Reinvestigation of Thermoelectric Properties of n- and p-Type Ba^sub 8-d^Au^sub x^Si^sub 46-x-y^ Clathrate

Issue Title: International Conference on Thermoelectrics 2013. Guest Editors: Jihui Yang, Hiroaki Anno, Matt Beekman, Ryoji Funahashi, Yuri Grin, Emmanuel Guilmeau, Jan Koenig, Bertrand Lenoir, Don Morelli, Lasse Rosendahl, James R. Salvador, Jeff Sharp, David Singh, Tsunehiro Takeuchi, Xiegeng Tang, Chunlei Wang, Hsin Wang, Wenqing Zhang, and Tie-Jun Zhu We have synthesized n- and p-type clathrates Ba^sub 8-d^Au^sub x^Si^sub 46-x-y^ with various Au contents (4.6 < x < 6.0) by arc-melting, annealing at 1173 K, and spark plasma sintering at 1073 K. The Au compositions found by wavelength-dispersive x-ray spectrometry for the synthesized samples were slightly lower than the nominal compositions. Ba^sub 7.8^Au^sub 4.6^Si^sub 41.4^ and Ba^sub 7.7^Au^sub 4.9^Si^sub 41.1^ samples showed n- and p-type conduction, respectively. According to the electron count (Ba^sup 2+^)^sub 8^Au(^sup 3-^)^sub 5.33^Si^sub 40.67^, the clathrate composition with x = 5.33 is expected to be an intrinsic semiconductor. Our experimental results show that increase of the Au composition causes a transition from n-type to p-type conduction between x = 4.6 and 4.9. We have also calculated the band structures of the Ba^sub 8^Au^sub x^Si^sub 46-x^ clathrate including a vacancy by ab initio calculation based on density functional theory with structure optimization. It was found that the vacancy behaves like an electron acceptor and the numbers of vacancies at 24k sites for the synthesized Ba^sub 8^Au^sub x^Si^sub 46-x-y^ clathrates can be estimated as 0.4 in a unit cell.[PUBLICATION ABSTRACT]