Doping Induced Abnormal Contraction and Significant Reduction of Lattice Thermal Conductivity of Open Framework Si24

Tao Ouyang 1, Email 

Qingyi Liu 1

Mingxing Chen 2

Chao Tang 1

Jin Li 1

Chunxiao Zhang 1

Chaoyu He 1

Hua Bao 3, Email

Jianxin Zhong 1

Ming Hu 4, Email

1 Hunan Key Laboratory for Micro-Nano Energy Materials and Device and School of Physics and Optoelectronics, Xiangtan University, Xiangtan 411105, Hunan, China
2 College of Physics and Information Science, Hunan Normal University, Changsha 410081, China
3 University of Michigan-Shanghai Jiao Tong University Joint Institute, Shanghai Jiao Tong University, Shanghai 200240, China
4 Department of Mechanical Engineering, University of South Carolina, Columbia, SC 29208, USA

Abstract

Effectively suppressing lattice thermal conductivity is a critical step for improving the thermoelectric performance. Taking open framework Si24, a potential thermoelectric material and a newly synthesized cage-like Si allotrope, as an example, we systematically investigate the lattice thermal conductivity of this new structure filled with alkaline-earth guest atoms ([email protected]24, X = Mg, Ca, Sr, and Ba) by combining first-principles calculation and phonon Boltzmann transport theory. The calculated lattice thermal conductivity is obviously decreased as guest atoms are inserted in the void sites of the Si24 framework, which is a common phenomenon for the guest-host systems. However, it is surprising to find that the thermal conductivity of this new filler structure presents a prominent element dependency. Inserting Ca into Si24 framework generally leads to 3 to 10 times lower thermal conductivity than that with other alkaline-earth atom fillers, and the value along zz crystal direction of 0.59 W/mK is even lower than that of amorphous silicon, despite the intrinsic thermal conductivity of pristine Si24 is as high as 21.25 W/mK. Such ultra-low thermal conductivity is found to be closely related to the strong harmonic interatomic interaction among guest and host atoms of [email protected]24 system. The strong interaction gives rise to anomalous contraction effect on the Si24 lattice (the volume abnormally decreases) and more dispersive phonon branches in low frequency range, which boosts the three-phonon scattering channels (reflected by the weighted phase space) and eventually suppresses the thermal conductivity of Si24. Finally, based on a simple and effective lattice chain model, we reproduce the abnormal thermal phenomenon observed in [email protected]24, and further demonstrate that the origin stems from the strong interaction between Ca and Si24 atoms. These findings shed light on a new physical mechanism for the reduction in thermal conductivity of [email protected]24, which offer a promising approach to improve the thermoelectric efficiency of Si related materials.

Doping Induced Abnormal Contraction and Significant Reduction of Lattice Thermal Conductivity of Open Framework Si24