Abstract
Based on global particle-swarm optimization algorithm and density functional theory methods, we predicted an alloyed Si2Ge compond with body centered tetragonal type VII clathrate (space group I4/mmm) built by a truncated octahedron fromed by six quadrangles and eight hexagons ([4668]). Si2Ge clathrate is 0.06 eV/atom lower than VII Si clathrate and thermally stable up to 1000 K. It has an indirect band gap of 0.23 eV, high p-doping Seebeck coefficient and n-doping electrical conductivity. It owns a low lattice thermal conductivity of 0.28 W/mK at 300 K because of its weak bonding and strong anharmonic interaction of longitudinal acoustic and low-lying optical phonons. The moderate electronic transport properties together with low lattice thermal conductivity results in a high optimal thermoeletric performance value of 2.54 (1.49) at 800 (1000) K in n (p)-doped Si2Ge.
Highlights
Based on global particle-swarm optimization algorithm and density functional theory methods, we predicted an alloyed Si2Ge compond with body centered tetragonal type VII clathrate built by a truncated octahedron fromed by six quadrangles and eight hexagons ([4668])
This phenomenon does exist in the clathrates, many researches reveal that alloyed inducing band convergence is responsible for the high Seebeck coefficients[19,20] and tuning the electrical properties as result of modifying the band structure[16], increasing the phonon scattering in order to reduce the thermal conductivity[20,21], and leading to a significant increase of TE performance in alloy compounds[22,23]
The lattice strain is mostly induced by the distorted tetrahedral coordination of SiGe alloy, or, alternatively, by the 90.2° (∠GeSiGe) and 89.7° (∠SiGeSi) of 4-membered (Si2Ge2) rings along c direction
Summary
Based on global particle-swarm optimization algorithm and density functional theory methods, we predicted an alloyed Si2Ge compond with body centered tetragonal type VII clathrate (space group I4/mmm) built by a truncated octahedron fromed by six quadrangles and eight hexagons ([4668]). SiGe alloyed clathrates exhibit a significant increase in TE performance from their high power factor (PF, S2σ) and low lattice thermal conductivity[16].
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