.; Huang, Q.; Ulutagay, M. U.S. Patent 6,890,500. (b) Huang, Q.; Ulutagay-Kartin, M.; Mo, X.; Hwu, S.-J. Mater. Res. Soc. Symp. Proc. 2003, 755, DD12.4. (5) (a) CU-4: Huang, Q.; Hwu, S.-J.; Mo, X. Angew. Chem., Int. Ed. 2001, 40, 1690-1693. (b) CU-2: Huang, Q.; Ulutagay, M.; Michener, P. A.; Hwu, S.-J. J. Am. Chem. Soc. 1999, 121, 10323-10326. (6) (a) CU-14: Mo, X.; Ferguson, E. Hwu, S.-J. Inorg. Chem. 2005, 44, 3121-3126. (b) CU-13: Mo, X.; Hwu, S.-J. Inorg. Chem. 2003, 42, 3978-3980. (c) CU-9 and CU-11: Huang, Q.; Hwu, S.-J. Inorg. Chem. 2003, 42, 655-657. (7) For example: (a) Hwu, S.-J.; Ulutagay-Kartin, M.; Clayhold, J. A.; Mackay, R.; Wardojo, T. A.; O’Connor, C. J.; Krawiec, M. J. Am. Chem. Soc. 2002, 124, 12404-12405. (b) Clayhold, J. A.; UlutagayKartin, M.; Hwu, S.-J.; Koo, H.-J.; Whangbo, M.-H.; Voigt, A.; Eaiprasertsak, K. Phys. ReV. B 2002, 66, 052403.

10.1021/cm0522230 CCC: $33.50 © 2006 American Chemical Society Published on Web 02/01/2006

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on the framework structure but, becaue of its insulating (ionic) nature, has little effect on the chemical and electronic properties of the resulting host oxide lattice. In general, the formation of these complicated mixedframework materials can be viewed as a result of structure “segregation” of chemically unique species, that is, inorganic versus organic for the examples shown in refs 1-3 and covalent versus ionic for those in refs 4-7. Because of the difference in chemical bonding, the electronic interaction at the interface is interrupted, thus leading to the observed lowdimensional physical properties. Therefore, the bulk materials show simplified and often superior “molecule-like” characteristics. The crystalline quantum dots, as mentioned above,3 reveal sharper emission spectra than do nanoparticles, which is attributed to the uniform size distribution of the semiconducting components. Recently, we have begun a systematic investigation of chalcogenide-based mixed-anion compounds to further explore new nanostructured materials via the concept of mixedframework solids. The idea has been to exploit anions that have inherent differences in chemical bonding due to distinct size and electronegativity. Prior studies on chalcogenidebased mixed-anion compounds include oxychalcogenides8 and chalcohalides.9 These two major families of chalcogenide-based derivatives have initially received scattered attention mostly because they were accidental discoveries during the exploratory synthesis of mixed-metal chalcogenides. These serendipitous discoveries arise from the incorporation of oxygen from air10f,h and inclusion of halides from the molten salt employed as a high-temperature solvent for crystal growth.11e It should be noted that chalcohalides (8) (a) LnCuOQ (Ln ) lanthanide, Q ) chalcogen): Ueda, K.; Hiramatsu, H.; Ohta, H.; Hirano, M.; Kamiya, T.; Hosono, H. Phys. ReV. B 2004, 69, 155305/1-155305/4. (b) La2CdO2Se2: Hiramatsu, H.; Ueda, K.; Kamiya, T.; Ohta, H.; Hirano, M.; Hosono, H. J. Mater. Chem. 2004, 14, 2946-2950. (c) La3CuO2S3: Ijjaali, I.; Haynes, C. L.; McFarland, A. D.; Van Duyne, R. P.; Ibers, J. A. J. Solid State Chem. 2003, 172, 257-260. (d) Bi2YO4Cu2Se2: Evans, J. S. O., Brogden, E. B., Thompson, A. L.; Cordiner, R. L. Chem. Commun. 2002, 912-913. (e) La4Ti2O4Se5 and La6Ti3O5Se9: Tougait, O.; Ibers, J. A. J. Solid State Chem. 2001, 157, 289-295. (f) Ln3.67Ti2O3Se6 (Ln ) Ce, Nd, Sm): Tougait, O.; Ibers, J. A. Chem. Mater. 2000, 12, 2653-2658. (g) A2Cu2CoO2S2 (A ) Sr, Ba): Zhu, W. J.; Hor, P. H.; Jacobson, A. J.; Crisci, G.; Albright, T. A.; Wang, S.-H.; Vogt, T. J. Am. Chem. Soc. 1997, 119, 12398-12399. (h) Ln2Ta3Se2O8 (Ln ) La, Ce, Pr, Nd): Brennan, T. D.; Aleandri, L. E.; Ibers, J. A. J. Solid State Chem. 1991, 91, 312-322. (9) (a) CuCr2Se4-xBrx: Lee, W.-L.; Watachi, S.; Miller, V. L.; Cava, R. J.; Ong, N. P. Science 2004, 303, 1647-1649. (b) MnSbS2Cl: Doussier, C.; Le´one, P. Moe¨lo, Y. Solid State Sci. 2004, 6, 13871391. (c) MnSbSe2I: Tougait, O.; Ibers, J. A.; Mar, A. Acta Crystallogr, Sect. C 2003, 59, i77-i78. (d) LnSbS2Br2 (Ln ) La, Ce) sulfobromide series: Gout, D.; Jobic, S.; Evain, M.; Brec, R. J. Solid State Chem. 2001, 158, 218-226. (e) LaCa2GeS4Cl3: Gitzendanner, R. L.; DiSalvo, F. J. Inorg. Chem. 1996, 35, 2623-2626. (f) CdSb6S8I4: Sirota, M. I.; Simonov, M. A.; Egorov-Tismenko, Y. K.; Simonov, V. I.; Belov, N. V. Kristallografiya 1976, 21, 64-68. (g) Cu3Bi2S4Br: Mariolacos, K.; KupcI¨ik, V. Acta Crystallogr., Sect. B 1975, 31, 1762-1763. (h) Cu3Bi2S4Cl: Lewis, J., Jr.; KupcI¨ik, V. Acta Crystallogr., Sect. B 1974, 30, 848-852. (10) (a) Harrington, J. A. Infrared Fibers and Their Applications; SPIE Press: Bellingham, WA, 2004; pp 83-104. (b) Marchese, D.; De Sario, M.; Jha, A.; Kar, A. K.; Smith, E. C. J. Opt. Soc. Am. B 1998, 15, 2361-2370. (11) For example: (a) CsLnMSe3 (Ln ) rare earth, M ) Zn, Cd, Hg): Mitchell, K.; Huang, F. Q.; McFarland, A. D.; Haynes, C. L.; Somers, R. C.; Can Duyne, R. P.; Ibers, J. A. Inorg. Chem. 2003, 42, 41094116. (b) Cs2Bi2CdS5: Huang, F. Q.; Somers, R. C.; McFarland, A. D.; Van Duyne, R. P.; Ibers, J. A. J. Solid State Chem. 2003, 174, 334-341.

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in their glass form have received recognition for their superior properties as infrared transmitting materials for optical fiber applications in telecommunication (optical amplifiers) and medical devices (surgical laser sources and laser power delivery).10 In the present work, we have chosen to incorporate the Cd2+ cation in combination with Sb3+ or Bi3+ because the chalcogenides of these cations show desired semiconducting properties.11 Our objective is to find a new class of lowdimensional semiconductors whose dimensionality and electronic properties can be varied in a controlled fashion. It is well-known that the stereochemical activity of the lone-pair electrons of Sb3+ and Bi3+ cations has played a major role during the formation of low-dimensional lattices. In the present work, we report the results of solid-state synthesis, crystal structure analysis, spectroscopy characterization, and electronic band structure calculations of new quaternary chalcohalide compounds CdSbS2X (X ) Cl (1), Br (2)), CdBiS2X (X ) Cl (3), Br (4)) and CdBiSe2X (X ) Br (5), I (6)). This new series of chalcohalide compounds adopts two structural types depending upon the specific combinations of chalcogenide and halide anions and is the first systematic study among mixed-anion compounds reported thus far showing the role of anions. Experimental Section General Procedures. All the reactants were loaded in a nitrogenfilled drybox. The reaction mixtures were sealed in a fused quartz tube under vacuum (∼10-4 Torr) using a methane torch. The materials were examined using powder X-ray diffraction (PXRD) patterns recorded on a SINTAG 2300 diffractometer equipped with a graphite monochromator (Cu KR1 radiation). Single-crystal X-ray structure analysis was carried out on a Rigaku AFC-8S four-circle diffractometer (Mo KR ) 0.710 73 Å) equipped with the mercury charge-coupled device area detector. Semiquantitative elemental analysis of single crystals was performed by energy-dispersive X-ray (EDX) analysis using a Hitachi S-3500 scanning electron microscope equipped with an OXFORD EDX microprobe. Syntheses. Crystals of CdBiS2Cl were initially obtained from the quaternary Bi/K/Ti/S reaction in an attempt to synthesize the Cd analogue of the misfit-layer compounds Bi6-xCaxTi5S16 (x ) 3.08) using the eutectic flux CdCl2/KCl.12 Elemental bismuth (Aldrich, 99.99%), potassium (Mallinckrodt, 98%), titanium (Aldrich, 99.9%), and sulfur (Aldrich, 99.99%) were mixed in a 2:4: 3:10 molar ratio. The potassium metal was added to produce highly reactive cadmium in situ via the reduction reaction CdCl2 + 2K f Cd + 2KCl at an elevated temperature. The reaction mixture was loaded in a carbon-coated quartz tube with a CdCl2 (Aldrich, 99+%) and KCl (Baker, reagent) eutectic flux (3:1 mole ratio, mp 390 °C)13 with a charge-to-flux ratio of 1:4 by weight. When a slow heating process was used to avoid an explosion due to the volatile sulfur (bp ) 754 °C), the reaction mixture was heated to 990 °C in three steps: first to 754 °C (0.5 °C/min), held for 1 day, and then to 990 °C (0.75 °C/min). The reaction was isothermed for 3 days and followed by slow cooling at a rate of 3 °C/h to room temperature. Reddish transparent, needle-shaped crystals of CdBiS2Cl in about 50% yield (based on Bi) were isolated from the flux. The final composition of the compound was determined first by (12) (a) Hung, Y.-C. Ph.D. Dissertation, Rice University, Houston, TX, 1994. (b) Hung, Y.-C.; Hwu, S.-J. Inorg. Chem. 1993, 32, 54275428. (13) Dergunov, E. P. Dokl. Akad. Nauk SSSR 1949, 64, 519.

New Mixed-Framework Chalcohalide Semiconductors

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Table 1. Crystollographic Data for the Single Crystals of CdMQ2X empirical formula formula weight (amu) crystal color, shape crystal dimensions (mm) crystal system space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z R1/wR2 (all data) GOF ∆Fmin/∆Fmax (e Å-3)

1

3

5

2

4

6

CdSbS2Cl 333.72 yellow, needle 0.02 × 0.02 × 0.36 orthorhombic Pnma (No. 62) 9.585(2) 3.9910(8) 12.443(3)

CdBiS2Cl 420.95 dark red, chunk 0.024 × 0.048 × 0.72 orthorhombic Pnma (No. 62) 9.541 (2) 3.9700(8) 12.545(3)

CdBiSe2Br 559.21 black, chunk 0.02 × 0.02 × 0.22 orthorhombic Pnma (No. 62) 10.025(2) 4.1190(8) 13.143(3)

475.9(2) 4 0.0246/0.0431 1.135 0.677/-0.789

475.2 (2) 4 0.0263/0.0604 1.113 2.14/-1.77

542.7(2) 4 0.0291/0.0588 1.012 3.111/-1.249

CdSbS2Br 378.18 red, needle 0.02 × 0.02 × 0.31 monoclinic C2/m (No. 12) 12.938(3) 3.9310(8) 9.6610(19) 91.11(3) 491.3(2) 4 0.0287/ 0.0551 1.153 0.0792/-1.152

CdBiS2Br 465.41 red, needle 0.01 × 0.01 × 0.29 monoclinic C2/m (No. 12) 12.977(3) 4.0120(8) 9.584(2) 91.07(3) 498.9(2) 4 0.0480/0.0989 1.196 2.718/-2.410

CdBiSe2I 606.20 black, needle 0.05 × 0.05 × 0.43 monoclinic C2/m (No. 12) 13.659(3) 4.1920(8) 10.193(2) 90.88(3) 583.6(2) 4 0.0270/0.0610 1.125 1.136/-1.848

Table 2. Atomic Coordinates of CdBiS2Cl (3) and CdBiS2Br (4)

X-ray single-crystal structure analysis and EDX, followed by a successful stoichiometric synthesis. The byproducts of the reaction were identified as BiTiS3 and KCl. To make large single crystals of CdBiS2Cl and CdBiS2Br, a vapor transport method with iodine was employed using the polycrystalline products synthesized by the methods described below, and the latter can be grown as large as 1 cm long. A sealed quartz tube (14 cm long) was heated in a horizontal two-zone tube furnace with a typical temperature gradient of 440 °C (charge zone) to 350 °C (growth zone). Efforts to grow crystals of CdBiSe2Br were not initially successful by using either the CdBr2/KBr eutectic flux methods or the iodine vapor transport but were eventually achieved via quenching at 600 °C from its own melt. We later found that the single crystals of CdBiS2Cl can also be grown by quenching the reaction melts at 500 °C, which is slightly below the decomposition temperature 550 °C based on thermogravimetric analysis. This implies that there is no glass formation up to the decomposition temperatures.14 Meanwhile, single crystals of all other compounds were directly grown by solid-state reactions (see below). Stoichiometric Synthesis. After the structural identification, the stoichiometric syntheses of this chalcohalide family were successfully carried out as follows:

Bi2Q3, and the ternary phases, SbSeI, SbSX (X ) Br, I),15 and CdBi2S4, were obtained. A stoichiometric yield of polycrystalline product was acquired by employing different starting materials according to eq 2 at temperatures as low as 450 °C.

3CdQ + 2M + MX3 + 3Q f 3CdMQ2X

CdQ + 2Bi + CdX2 + 3Q f 2CdBiQ2X

(1)

The reactants used, including their sources and purities, are as follows: Sb (Alfa Aesar, 99.5%), Bi (Alfa Aesar, 99.5%), CdS (Strem, 99.9+% Cd), CdSe (Alfa Aesar, 99.995%), SbCl3 (Alfa Aesar, 99.9%), SbBr3 (Alfa Aesar, 99.5%), BiCl3 (Alfa Aesar, 99.9%), BiBr3 (Alfa Aesar, 99%), BiI3 (Aldrich, 99.999%), and chalcogens (S and Se, Aldrich, 99.99%). For the reactions leading to sulfur-containing compounds, the reactants were first slowly (0.5 °C/min) heated to 250 °C and isothermed for 10 h and then heated at 1 °C/min to 400-430 °C and isothermed for 4 days. The reactions were then slowly cooled (-0.1 °C/min) to room temperature. For the reactions leading to the selenium-containing compounds, the reactants were heated directly to 430 °C (1 °C/min) and isothermed for 4 days followed by slow cooling to room temperature. All phases, including CdSbSe2I, can be synthesized stoichiometrically in polycrystalline form at the given temperature range mentioned above. All attempts to synthesize other members of the chalcohalide family, including “CdMS2I”, “CdMSe2Cl”, “CdSbSe2Br”, and the tellurium analogues, failed. Instead, the binary phases, Sb2Q3 and (14) To investigate any possible glass formation, the grounded polycrystalline samples were sealed in a quartz tube under vacuum, and the powder was heated to the temperatures below the decomposition point. After dwelling at high-temperature overnight, the reaction was quenched in a water bath. No glass formation was observed; instead, sizable crystals of the polycrystalline reactants were isolated.

atom

Wyckoff notation/sof

x

y

z

Uisoa (Å2)

Cd Bi S(1) S(2) Cl

4c 4c 4c 4c 4c

CdBiS2Cl 0.49516(7) 3/4 0.30465(3) 1/4 0.2517(2) 3/4 0.4490(2) 3/4 0.4261(3) 1/4

0.26490(5) 0.02903(2) 0.1698(2) 0.8872(2) 0.3943(2)

0.0168(4) 0.0152(2) 0.0125(4) 0.0117(4) 0.0186(5)

Cd(1) Cd(2) Bi S(1) S(2) Br

2c 2i 4i 4i 4i 4i

CdBiS2Br 0 0 0 0 0.78313(5) 1/2 0.9196(4) 1/2 0.8567(3) 1/2 1.1379(2) 1/2

1/2 0 0.69591(8) 0.7441(5) 0.4466(5) 0.9276(2)

0.0201(5) 0.0186(5) 0.0182(4) 0.0192(5) 0.0140(9) 0.0138(9)

a Equivalent isotropic U defined as one-third of the trace of the orthogonalized Uij tensor.

(2)

When the same reaction mixture is used, larger single crystals (ca. 10-20%) were grown by first heating the reactants to 900 °C followed by slow cooling to room temperature. The identified crystalline byproducts were the binary phases Bi2Q3, Sb2Q3, and CdQ. X-ray Crystallography. The needle-shaped single crystals were mounted on the quartz fiber for structural analysis. Single-crystal X-ray diffraction data were collected at the room temperature (295 K). The structures were solved with the direct methods and refined on F2 with the full-matrix, least-squares method by SHELXL-97 of the SHELXTL program suite.16 Each final refinement included anisotropic displacement parameters and a secondary extinction correction. Additional experimental details and crystallographic data are listed in Table 1. The atomic coordinates and selected bond distances and bond angles of representative compounds CdBiS2Cl (type I) and CdBiS2Br (type II) are in Tables 2 and 3, respectively. PXRD patterns of all stoichiometrically synthesized products (15) (a) Inushima, T. J. Phys. Chem. Solids 1999, 60, 587-598. (b) Lukaszewicz, K.; Pietraszko, A.; Stepen-Damm, Y.; Kajokas, A. Pol. J. Chem. 1997, 71, 1852-1857. (c) Ibanez, A.; Jumas, J. C.; OlivierFourcade, J.; Philippot, E.; Maurin, M. J. Solid State Chem. 1983, 48, 272-283. (16) Sheldrick, G. M. SHELXTL DOS/Windows/NT, Version 6.12; Bruker Analytical X-ray Instruments, Inc.: Madison, WI, 2000.

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Table 3. Selected Bond Distances (Å) and Angles (deg) of CdBiS2Cl (3) and CdBiS2Br (4) Type I Cd-S(1)a Cd-S(1)b S(1)b-Cd-S(1)a S(1)b-Cd-Cl S(1)b-Cd-S(2)b S(1)a-Cd-Cl S(1)-Cd-S(2)b

CdS4Cl2 Octahedron 2.611(2) Cd-S(2) (×2) 2.581(2) Cd-Cl (×2) 171.32(4) S(2)b-Cd-S(2)a 92.37(6) S(2)b-Cd-Cl 92.04(5) S(2)b-Cd-Cl 93.37(6) S(2)b-Cd-S(2)a 81.86(5)

Bi-S(1) (×2) Bi-Cl (×2) Bi-Cla S(1)b-Bi-S(1) S(2)b-Bi-S(2)a S(2)b-Bi-S(2)a

BiS5Cl3 btp 2.705(1) Bi-S(2)b (×2) 3.412(2) Bi-S(2)a 3.737(3) 94.44(6) S(2)b-Bi-S(1)b 79.82(5) S(2)b-Bi-S(1)a 82.83(5) S(2)b-Bi-S(1)a

2.804(1) 2.648(1) 90.13(6) 86.22(4) 174.36(6) 90.13(6)

3.001(1) 2.575(2) 84.49(5) 89.28(4) 163.45(6)

Type II Cd(2)-S(1) (×2) S(1)e-Cd-S(1) S(2)e-Cd-S(2)g S(2)e-Cd-S(2)g

CdS6 Octahedron 2.57(2) Cd(2)-S(2) (×4) 180.00(0) S(1)e-Cd-S(2)g 96.10(4) S(2)e-Cd-S(2)f 87.40(7)

Cd(2)-S(1) (×2) S(1)h-Cd-S(1) S(1)-Cd-Br Br-Cd-Bri

CdS2Br4 Octahedron 2.673(3) Cd(2)-Br (×4) 180.00(0) S(1)-Cd-Brh 91.25(1) Brh-Cd-Bri 97.59(2) Brh-Cd-Br

Bi-S(1)a (×2) Bi-S (×1) S(1)-Bi-S(1)a S(2)c/d-Bi-S(2) S(2)c-Bi-S(2)d

BiS5Br3 btp 2.708(2) Bi-S(2)c/d (×2) 2.573(3) Bi-Br 95.65(6) S(2)-Bi-S(1)a 78.95(2) S(2)c/a-Bi-S(1)c 82.63(3) S(2)c-Bi-S(1)a

2.78(2) 83.90(5) 92.60(2)

2.780(2) 88.75(3) 92.41(2) 180.00(0) 3.041(2) 3.765(4) 85.20(50 88.77(4) 163.14(6)

a-i Symmetry codes: (a) x, y, z; (b) -1/2 + x, 1/2 - z; (c) 1/2 - x, -1/2 - y, 1 - z; (d) 1/2 - x, 1/2 - y, 1 - z; (e) -x, -y, 1 - zl; (f) -1/2 + x, -1/2 + y, z; (g) 1/2 - x, 1/2 - y, -z; (h) 1/2 - x, -1/2 - y, -z; (i) -x, -y, -z.

confirm the purities of the bulk samples; see an example in Supporting Information, Figure S2. Diffuse Reflectance UV-Vis Spectrum. UV-vis spectra were recorded on a Shimadzu 3901 spectrophotometer in the range of 0.5-6.5 eV (190-2400 nm) using the diffuse reflectance measurement. Ground polycrystalline samples were applied on the surface of a BaSO4 sample holder. The absorption data (R/S) were calculated from the reflectance mode using the Kubelka-Munk function R/S ) (1 - R)2/2R, where R is the reflectance at a given energy, R is the absorption, and S is the scattering coefficient.17 The energy gap was determined as the intersection point between the energy axis at the absorption offset and the line extrapolated from the linear portion of the absorption edge in the R/S versus E (eV) plot. For comparison, the UV-vis spectra of CdQ, Bi2Q3, and Sb2Q3 were also recorded. Fourier Transform Infrared Spectrum. The IR spectra on polycrystalline samples were collected using a Nicolet Magan-IR 550 spectrophotometer, in the 4000-400 cm-1 region. Powder samples were pressed into pellets with dry KBr (10 mm diameter, 0.5 mm thick). Single-crystal infrared transmittance investigations were performed using a Spectra Tech in-plan spectrometer, which was equipped with a microscope attachment. This enabled the investigations of materials with a sampling area as small as 10 µm. Spectra were taken on small transparent crystals of 3 and 4, which (17) (a) Wendlandt, W. W.; Hecht, H. G. Reflectance Spectroscopy; Interscience: New York, 1966. (b) Kortu¨m, G. Reflectance Spectroscopy; Springer-Verlag: Berlin/New York, 1969. (c) Kortu¨m, G.; Braun, W.; Herzog, G. Angew. Chem., Int. Ed. Engl. 1963, 2 (7), 333-404.

were placed on a KBr plate. A 32× magnification head with an aperture size of 26 × 26 µm was used during the studies. These measurements indicate that these mixed-framework chalcohalides are transparent in the mid-IR region (1400-4000 cm-1, or 25007000 nm). Spectra are given in Figure S1 of Supporting Information. Electronic Band Structure Calculations. Extended Hu¨ckel tight-binding (EHTB) electronic band structure calculations18 were carried out for CdSbS2Cl, CdS, and Sb2S3 using the CASEAR19 program. The atomic orbital parameters employed in our calculations are listed in Table S1 of Supporting Information.

Results and Discussion Crystal Structure. The new series of chalcohalides containing chalcogenide (Q2-) and halide (X-) anions can be represented by the general formula CdMQ2X (M ) Sb, Bi; Q ) chalcogen; X ) halogen). These solids form two structure types depending upon the combination of chalcogenide and halide anions. CdSbS2Cl, CdBiS2Cl, and CdBiSe2Br crystallizing in the orthorhombic space group Pnma (No. 62) adopt the type I structure, in which both anions belong to the same period, that is, Q2-/X- ) S2-/Cl- and Se2-/ Br-. CdSbS2Br, CdBiS2Br, and CdBiSe2I crystallizing in the monoclinic space group C2/m (No. 12) possess the type II structure, in which the halide anions lie one period below the chalcogenide anions, that is, Q2-/X- ) S2-/Br- and Se2-/ I-. As already mentioned in Experimental Section, attempts to synthesize compounds of other combinations of chalcogenide and halide anions, including Te2-/I-, have not yet produced any chalcohalide phases of the title series. It is noted that the β angles in the type II structure deviate from 90° only slightly (Table 1), which could be due to an increased size difference between the Q2- and X- anions. The two examples reported in prior studies, MnSbS2Cl (Pnma)9b and MnSbSe2I (C2/m),9c adopt the type I and II structures, respectively. This is consistent with our observation concerning the anion combinations. Types I and II can be viewed as layered type structures that share common features with respect to the CdQ6-xXx slabs. Each slab is made of an extended array of cornerand edge-sharing Cd-centered octahedra adopting the (110) plane of a distorted NaCl-type structure. The fused CdQ6-xXx octahedra share trans edges along one direction and a corner along the orthogonal direction. As shown in Figure 1, the M3+ (M ) Sb, Bi) cations form distorted square pyramids, MQ5, as found in the Sb2Se3-type substructure in the region between the CdQ6-xXx slabs. The asymmetrical environments of the M3+ cations show that their lone-pair electrons are stereochemically active and point toward the halide anions with the extra long M-X distances, for example, 3.508(1)3.814(2) Å for Sb-Cl (vs 2.61 Å, the sum of Shannon crystal radii),20 3.602(4)-3.887(3) Å for Sb-Br (vs 2.76 Å), 3.412(2)-3.737(3) Å for Bi-Cl (vs 2.77 Å), 3.590(4)-3.768(3) Å for Bi-Br (vs 2.92 Å), and 3.796(4)-4.082(3) Å for Bi-I (vs 3.16 Å). As shown in Figure 2, each MQ5 unit links to (18) Whangbo, M.-H.; Hoffmann, R. J. Am. Chem. Soc. 1978, 100, 60936098. (19) Our calculations were carried out by employing the CAESAR2 program package. Dai, D.; Ren, J.; Liang, W.; Whangbo, M.-H. http:// chvamw.chem.ncsu.edu/ (accessed 2002). (20) Shannon, R. D. Acta Crystallogr., Sect. A 1976, 32, 751-767.

New Mixed-Framework Chalcohalide Semiconductors

Chem. Mater., Vol. 18, No. 5, 2006 1223

Figure 1. Perspective views of the representative unit-cell drawings of (a) CdBiS2Cl (3), (b) CdBiS2Br (4), and for comparison, (c) FeUS3; see text. Table 4. BVS Values for the CdBiS2Cl (3) Phase21

Figure 2. Distorted bicapped trigonal prismatic (a) BiS5Cl3 and (b) BiS5Br3 units observed in the structures of 3 and 4, respectively. Two units are fused via sharing chalcogenide anions; see text.

additional halide anions (X) to form a distorted bicapped trigonal pyramid MQ5X3. Furthermore, two MQ5X3 units share the vertex chalcogenide anions in a M2Q8X6 dimeric unit. These M2Q8X6 units share the remaining chalcogenide anions to further extend the structure along the b-axis direction. The type I and type II structures are further described in detail by considering CdBiS2Cl and CdBiS2Br. Type I: CdSbS2Cl (1), CdBiS2Cl (3), and CdBiSe2Br (5) (Pnma, No. 62). 1, 3, and 5 are isostructural, and for

Cd

Bi

BVS

S(1) S(2) Cl (as a S)

0.44 + 0.41 0.24 × 2 0.31 × 2 (0.37 × 2)

0.69 × 2 0.31 × 2 + 0.99 0.08 × 2 + 0.04 (0.10 × 2 + 0.04)

2.23 2.09 0.82 (0.98)

BVS

1.95 (2.07)

3.19 (3.23)

simplicity, only the structure of CdBiS2Cl (3) is discussed in detail below. Figure 1a shows the connectivity in a unit cell where the extra long Bi-Cl bond (>3.41 Å) is omitted for clarity. The Cd2+ cations, which reside on the mirror planes at z ) 1/4 and z ) 3/4, are six-coordinate each with four S2- and two Cl- anions. The [CdS4Cl2] octahedral units are connected by sharing an edge made up of equatorial S(2) and Cl in the [010] direction and a vertex through S(1) in the [100] direction. As already mentioned, the trivalent Bi3+ cations are located between the [CdS4Cl2] slabs and adopt a bicapped trigonal prismatic coordination with respect to the sulfur and chlorine atoms (Figure 2). Compounds of this structure type consist of anions that differ in their atomic numbers by one and, hence, have almost undistinguishable X-ray scattering factors. We rely on the combination of EDX analysis, stoichiometric synthesis, and bond valence sum (BVS) analysis21 to establish the chargebalanced chemical composition and atomic site assignments. During the single-crystal structure refinement, the three crystallographically unique sites S(1), S(2), and Cl were finalized based on the calculated BVS values of the cations and anions (Table 4). In particular, the BVS value for the Cl site is calculated to be only 0.98 if it were refined as S (see the number in parentheses). It should be noted that, although some S/Cl-containing compounds show mixed anion sites,22 most of the reported structures exhibit distinct S and Cl sites.9 We have, therefore, concluded that the sulfur and chlorine each resides in a distinct crystallographic site, which is consistent with that of the reported MnSbS2Cl (21) (a) Brese, N. E.; O’Keeffe, M. Acta Crystallogr., Sect. B 1991, 47, 192-197. (b) Brown, I. D. J. Appl. Crystallogr. 1996, 29, 479-480. (22) (a) AgIn2S3Cl: Range, K.-J.; Huebner, H. J. Z. Naturforsch., B: Anorg. Chem. Org. Chem. 1983, 38, 155-160. (b) Bi4S5Cl2: Kraemer, V. Acta Crystallogr., Sect. B 1979, 35, 139-140.

1224 Chem. Mater., Vol. 18, No. 5, 2006

structure.9b The calculated oxidation states of Cd2+, Bi3+, S2-, and Cl- ions are 1.95, 3.19, 2.16 (averaged), and 0.82, respectively. The extended structure can be viewed as parallel [CdS2Cl]∞3- slabs interlinked by the Bi3+ cations (Figure 1a). The structural formula for the [CdS2Cl]∞3- slab can thus be written as [CdS(1)2/2S(2)2/2Cl2/2]∞ (≡ [CdS2Cl]∞). The Bi3+ cations interconnect the close-shell, semiconducting [CdS2Cl]∞3- slabs (see below) to keep the charge balanced. As stated above (Figure 2a), the local coordination environment around the Bi3+ cations can be described as the [BiS5Cl3] bicapped trigonal prism (btp). However, the Bi-Cl bond distances are rather long so that the semiconducting slabs are held together primarily by the Bi-S bonds [2.705(1) Å for Bi-S(1) and 3.001(1) Å for Bi-S(2) (Table 3)]. The Bi-S(2) bond shared by the btp dimers adopts the shortest distance, 2.575(2) Å. Type II: CdSbS2Br (2), CdBiS2Br (4), and CdBiSe2I (6) (C2/m, No.12). 2, 4, and 6 are isostructural and, for simplicity, only the structure of CdBiS2Br (4) is discussed in detail below. The type II structure also possesses the Cdcontaining chalcohalide slabs adopting the (110) plane of a distorted NaCl-type structure that are interlinked by the Sb3+ and Bi3+ cations. Unlike type I, the Cd-containing chalcohalide slab in the type II structure consists of two types of coordination, CdS6 and CdS2Br4, possibly because of the increased difference in the anion sizes, that is, S2-/Cl- ) 1.70/1.67 Å in 3 versus S2-/Br- ) 1.70/2.06 Å in 4. As shown in Figure 1b, the two kinds of octahedra each share the trans S or Br atoms, respectively, to form pseudo-onedimensional chains parallel to [010]. The alternating CdS6 and CdS2Br4 chains are interlinked by sharing the vertex atoms S(1) to extend the corrugated slab along the bc plane. The structural formula concerning the unique chains can thus be written as {[CdS(1)2/2S(2)4/2][CdS(1)2/2Br4/2]}∞ (≡ [Cd2S4Br2]6-∞). The second distinct feature of the type II structure is the connectivity between the Cd-containing chalcohalide slabs through the M3+ cations. The Bi3+ cations of 4 have stronger covalent interactions with S2- than with Br-, and the extended structure shown in Figure 1b can be viewed as the [CdS2/2Br4/2] (≡ CdSBr2) slabs along the (002) plane alternately packed with the all-sulfur slabs made up of the [CdBi1/1Bi2/2S2/2S4/2] (≡ CdBi2S3) units along (001). The local environment of the Bi-centered btp (Figure 2b) in 4 is distorted. As found for 3 (type I structure), the BiS(2) distance, 2.573(3) Å, between the btp’s is considerably shorter than the intra-btp Bi-S distances, for example, 2.708(2) Å and 3.041(2) Å. This once again is due to the stereochemically active lone-pair electrons of the trivalent Bi3+ cation. As shown by the projected view in Figure 3, both the type I and the type II structures adopt structural units commonly seen in post-transition metal chalcogenides, for example, Sb2Se3. In forming the extended network structure, the CdQ6-xXx and MQ5X3 units share solely the chalcogenide anions. The type II structure is isostructural with FeUS3, where the corresponding polyhedral units, the FeS6 octahedron, and US8 btp, are less distorted than CdS6/CdS2Br4 and BiS5Br3 observed in 4 as a result of the missing of lone-pair electrons (Figure 1c).

Wang et al.

Figure 3. Projected views of (a) CdBiS2Cl and (b) CdBiS2Br; each represents structure types I and II, respectively. Circled units are commonly seen in the post-transition metal chalcogenides; see text.

It is worth mentioning that 1 and 3 form a solid solution series CdSb1-xBixS2Cl, which, as expected, adopts the type I structure. Diffuse reflectance UV-vis spectra of the solid solution series showing the blue shift of the band edge as the concentration of the Bi3+ cation (x) increases (see Supporting Information, Figure S3). The attempt to synthesize a mixed-halide solid solution series CdBiS2Cl1-xBrx failed; instead, mixed phases of CdBiS2Cl and CdBiS2Br were found. Optical Properties. The reflectance spectra of the title series and CdQ are compared in Figure 4. The reported band gaps Eg of CdS and CdSe are 2.41-2.50 and 1.70-1.75 eV, respectively.23 These values are significantly greater than the band gaps of Sb2S3 and Sb2Se3 (1.95-2.20 and 1.11 eV, respectively)24 and those of Bi2S3 and Bi2Se3 (1.30 and 0.30 eV, respectively).25 Figure 4 shows that the absorption edge of CdMQ2X has a noticeable red shift with respect to that of CdQ (Q ) S, Se). The optical band gaps of CdMQ2X become smaller when Cl- is replaced with Br- and also when Br- is replaced with I-, regardless of the structure types. This suggests that the orbitals of the halide ions contribute to the top of the valence bands (VBs). This view is consistent (23) (a) Bernard, J. E.; Zunger, A. Phys. ReV. B 1987, 36, 3199-3228. (b) Yeh, C. Y.; Lu, Z. W.; Froyen, S.; Zunger, A. Phys. ReV. B 1992, 46, 10086-10097. (c) Zhang, S. B.; Wei, S. H.; Zunger, A. Phys. ReV. B 1995, 52, 13975-13982. (24) (a) Fujita T.; Kurita K.; Tokiyama K.; Oda, T. J. Phys. Soc. Jpn. 1987, 56, 3734-3739. (b) Wood, C.; Shaffer, J. C.; Proctor, W. G. Phys. ReV. Lett. 1972, 29, 485-487. (25) Black, J.; Conwell, E. M.; Seigle, L.; Spencer, C. W. J. Phys. Chem. Solids 1957, 2, 240-251.

New Mixed-Framework Chalcohalide Semiconductors

Figure 4. UV-vis diffuse reflectance spectra showing a red shift in (a) CdSbS2X (X ) Cl, Br), (b) CdBiS2X (X ) Cl Br), and (c) CdBiSe2X (X ) Br, I). For comparison, the spectra of the corresponding CdQ is included in parts a-c. Substituting Bi3+ for Sb3+ also exhibits an expected shift in part d; see text.

Figure 5. Plots of the total and partial density of states calculated for CdSbS2Cl. The dashed line indicates the position of the Fermi level.

with the reported electronic band structures of the sulfobromides LnSbS2Br2 (Ln ) La, Ce) in that the VB is primarily composed of the s and p orbitals of the Br- and S2- anions.9d Electronic Band Structure. The total and partial density of states calculated for CdSbS2Cl are presented in Figure 5, while the corresponding plots calculated for CdS and Sb2S3 are shown in Supporting Information Figures 4S and 5S, respectively. In agreement with the experiment, both CdS and Sb2S3 are calculated to be semiconductors. In CdS the

Chem. Mater., Vol. 18, No. 5, 2006 1225

VBs are given by the Cd-S bonding levels, and the conduction bands (CBs) are given by the Cd-S antibonding levels (Supporting Information, Figure S4). Sb2S3 consists of two kinds of Sb atoms in Sb2S3, that is, three-coordinate Sb(1) and five-coordinate Sb(2). The local coordination environment of the Sb in CdSbS2Cl is close to that of the five-coordinate Sb(2) of Sb2S3. The VBs of Sb2S3 are given by the lone-pair levels of the Sb3+ ions, which make antibonding interactions with the orbitals of the surrounding S2- ions, and the CBs are given by the Sb-S antibonding levels (Supporting Information, Figure S5). Thus, the band gap is smaller for Sb2S3 than for CdS. It should be noted that the CBs of CdS lie lower in energy than those of Sb2S3. In CdSbS2Cl, the highest-lying VBs are largely given by the lone-pair levels of the Sb3+ ions, which make antibonding interactions with the orbitals of the surrounding five S2- and three Cl- ions and the lowest-lying CBs by the Cd-S antibonding levels (Figure 5). Consequently, the band gap of CdSbS2Cl is smaller than that of CdS but comparable with those of Sb2S3. Conclusions A new family of layered chalcohalides was synthesized by conventional solid-state reactions at an intermediate temperature range. These compounds adopt one of the two related structure types, that is, the type I (orthorhombic) in which the chalcogenide and halide anions belong to the same period and the type II (monoclinic) in which the halide anions lie one period below the chalcogenide anions. This new series of chalcohalide compounds represents the first systematic study, as far as we know, among mixed-anion compounds reported thus far showing the role of anions. The UV-vis spectra indicate that these solids are midrange semiconductors with a band gap of 1.3-2.2 eV. The UVvis diffuse reflectance spectra of CdMQ2X exhibit a red shift with respect to those of CdQ. This is due to the electronic transition from the VB made up of p orbitals of the largely distorted MQ5 square pyramids to the bottom of the CB made up of the Cd-Q antibonding levels. By means of the chemical substitution of Sb3+ for Bi3+, the band gap can be fine-tuned. Meanwhile, the preliminary optical property studies revealed that these materials are transparent in a broad IR range, which offers potential applications as window materials for laser delivery media. Further studies on optical properties such as the reflective index and laser delivery threshold would be worth investigating. Acknowledgment. Financial support for this research (DMR0077321, 0322905) and the purchase of a single-crystal X-ray diffractometer (CHE-9808165) from the National Science Foundation is gratefully acknowledged. M.-H.W. thanks the financial support from the Office of Basic Energy Sciences, Division of Materials Sciences, U.S. Department of Energy, under Grant DE-FG02-86ER45259. The authors are grateful to Dr. D. VanDeveer for his help on structure analysis. Supporting Information Available: Parameters for EHTB calculations; IR, XRD, and UV-vis spectra; plots of the total and partial density of states calculated for CdS and Sb2S3 (PDF); and X-ray crystallographic file, in CIF format, are available free of charge via the Internet at http://pubs.acs.org. CM0522230