Molten gallium flux synthesis of known thermoelectric and novel magnetic inorganic clathrate compounds: Improving thermoelectric performance

Molten gallium metal has been used as a solvent to grow large single crystals of known inorganic thermoelectric clathrates Sr₈Ga ₁₆Ge₃₀, Ba₈Ga₁₆Ge₃₀, and Ba₈Ga₁₆Si₃₀. X-ray diffraction, thermal analysis, electron microprobe, Glow Discharge Mass Spectrometry, temperature dependent electrical conductivity and Seebeck coefficient measurements characterized the single crystals. The Thermoelectric performance was shown to be heavily dependent on the synthetic conditions including container choice, thermal history and impurity concentration.; Inorganic Clathrates have attracted intense interest in last several years as potential new materials for thermoelectric devices. If a small to moderate increase in thermoelectric performance over the currently used materials is realized, substantial environmental and technological gains could be achieved. Since thermoelectric refrigeration modules require no moving parts or heat exchange gas (freon) they offer significant advantages over conventional refrigeration technology that tends to fail due to the finite lifetime of the pumping equipment. High temperature devices are also extremely useful for power generation in harsh unforgiving environments where excess heat is available.; The thermoelectric performance, primarily at room temperature, of these compounds was found to be heavily dependent on the synthetic procedures used to obtain them. A flux growth procedure was developed to overcome the problems of the traditional melt-quench-anneal solid-state chemical approach. This procedure yielded large single crystals of the Sr₈Ga₁₆Ge ₃₀, Ba₈Ga₁₆Ge₃₀ and Ba₈Ga ₁₆Si₃₀ compounds which ready facilitated their chemical and electronic study. Finally, an outlook on the application of these compounds as thermoelectric devices is given.; Application of the flux method to other systems was also successful in the discovery of two new inorganic clathrate compounds: type IV Eu₄Ga ₈Ge₁₆ and type V Yb₈Ga₁₆Ge₁₄. The Eu₄Ga₈Ge₁₆ compound was found to be antiferromagnetic with a T_c of 8 K. The compound was investigated by orientated single crystal magnetic susceptibility, heat capacity, Mössbauer spectroscopy and polarized magnetic neutron diffraction. The compound exhibited a nearest-neighbor 1-dimensional ferromagnetic interaction in an overall 3-dimensional ordered antiferromagnetic state. Spin-flop experiments were used to extract the antiferromagnetic exchange coupling constant. The adherence to the Zintl concept was investigated by Mössbauer spectroscopy.