Article pubs.acs.org/cm
Sb and Se Substitution in CsBi4Te6: The Semiconductors CsM4Q6 (M = Bi, Sb; Q = Te, Se), Cs2Bi10Q15, and CsBi5Q8 Duck Young Chung,† Ctirad Uher,‡ and Mercouri G. Kanatzidis*,†,§ †
Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, United States Department of Physics, University of Michigan, Ann Arbor, Michigan 48109, United States § Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States ‡
S Supporting Information *
ABSTRACT: The solid solutions of CsBi4Te6, a high ZT material at a low temperature region, with Sb and Se were synthesized with general formulas CsBi 4‑x Sb x Te 6 and CsBi4Te6‑ySey. The introduction of Sb and Se in the lattice of CsBi4Te6 is possible but only to a limited extent. The Sb and Se atoms substituted are not uniformly distributed over all crystallographic sites but display particular site preferences. The structure of new Sb/Bi solid solutions retains the original framework of CsBi4Te6 composed of NaCl-type Bi/Te slabs interconnected by characteristic Bi−Bi bonds and Cs atoms located in the interlayer space. A structurally modified phase in Se/Te solid solutions was found from the reactions targeted for 0.2 < y < 2.4 with the formula of CsBi5Te7.5‑ySey (or Cs2Bi10Q15, (Q = Se, Te)). The new structure is constructed by the same structural motif with an extended Bi/Te slab (29 Å) compared to that in CsBi4Te6 (23 Å). The CsBi5Te7.5‑ySey possesses Bi/Te slabs that extend by an additional “Bi2Te3” unit compared to the structure of CsBi4Te6, which implies the existence of a phase homology of compounds with the adjustable parameter being the width of the Bi/Q slab. In the reactions targeted for the compounds with higher y, a new phase CsBi5Te3.6Se4.4 with a different type of framework was found. The electrical conductivity and thermopower for the selected samples show p-type conduction with metallic behavior. The room temperature values measured are in the range of 300−1100 S/cm and 100−150 μV/K for Sbsubstituted samples and 20−500 S/cm and 70−140 μV/K for Se-substituted samples, respectively. Thermal conductivities of these samples are in the range of 0.9−1.2 W/m·K at room temperature. Tailoring the transport behavior of these materials for thermoelectric applications may be achieved by doping, as is possible for the parent compound CsBi4Te6. KEYWORDS: homologous series, chalcogenide, narrow bandgap semiconductors, crystal growth, electronic materials
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INTRODUCTION Complex bismuth chalcogenides tend to adopt an impressive array of low-dimensional structures, many of which can be organized under homologous series such as Am[M1+lSe2+l]2m[M2l+nSe2+3l+n] (A = K, Rb, Cs, Sr, Ba; M = Bi, Sb, Pb, Sn),1 A2[M5+nSe9+n] (A = Rb, Cs; M = Bi, Ag, Cd),2 and CsPbmBi3Te5+m.3 The structures of sulfide and selenide compounds within a homologous series evolve by varying the size and shape of two primary building blocks, NaCl- and Bi2Te3-type units, to form 3-dimensional frameworks, e.g. KBi6.33S10,4 α-, β-, γ-K2Bi8Q13 (Q = Se,5,6 S4), K2.5Bi8.5Se14,6 and KM4Bi7Se15 (M = Sn, Pb).7 The telluride compounds, however, are known to be built based on only a single structural motif, a slice of NaCl-type block. The tellurides form layered structures that sandwich alkali metals. Examples include RbBi3.67Te6,8 CsMBi3Te6, CsM2Bi3Te7,9 CsM3Bi3Te8, and CsM4Bi3Te9 (M = Pb, Sn),3 and CsBi4Te6.10,11 Among these tellurides, CsBi4Te6, a promising thermoelectric material, has a salient structural feature by direct Bi−Bi © 2012 American Chemical Society
bonding that interconnects Bi/Te slabs to form infinite layers. The Bi−Bi bond formed by reduced Bi2+ is a rare observation in bismuth chemistry. This feature in CsBi4Te6 was found to be important in allowing the compound to achieve a narrower band gap (80 meV) than that of Bi2Te3 itself (150 meV).12 In this work we examine solid solutions of CsBi4Te6 with Sb and Se substitution on the Bi and Te sites, respectively. Because the fully substituted end members CsSb4Te6 and CsBi4Se6 are not stable, we find that solid solutions over the entire range are not possible. Here we establish the respective limits of x and y of CsBi4‑xSbxTe6 and CsBi4Te6‑ySey stabilized in the CsBi4Te6 structure and probe whether the substitution is random or preferential among the lattice sites. Our work shows that substitutions in the CsBi4Te6 structure are possible only to a limited extent. In the case of Sb, the x Received: February 13, 2012 Revised: April 9, 2012 Published: April 23, 2012 1854
dx.doi.org/10.1021/cm300490v | Chem. Mater. 2012, 24, 1854−1863
Chemistry of Materials
Article
Table 1. Comparison of EDS/SEM Data for the Crystals of CsBi4‑xSbxTe6 and CsBi4Te6‑ySey Solid Solutions with the Nominal Composition Applied in Synthesis and Approximate Yield of Needle Phase starting material Bi2‑aTe3, Bi2Te3‑b a a a a a a a b b b b b b b b b b b
= = = = = = = = = = = = = = = = = =
0.1 0.2 0.3 0.4 0.6 0.8 1.0 0.1 0.2 0.3 0.4 0.45 0.5 0.6 0.6 0.8 0.9 1.5
nominal composition
analyzed composition of needle product
structure typea
approximate yield (%)
CsBi3.8Sb0.2Te6 CsBi3.6Sb0.4Te6 CsBi3.4Sb0.6Te6 CsBi3.2Sb0.8Te6 CsBi2.8Sb1.2Te6 CsBi2.4Sb1.6 Te6 CsBi2.0Sb2.0Te6 CsBi4Te5.8Se0.2 CsBi4Te5.6Se0.4 CsBi4Te5.4Se0.6 CsBi4Te5.2Se0.8 CsBi4Te5.1Se0.9 CsBi4Te5.0Se1.0 Cs0.8Bi4Te4.8Se1.2 CsBi4Te4.8Se1.2 Cs0.8Bi4Te5.2Se0.8 CsBi4Te4.2Se1.8 CsBi4Te3.0Se3.0
Cs0.96Bi3.47 Sb0.23Te6 Cs0.84Bi3.45 Sb0.35Te6 Cs1.20Bi3.18 Sb0.63Te6 Cs1.08Bi3.44 Sb0.70Te6 Cs1.06Bi2.72 Sb1.03Te6 Cs1.21Bi2.59 Sb1.31Te6 Cs0.93 Bi2.32Sb2.02 Te6 Cs1.05Bi4Te6.19Se0.18 Cs1.01Bi4Te5.99Se0.33 Cs0.99Bi4Te5.76Se0.44 Cs1.12Bi4Te5.58Se0.64 Cs0.96Bi4Te5.06Se0.94 Cs1.07Bi4Te4.94Se1.06 Cs0.64Bi4 Te4.74Se1.26 Cs1.17Bi4Te5.04Se1.16 Cs1.12Bi4Te5.58Se0.64 Cs0.97Bi4Te4.32Se2.02 Cs1.15Bi5Te4.25Se4.33
original original original original original original original original original original original original original extendedb original extended original newc
100 100 100 100 40 20 10 100 100 100 100 100 90 90 70 70 70 70
a
The CsBi4Te6 structure is referred to here as original. bThe structure has an extended Bi/Te building block (29 Å wide). cThe new structural compound, Cs0.75Bi3.75Te2.70Se3.30.
limit for solid solution is ∼2.0. In the case of Se, the y limit is ∼3.0. In the process of Se substitutions we discovered a new compound, CsBi5Te7.5‑ySey (or Cs0.8Bi4Te6‑ySey), which has similar bonding features to CsBi4Te6 but more extended building blocks. Further substitution of Se led to another new compound, the 3-dimensional CsBi5Te3.6Se4.4 (Cs0.75Bi3.75Te2.70Se3.30). In addition to the structures and fundamental physicochemical characterization, we report charge transport and thermal transport properties measured over a wide temperature range on ingot samples of CsBi4‑xSbxTe6 and CsBi4Te6‑ySey.
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sealed under vacuum while keeping the bottom of the tube in liquid N2. The mixture was then melted by a weak torch flame for about 1 min, followed by quenching it in air. Method B. To avoid the exothermic spark caused by a direct contact of Cs metal with Bi2Te3‑bSeb, an H-shaped quartz tube (13 mm O.D. × 11 mm I.D.) with two bottom ends closed was used. For the synthesis of CsBi4Te5.2Se0.8, for instance, molten Cs metal (0.694 g, 5.222 mmol) was placed in one bottom end and Bi2Te2.6Se0.4 (8.0 g, 10.239 mmol) was separately placed in the other end. The two open ends of the H-shaped tube were sealed under vacuum ( 1.0), Method A which involves quenching provided higher yields of the target solid solutions than Method B which applies slow cooling. The latter method, however, gave better crystal quality more suitable for the crystallographic studies and property measurements. In both methods, a slight excess (0.05 mol eq.) of Cs was used to suppress the formation of Bi2Te3type impurity crystals and to compensate for any loss of Cs possibly occurring during the preparation reactions. In the range of a ≤ 0.4 with Bi2‑aSbaTe3, crystals of CsBi4‑xSbxTe6 were synthesized by both Method A and B. When a > 0.4, unreacted or recrystallized Bi2‑aSbaTe3 phase began to emerge in the products, and its fraction increased as more Sb was involved. In the range of a > 1.2, only plate shaped crystals of Bi2‑aSbaTe3 were observed. For the Se solid solutions, CsBi4Te6‑ySey was obtained when b in the Bi2Te3‑bSeb starting material was
