Abstracts Earth and Environmental Sciences

SYNTHESIS AND INVESTIGATING THERMOELECTRIC CHARACTERISTICS OF THE RECuQ2 (RE= Pr, Sm, Gd, Dy, Er AND Q= Se, Te)

by Mehdi Esmaeili

Results of this research are available online in two published papers. The main focus of this research was to synthesize and then to characterize the potential high-performance thermoelectric materials. In this regard, we have prepared a series of pure RECuSe2 (with RE = Pr, Sm, Gd, Dy and Er) and RECuTe2 (with RE = Er, Dy and Gd) and analyzed their crystal structure, electronic and physical properties. We used powder and single crystal X-ray diffraction techniques to analyze their crystal structures and employed energy dispersive X-ray spectrometry (EDS) to verify their chemical compositions. The temperature stability of the synthesized samples was examined by differential thermal and gravimetrical analysis. The high-purity consolidated pellets were prepared for physical properties measurements. We analyzed the relationship between their crystal structures and pertinent electronic properties through the LMTO calculations. The RECuSe2 phases adopt two structures, monoclinic and trigonal. The monoclinic structure (P21/c, z = 4) is observed for lighter rare earths (RE = Pr, Sm and Gd) and Cu-disordered trigonal structure for heavier rare earths (P m1, z = 1, RE = Dy and Er). The resistivity and Seebeck coefficient measurements indicate that the studied selenides are p-type semiconductors with relatively small activation energies (0.045-0.12 eV). However, their electrical resistivities are too high (0.49-220 Ohmcm at room temperature) to make them competitive thermoelectric materials. Electronic structure calculations indicate presence of a band gap in the RECuSe2 phases. The synthesized RECuTe2 phases (RE = Er, Dy and Gd) adopt a monoclinic-distorted variant (C2/m, z = 2) of the trigonal structure (P m1, Z= 1) observed for the RECuSe2 (with RE = Dy, Er). While such disorder may be beneficial for lowering their thermal conductivity, large values of electrical resistivity (0.02-0.87 Ohmcm at room temperature) make these phases unsuitable for practical applications. Comparing to the corresponding semiconducting selenides, the tellurides have lower resistivities, and display a metallic type resistivity. Such behavior stems from the closure of band gaps, which is verified by the electronic structures calculations. Structurally the RECuTe2 phases (with RE = Er, Dy and Gd) are similar to RECuSe2 with the P m1 structure. The monoclinic distortion in RECuTe2 is driven by Cu displacement inside the larger tetrahedral voids in the hexagonal close packing of the Te atoms. Most likely, Cu shifts to one side of the Te tetrahedra to optimize the Cu-Te interactions. Thesis Candidate in Philosophy