Electronic structure and thermoelectric properties on transition-element-doped clathrates

Transition element(TM) doping for group IV clathrates is very interesting from the viewpoint of p-type thermoelectric materials. Group IV clathrates are candidates of high performance thermoelectric materials, because they show low thermal conductivity and high carrier mobility. But almost all clathrate semiconductors show n-type conduction due to excess electrons brought by alkali or alkaline-earth metal elements. We have studied the doping effects of noble metal elements on the electronic structure and thermoelectric properties by means of computational approaches. The electronic structure is calculated by the Full-potential Linearized Augmented Plane Wave (FLAPW) method with the Generalized Gradient Approximation (GGA) based on the density functional theory. The calculated electronic structure shows that TM-substituting clathrates Ba₈M₆X₄₀(M=Cu, Ag, Au; X=Si, Ge) are p-type semiconductors and have large thermoelectric power(α) at room temperature. The calculated energy of the band gap E_g is 302 meV in Ba₈Au₆Ge₄₀, which is smaller than that in Ba₈Ga₁₆Ge₃₀(E_g=513 meV ). When La atoms are doped at guest sites, the band gap becomes large: E_g=353 meV(La₂Ba₆Au₆Ge₄₀). By using a rigid band and a constant relaxation time approximation, we have calculated the thermoelectric properties. For La₂Ba₆Au₆Ge₄₀, we obtained α=240 μV/K at a hole concentration n_h=10²⁰/cm³ and at 300 K.