First principles study of electronic structures of dopants in Mg 2 Si
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First principles study of electronic structures of dopants in Mg2Si K. Xionga*, S. Sobhania, R. P. Guptaa, W. Wanga, B. E. Gnadea and Kyeongjae Choa,b,† a
Materials Science & Engineering Dept, The University of Texas at Dallas, Richardson, TX 75080, USA b Physics Dept, The University of Texas at Dallas, Richardson, TX 75080, USA *[email protected] † kjcho @utdallas.edu
ABSTRACT We investigate the impact of various dopants (Na, Ag, Cd, Zn, Al, Ga, In, Tl, Ge, and Sn) on the electronic structure of Mg2Si by first principles calculations using a hybrid functional that does not need a band gap correction. We find that for Na and Ge in Mg2Si, the impurity-induced states do not affect the density of states at both edges of the valence band and the conduction band. Agand Sn affect slightly the density of states at the valence band edge, while Cd and Zn affect slightly the density of state at the conduction band edge. Al and In could modify significantly the density of states at the conduction band edge. Ga introduces states just at the bottom of the conduction band. Tl introduces states in the band gap. This study provides useful information on optimizing the thermoelectric efficiency of Mg2Si. INTRODUCTION Thermoelectric (TE) materials have attracted much attention for being used in energy harvesting applications because they can convert heat into electricity. However, developing TE devices for commercial applications is hindered by the low energy conversion efficiency of the TE materials [1-2]. The effectiveness of a TE material, or so-called “figure of merit”, can be expressed as: ZT=α2σ/κ, where S is the Seebeck coefficient, σ is the electrical conductivity, and κ is the thermal conductivity. Clearly, to have a high ZT value the candidate material should have a large S, a high σ, and a low κ. Mg2Si shows high thermoelectric performance at mid-temperature range (500-800K) [3]. Compared to conventional TE materials such as Bi2Te3 and PbTe, Mg2Si has advantages of abundant, light and non-toxic, making it a promising TE material for use in industrial waste heat recovery, solar thermal power conversion and geothermal power generation. Since doping impurities into TE materials can enhance the TE performance, this has led to considerable effort on improving the ZT of Mg2Si by introducing various dopants [4-11]. However, to our knowledge, a thorough understanding at an atomic level of the impact of dopants on the electronic structures of Mg2Si is still missing. Moreover, we are also interested in the impurityinduced energy level in the band structure of the doped Mg2Si. Recent experimental work has demonstrated that doping Tl into bulk PbTe could double the ZT (~1.4) as compared to that of the undoped PbTe [12]. This ZT enhancement is attributed to the enhancement of the Seebeck coefficient (S) caused by the Tl-induced resonant states in the valence band of PbTe. This finding sheds new light on the TE efficiency improvement, which motivates us to explore suitable dopants that could bring similar effects to Mg2Si. In this
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