X-ray Absorption Fine Structure Simulation Of Skutterudite Thermoelectric Compounds

Skutterudite is a kind of materials with excellent thermoelectric properties, butthe high lattice thermal conductivity limits its application and development. Thelattice thermal conductivity depends on the energy band structure and crystalstructure of the material. Theoretical calculations indicate that the thermoelectrictransport properties of n-type filled skutterudites depends on Sb5p and Co3delectronic band structure, which are located at the bottom of conduction band. In thisthesis, the electronic structure of CoSb3was investigated by X-ray absorption nearedge structure (XANES) spectrum through multiple-scattering theory. The atomicstructure information of Ba and In filled skutterudites was investigated by extendedX-ray absorption fine structure (EXAFS) spectrum via single-scattering theory. Themain results obtained are as follows:(1) The Co and Sb K-edge XANES spectrum of CoSb3were calculated by fullpotential multiple-scattering theory of real space. The theoretical parameters wereoptimized to make agreements between theoretical and experimental XANESspectra. The effects of cluster sizes on Co and Sb K-edge XANES spectra werestudied. The results indicated that optimal cluster sizes were5.74and5.60,respectively. The effect of core-hole lifetimes on Co K-edge XANES spectra werealso studied. The results indicated that optimal core-hole lifetime was1.3eV. Thelocal density of states of Co and Sb indicated that Co4s, Co4p and Sb5s orbitaloccupied the bottom of the valence band, and Co3d and Sb5p orbital occupied thetop of valence band and the bottom of the conduction band.(2) The Co K-edge EXAFS spectra of CoSb3、Ba0.3Co4Sb12、In0.25Co4Sb12andBa0.3In0.2Co4Sb12were calculated by single-scattering theory. The results indicatedthat the bond lengths in CoSb6octahedron were increased due to the filling of Ba andIn. The thermal vibration parameters of Co-Sb bond of the first three shells wereincreased in Ba and In filled skutterudites, but they were still relatively small. Itshowed that the lattice thermal disorder of skutterudite compounds were increaseddue to the lattice distortion caused by Ba and In filling, but the crystal lattice werestill highly ordered. (3) The Sb K-edge EXAFS spectra of CoSb3、Ba0.3Co4Sb12、In0.25Co4Sb12andBa0.3In0.2Co4Sb12were calculated by single-scattering theory. The results indicatedthat the lengths of long and short Sb-Sb bonds both decreased and the difference inbond lengths between long and short bonds in Sb4ring decreased. It was showed thatthe Sb4ring was transformed from rectangular to square in Ba and In filledskutterudites. On the other hand, the thermal vibration of long bond is higher thanshort bond of Sb4ring in four skutterudite compounds.