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This book explores synthesis, structural changes, properties, and potential applications of transition metal (TM) compounds. Over three sections, chapters cover such topics as the synthesis of pentoxide vanadium (V2O5), the effect of TM compounds on structural, dielectric properties and high-temperature superconductors, and TM-doped nanocrystals (NCs).
The crystal structures of the new intermetallic compounds Eu2ZnPn2 (Pn = Sb, Bi) and Sr2ZnPn2 (Pn = Sb, Bi) are reported. They have been synthesized from their corresponding elements through high-temperature reactions using the flux-growth method. The structures for Eu2ZnPn2 (Pn = Sb, Bi) and Sr2ZnPn2 (Pn = Sb, Bi) have been established by single-crystal X-ray diffraction. In those cases, the X-ray patterns can be successfully indexed based on a hexagonal cell with unit cell parameters in the range a=4.6-4.7 Å and c=8.2-8.5 Å. Structure solutions in the space group P63/mmc suggest the defect ZrBeSi type (Pearson's symbol hP6; 3 unique positions) as the likely model; however, subsequent refinements indicate nearly 50% occupancy on the Zn site. Based off the evidence that I will present in the following paper, I believe it is plausible that the crystal structures of the reported compounds have a long-range order of zinc-vacancies. These systematic vacancies further suggest the compounds may have thermoelectric properties. Evidence for such was sought using powder X-ray diffraction, single crystal X-ray diffraction, electron diffraction, X-ray dispersive spectroscopy, and magnetic susceptibility measurements.
A high efficiency thermoelectric material requires being a "phonon glass -electron crystal". Zintl phase compounds can be engineered to combine a "phonon glass" with an "electron crystal" by selectively doping the system to optimize the electronic properties. Current research on thermoelectrics is concentrated on (I) chemical or physical changes to improve the existing materials such as doping and reducing particle size to nano-size and (II) new materials with superior thermoelectric properties. Chapters 2 and 3 have been focused on tuning the transport properties of existing materials to improve the thermoelectric performance. In chapters 4 and 5, new Zintl phases have been synthesized and investigated with respect to their thermoelectric properties. In chapter 2, silicon nanoparticles embedded Mg2Si/xSi nanocomposites have been synthesized at 623 K from MgH2 and Bi containing Si nanoparticle powders. This synthetic route avoids the production of oxides and lowers the formation temperature of Mg2Si. It also provides a route to homogeneously mixed Si nanoparitcles within a Mg2Si matrix. Powder X-ray diffraction (XRD), thermogravimetry/differential scanning calorimetry (TG/DSC), electron microprobe analysis (EMPA), and scanning transmission electron microscopy (STEM) are applied to characterize the phase and micro-structure. Thermoelectric properties measurements indicate that the thermal conductivity is reduced by a small amount of Si nano-inclusions, which is in agreement of our theoretical calculations. A dimensionless figure of merit zT ~ 0.7 is obtained at 775 K for 1% Bi doped Mg2Si/x Si with x = 0 and 2.5 mol.%. In chapter 3, magnetic and transport properties of a series of Te doped Yb14MnSb11 samples prepared by Sn flux method have been studied. Increasing amounts of Te increases the saturation moment of Yb14MnSb11−(x)Te(x) and magnetoresistence effect. Both Seebeck coefficient and electrical resistivity increase with increasing amount of Te as a result of decreasing carrier concentration. Approximately 12% improvement of zT has achieved for x = 0.07 at 1240 K. Thermoelectric properties of the compounds Yb11MSb9 (M = Ga, In) via self-flux synthesis, closely structure related to Yb14MnSb11, have been investigated in chapter 4. Particularly low lattice thermal conductivity values, less than 0.6 W/m*K, are obtained for both compounds. The low lattice thermal conductivity suggests that Yb11MSb9 (M = Ga, In) has the potential for high thermoelectric efficiency at high temperature if charge carrier doping can be optimized. A two-step solid-state method is developed to fabricate two rare-earth containing ternary phosphides, Eu3Ga2P4 and Eu3In2P4, and their thermoelectric properties are investigated in chapter 5. The powder XRD and TG-DSC are employed to characterize the phase purity and thermal stability. Electronic structures of both compounds are calculated and provide band-gaps of 0.60 and 0.29 eV for Eu3Ga2P4 and Eu3In2P4, respectively. Transport properties measurements suggest that these Zintl phosphides have the potential to be good high temperature thermoelectric materials with optimization of the charge carrier concentration by appropriate extrinsic dopants.