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 Ba 8 M 6 X 40 (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 302meV in Ba 8 Au 6 Ge 40 , which is smaller than that in Ba 8 Ga 16 Ge 30 ( E g =513meV ). When La atoms are doped at guest sites, the band gap becomes large: E g =353meV(La 2 Ba 6 Au 6 Ge 40 ). By using a rigid band and a constant relaxation time approximation, we have calculated the thermoelectric properties. For La 2 Ba 6 Au 6 Ge 40 , we obtained α=240 μV/K at a hole concentration n h =10 20 /cm 3 and at 300K.
Sr2Ta2O7, a layered perovskite compound, has been reported to possess most excellent photocatalytic properties among the layered perovskite materials. Recently, we have successfully demonstrated that Ba5Ta4O15 that was prepared under Ta rich atmosphere has high photocatalytic performance as well as Sr2Ta2O7. In this study, several amount of Ba was doped into Sr2Ta2O7, and the photocatalyst samples with a mol ratio of Sr: Ba: Ta = (1-x): x: 1 were prepared by the polymerized complex method to investigate the effect of Ba substitution for the Sr site on photocatalytic activity. The maximum photocatalytic performance was obtained for x = 0.2 near the solid solubility limit, which is three times as high as that of undoped Sr2Ta2O7 and is in the highest level in a series of tantalum photocatalysts reported so far. The increase of photocatalytic activity would be caused by crystal distortion due to doping of Ba ion.
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