Electronic Structure and Thermoelectric Properties of A x Mo 3 Sb 5 Te 2
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Electronic Structure and Thermoelectric Properties of AxMo3Sb5Te2 Navid Soheilnia and Holger Kleinke* Department of Chemistry, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1 E-mail: [email protected] ABSTRACT Mo3Sb7 may be chemically modified to become semiconducting by replacing two Sb atoms with two Te atoms (per formula unit). This material may be an attractive candidate for the thermoelectric energy conversion, as its thermal conductivity may be lowered by creating the rattling effect upon intercalation of small cations, and its band structure may be tailored, i.e. the band gap size modified. The higher the Te content and the higher the cation amount, the smaller is the band gap, which can virtually reach any value below 0.5 eV. INTRODUCTION While investigating transition metal antimonides for their nonclassical bonding in linear Sb atom chains [1-4], Mo3Sb7 [5] caught our attention as a potential thermoelectric material. Thermoelectrics may convert electricity into a temperature gradient (Peltier effect) or vice versa (Seebeck effect). The materials commercially used are usually narrow band gap semiconductors comprising heavy elements, such as Bi2Te3 and PbTe [6]. Currently under serious investigations are also antimonides, namely the filled skutterudites LnδM4Sb12 [7-15]. Therein, Ln is a lanthanoid atom, and M a late transition element such as Fe, Co, and Ni. While the parent compound, LaFe4Sb12, is metallic like Mo3Sb7, and thus unsuitable for the thermoelectric energy conversion, LaFe3CoSb12 exhibits outstanding thermoelectric properties, for its high thermopower and electrical conductivity are combined with an extraordinarily low thermal conductivity. The filled skutterudites come close to being a phonon-glass, electron-crystal material [16, 17], with the rattling effect arising from the Ln atom vibrations in a large "cage" of Sb atoms. Research into other new materials [18] concentrate on (among others) tin- and antimony-based half-Heusler compounds [19-23], germanium and tin-based clathrates [24-27], and naturally bismuth chalcogenides [28-32]. To render Mo3Sb7 useful, its metallic properties have to be changed into semiconducting ones by increasing its valence-electron concentration from 53 to 55 valence-electrons per formula unit [33]. This was achieved by a formal Sb/Te substitution, i.e. Mo3Sb5Te2 is indeed a semiconductor [34]. This contribution deals with the intercalation of cations A into Mo3Sb5Te2, and the consequences thereof for the electronic and physical properties. EXPERIMENTAL SECTION AxMo3Sb5Te2 and AxMo3Sb7 may be prepared in either a one-step or two-step synthesis. We obtained the best, i.e. most homogeneous samples by synthesizing Mo3Sb7 and Mo3Sb5Te2 first at 700 °C in fused silica tubes, starting from the elements in the stoichiometric ratios in powder form. The second step then is to add the cation A, again in powder form, and anneal that mixture at 600 °C in fused silica tubes. While this requires more actual work than the direct synthesis starting from all fo
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