Thermoelectric Properties of the Semiconducting Antimonide-Telluride Mo 3 Sb 5-x Te 2+x
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Thermoelectric Properties of the Semiconducting Antimonide-Telluride Mo3Sb5-xTe2+x Enkhtsetseg Dashjav and Holger Kleinke* Department of Chemistry, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1 E-mail: [email protected] ABSTRACT Typically, useful thermoelectrics are small-gap semiconductors. Mo3Sb7 would be an interesting candidate, if it were not metallic. Electronic structure calculations reveal that metallic Mo3Sb7 can be made semiconducting by heavy doping, e.g., by replacing Sb in part with Te. We succeeded in the preparation of semiconducting Mo3Sb5-xTe2+x with enhanced thermoelectric properties. Furthermore, we incorporated small M atoms into the cubic Sb/Te cage in an attempt to create the rattling effect as found in the filled skutterudites that have attracted wide interest for their outstanding thermoelectric properties. INTRODUCTION Thermoelectric materials can convert heat into electricity and vice versa. This fascinating energy conversion is commercially in use, but due to its low efficiency restricted to niche technologies, such as small-scale refrigeration or power generations in remote locations (e.g. in spacecrafts, subsea, the Rockies, …). Thermoelectric devices are usually comprised of semiconductors [1]. Their performance is indicated by the figure-of-merit ZT, which is defined as ZT = TS2σ/κ. Here, T is the temperature, S the Seebeck coefficient (thermopower), and σ and κ are the electrical and the thermal conductivities, respectively. The commercially used materials such as Bi2Te3 may exhibit ZT values around 1 at the ideal operating temperature; the higher ZT, the better the thermoelectric performance [2]. In the past years, the filled skutterudites [3] - among other materials such as β-Zn4Sb3 [4] and CsBi4Te6 [5] - have attracted wide interest because of their outstanding thermoelectric properties, which were described in 1996 [6]. Many investigations into this structure family followed subsequently [7-9]. The general formula is LnxM4Sb12 with x ≤ 1, where Ln is a lanthanoid and M a valence-electron rich transition element such as Fe, Co, Ni, … While the parent compound, LaFe4Sb12, is metallic, LaFe3CoSb12 exhibits excellent thermoelectric properties based on its experimentally determined figure-of-merit ZT, which may become as high as 1.4 at 730 °C, for its good thermopower and electrical conductivity are combined with an extraordinarily low (thus perfect) thermal conductivity. The latter stems from the high vibrations of the La atom situated in a large "cage" of Sb atoms, a phenomenon usually referred to as rattling. In the past, we investigated valence-electron poor transition metal antimonides [10,11], one direction being the thermoelectric energy conversion [12,13]. In some sense, the antimonide Mo3Sb7 [14] is a material similar to the skutterudites: its structure contains voids that might be filled with (small) cations, and its band structure comprises a band gap of the right magnitude. Its cubic symmetry is also an advantage, for the thermopower depends on the ef
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