Thermoelectric properties of Tl 9 BiTe 6 / Tl 9 BiSe 6 solid solutions
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Thermoelectric properties of Tl9BiTe6 / Tl9BiSe6 solid solutions Bernd Wölfing1, Christian Kloc1 and Ernst Bucher1,2 1 Department of Materials Physics Research, Bell Laboratories, Lucent Technologies, Murray Hill, NJ 07974, U.S.A. 2 Lehrstuhl für angewandte Festkörperphysik, Universität Konstanz, Konstanz, Germany
ABSTRACT The compounds Tl9BiTe6 (TBT) and Tl9BiSe6 (TBS) crystallize in the tetragonal space group I4/mcm. Tl9BiTe6 has a thermopower of 185 µV/K and an electrical resistivity of 5.5 mΩcm at 300K, resulting in a power factor of S2/ρ = 0.6 mW/mK2. Compared to Bi2Te3 which is the state of the art material at this temperature this is about a factor of 7 lower. At 300 K TBS has a thermopower of 750 µV/K but a high resistivity of 130 Ωcm. To optimize the thermoelectric properties of TBT solid solutions have been formed with TBS. The resistivities and have been measured on Tl9BiTe1-xSex with x = 0.05, 0.08, 0.2 and 0.5. In addition to the electrical properties the lattice constants have been measured by X-ray diffraction. The dependence of the lattice constants on the Te/Se ratio clearly deviates from Vegard’s law. Different affinities of Te and Se towards the two chalcogenide sites in the crystal can explain this behavior.
INTRODUCTION TBT and TBS belong to a larger family of ternary compounds. This family contains all combinations of Tl9MVQ6 (MV = Sb, Bi; Q = Se, Te) and Tl8MIV2Q6 (MIV = Sn, Pb; Q = Se, Te) except Tl8Sn2Se6. The 7 compounds in this family are completely miscible. This group of compounds is derived from the binary Tl5Te3. Assuming that Tl5Te3 is a semi-metal, the Tl atoms exhibit two different valence states: TlI and TlIII [1]. The expanded formula can thus be written as TlI8(TlI, TlIII)Te6 where the atoms in brackets are on the cystallographic 4c site. TlIII can be replaced by trivalent metals Bi and Sb resulting in compounds with a disordered 4c site. Alternatively both atoms on this site can be substituted by Pb or Sn. In all but the Sn-compound Te can be substituted by Se. The existence of these compounds suggest that this simple valence model is accurate to a certain extent. The quality of the electrical properties of a thermoelectric material is described by the power factor S2/ρ where S is the thermopower and ρ the electrical resistivity. In comibination with the thermal conductivity κ the figure of merit Z = S2/ρκ is obtained which describes the overall quality of a thermoelectric material. Of all compounds in this family TBT has the highest power factor while TBS exhibits the highest thermopower. By forming solid solutions between these two compounds we aimed at increasing the thermopower without sacrificing too much electrical to obtain a solid solution with a power factor exceeding that of pure TBT.
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EXPERIMENTAL TBT and TBS are reported to melt congruently at temperatures of 540oC and 519oC [2,3]. Stoichiometric amounts of the elements were sealed in quartz tubes and heated to 600oC for several hours. Then the melt was furnace-cooled to room temperature. X-ray powder diff
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