Elsevier

Materials Today Physics

Volume 21, November 2021, 100566
Materials Today Physics

Significant phase-space-driven thermal transport suppression in BC8 silicon

https://doi.org/10.1016/j.mtphys.2021.100566Get rights and content

Highlights

  • BC8 silicon possesses unusually low thermal conductivity, i.e., 1-2 orders of magnitude lower than that of DC-Si.

  • The origin of the strikingly different thermal conductivity is revealed by INS measurement and DFT simulation.

  • Flat phonon bands in the middle energy range act as a scattering bridge between the high and low energy phonons.

Abstract

The BC8 silicon allotrope has a lattice thermal conductivity 1–2 orders of magnitude lower than that of diamond-cubic silicon. In the current work, the phonon density of states, phonon dispersion, and lattice thermal conductivity are investigated by inelastic neutron scattering measurements and first-principles calculations. Flat phonon bands are found to play a critical role in the reduction of lattice thermal conductivity in BC8–Si. Such bands in the low-energy range enhance the phonon scattering between acoustic and low-energy optical phonons, while bands in the intermediate-energy range act as a scattering bridge between the high- and low-energy optical phonons. They significantly enlarge the phonon-phonon scattering phase space and reduces the lattice thermal conductivity in this novel silicon allotrope. This work provides insights into the significant reduction of the lattice thermal conductivity in BC8–Si, thus expanding the understanding of novel silicon allotropes and their development for electronic devices.

Introduction

With the development of novel synthesis techniques [1] including extreme synthesis conditions [2,3], numerous silicon allotropes were discovered in recent years, including Si-II [4], Si–V [5], Si-VI [6], Si-VII [5], Si-VIII [7], Si-X [8], Si-XI [9,10], Si-XIII [11], etc. Among them, Si-III (BC8–Si) [12], Si-IV (HD-Si) [11], Si-XII (R8-Si) [13,14], zeolite types Si136 [15], and Si24 [2],are recoverable phases that remain metastable at ambient conditions. Many of these allotropes have novel properties beyond those of diamond-cubic silicon (DC-Si). For example, Si24 [2], and tI16-Si [16] have a quasi-direct band gap near 1.4 eV and a direct band gap of 1.25 eV, respectively, which is close to the optimum value for solar conversion, while maintaining large carrier mobility and low mass density [17]. Additionally, the thermal conductivity of Si24 is 1/13th that of DC-Si [18], suggesting promise in thermoelectric applications. While several new silicon allotropes [19,20] with novel electronic properties [17] have been investigated, experimental studies of thermal transport properties are rare, mainly due to the small sample sizes limited by high-pressure synthesis.

Recently, large and phase-pure samples of BC8–Si (cI16) were successfully synthesized by the multi-anvil press method [12], opening the possibility for novel measurements. BC8–Si is a narrow-gap semiconductor with an electrical conductivity of 76 S/cm at 300 K thanks to its high carrier density. Moreover, a recent study revealed that BC8–Si has a remarkably low thermal conductivity: 1–2 orders of magnitude lower than that of DC-Si depending on the temperature. This result appears highly unusual since BC8–Si and DC-Si both have cubic structures [20]. The microscopic origin of this thermal transport difference is still unclear due to the lack of knowledge on the lattice dynamics of BC8–Si.

In this work, the primary origin of the strikingly different thermal conductivities of BC8–Si and DC-Si is revealed by combining inelastic neutron scattering (INS) measurements and first-principles simulations. INS experiments were performed on DC- and BC8–Si samples to obtain the phonon density of states (DOS) at ambient temperature and the results reveal softer phonons in BC8–Si. The first-principles simulations indicate that the reduction of lattice thermal conductivity in BC8–Si mainly results from the enlarged phonon-phonon scattering phase space induced by flat phonon bands, which act as a bridge between low-energy and high-energy optic branches. The results offer microscopic insights into thermal transport in BC8–Si and further exploration of the thermal properties of silicon allotropes.

Section snippets

Experimental and computational methods

Bulk samples of BC8–Si used for the INS measurements were synthesized via direct transformation of elemental silicon using the multi-anvil press method [12]. The recovered samples were determined to be homogeneous and phase-pure through characterization using X-ray diffraction, Raman spectroscopy, and nuclear magnetic resonance spectroscopy [21]. A total sample mass of 10 mg of BC8–Si was used for INS and 5 g of crystalline DC-Si sample was used for comparison.

INS measurements were performed on

Results and discussions

Silicon allotrope BC8–Si is obtained by compressing DC-Si to 11–14 GPa at room temperature to form metallic β-Sn-Si (Si-II) and then slowly decompressing and unloading Si-II to atmospheric pressure leads to BC8–Si. BC8–Si (cI16) crystallizes in a body-centered cubic structure with 16 atoms in its conventional cell, as shown in Fig. 1. The cubic structure consists of distorted Si tetrahedra and Si atoms is fourfold coordinated with one Si–Si distance 2.34 Å directed along a threefold cubic axis

Conclusion

In summary, by combining experimental INS measurements and first-principles calculations based on the Boltzmann transport theory, the origin of the remarkable difference between lattice thermal conductivity of DC and BC8 silicon is revealed to be the dramatically larger phonon scattering phase space of BC8–Si. The “flat” optical phonon bands contribute to this large scattering phase space. Such bands between 10 and 20 meV enhance the scattering between acoustic and low energy optical phonons,

Author contributions

Junyan Liu: Calculations; Writing-reviewing & editing; Timothy A. Strobel: Preparing samples; Reviewing the manuscript; Haidong Zhang: Preparing and providing samples; Doug Abernathy: Neutron inelastic scattering measurement; Reviewing the manuscript; Chen Li: Project administration supervisor; Neutron inelastic scattering measurement; Guiding the experiments; Funding acquisition; Writing- reviewing & editing; Jiawang Hong: Project administration supervisor; Guiding the calculations; Funding

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work is supported by the National Science Foundation of China (Grant No. 1217021241), Beijing Natural Science Foundation, China (Grant No. Z190011). CL acknowledges support by the National Science Foundation under Grant No. 1750786. TAS acknowledges support from NSF-DMR under Grant No. 1809756. This research used resources at the Spallation Neutron Source, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory. Theoretical calculations were performed using

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