| The major task of this work was synthesis and characterization of solid-state inorganic materials for thermoelectric applications. Thermoelectric (TE) materials can convert thermal energy into electricity and can be also used for heat pumping or refrigeration. Promising materials are small band gap semiconductors composed of heavy elements, where high electrical conductivity as well as low thermal conductivity enhances the efficiency.; Since main group antimonides belong to the best current thermoelectrics, early transition metal antimonides were studied. Furthermore, binary lead, bismuth and antimony chalcogenides have been commercially used as TE materials for a long time. Accordingly I also focused on the ternary and quaternary variants of these systems.; The early transition metal antimonides are composed of heavy elements and exhibit complex crystal structures of high symmetry as well as high thermal stability. Nevertheless, all the hitherto known compounds in this class exhibit metallic behavior, which inhibits their application as TE materials. We performed extensive electronic structure calculations on the binary antimonides and found out that NbSb2, which crystallizes in the OsGe2 structure type, exhibits a low Density Of States at the Fermi level. Subsequently we decided to prepare compounds with the same structure type and the same valence-electron concentration, where the M--M interactions are polarized and the Density Of States at Fermi level is zero. We successfully synthesized HfMoSb4, which crystallizes in the OsGe2 structure type, and our electronic structure calculation on the ordered model predicted nonmetallic behavior for this compound. The electrical conductivity measurements were in agreement with the theoretical predictions. We also synthesized (Ti,Mo)Sb2 and (Ti,Mo)As2 that crystallize in the same structure type. However the Mo:Ti ratio was not 1:1, a condition required to keep the same valence-electron concentration as in NbSb2.; With respect to new ternary and quaternary chalcogenides, we synthesized the new ternary selenide Pb4Sb6Se13, which crystallizes in the monoclinic space group I2/m, as well as Pb6Sb6Se17, crystallizing in the orthorhombic space group P21212. The electronic structure calculations predicted semiconducting behavior for both compounds. The electrical conductivity measurement as a function of temperature for Pb4Sb 6Se13 was in agreement with our calculation. We also synthesized the known quaternary sulfide FePb4Sb6S14, for which our electronic structure calculation and the physical property measurements revealed semiconducting behavior. We consequently doped this material with Co and Sn to enhance the thermoelectric properties of this material. (Abstract shortened by UMI.)... |
