Strain effect on phonon transport in open framework Si24: A first-principles study

Abstract

Strain engineering is one of the most efficient methods to modulate the thermal transport property due to its flexibility and easy realization in experiments. In this paper, taking the newly synthesized open framework Si24 as an example, we systematically investigated the strain effect on phonon transport by means of the first-principles study. The phonon thermal conductivity (κ) of bulk material usually increases under compression and decreases under tension. However, our calculations show that the κ of Si24 decreases abnormally in the range of ε = −4% (compression, P = 4 GPa) to ε = 5% (tension, P = -4 GPa). When the tensile strain increases further, the κ begins to decrease and finally reaches the value (31.6 W/mK at ε = 19%) below the pristine κ (45.3 W/mK). By analyzing the key factors of κ at the phonon mode level, we find that this abnormal variation on κ of Si24 is mainly depending on the competition between the phonon relaxation time (τ) and other phonon parameters which include volume, specific heat capacity, and phonon group velocity. The abnormally increased κ in the strain range of ε = −4% to ε = 5% is mainly attributed to the increase of phonon relaxation time (τ), which can be further explained by phonon scattering intensity. However, as for tension ε > 5%, the reduction of κ under tensile strain mainly comes from the negative effect of phonon group velocity, specific heat capacity, and crystal volume on κ. The findings presented in this work are expected to deepen our understanding of phonon transport in the open framework Si24 and uncovers the underlying mechanism behind the anomalous strain-dependent κ of Si24, which is helpful for further research of Si allotropes, such as improve the thermoelectric performance.