Effect of local atomic and electronic structures on thermoelectric properties of chemically substituted CoSi

¹ National Synchrotron Radiation Research Center - Hsinchu 30076, Taiwan
² Institute of Physics, Academia Sinica - Taipei 11529, Taiwan
³ Department of Physics, Tamkang University - Taipei 25137, Taiwan
⁴ Department of Physics, National Dong Hwa University - Hualien 97404, Taiwan
⁵ Department of Physics, National Cheng Kung University - Tainan 70101, Taiwan

^(a) clchen@phys.sinica.edu.tw
^(b) dong.cl@nsrrc.org.tw

Received: 12 February 2014
Accepted: 17 April 2014

Abstract

We report the effects of Ge partial substitution for Si on local atomic and electronic structures of thermoelectric materials in binary compound cobalt monosilicides (). Correlations between local atomic/electronic structure and thermoelectric properties are investigated by means of X-ray absorption spectroscopy. The spectroscopic results indicate that as Ge is partially substituted onto Si sites at , Co in CoSi_1−xGe_x gains a certain amount of charge in its 3d orbitals. Contrarily, upon further replacing Si with Ge at , the Co 3d orbitals start to lose some of their charge. Notably, thermopower is strongly correlated with charge redistribution in the Co 3d orbital, and the observed charge transfer between Ge and Co is responsible for the variation of Co 3d occupancy number. In addition to Seebeck coefficient, which can be modified by tailoring the Co 3d states, local lattice disorder may also be beneficial in enhancing the thermoelectric properties. Extended X-ray absorption fine structure spectrum results further demonstrate that the lattice phonons can be enhanced by Ge doping, which results in the formation of the disordered Co-Co pair. Improvements in the thermoelectric properties are interpreted based on the variation of local atomic and electronic structure induced by lattice distortion through chemical substitution.