Reducing lattice thermal conductivity in schwarzites via engineering the hybridized phonon modes

The cage structure and low lattice thermal conductivity k(L) make the schwarzites a suitable candidate for building the host-guest system favorable for thermoelectric applications. In host-guest system, the guest atoms in cages play a crucial role for reducing k(L). However, the underlying mechanism on the thermal conductivity reduction remains unclear. In this work, the authors unveil, through atomic simulations, the working principles of guest atoms in schwarzites by highlighting the relationship between thermal transport properties and rattling motions of guest atoms. It has been found that local vibration of the on-center guest atoms at low temperatures gives rise to a single hybridized mode, while the positions of guest atoms at high temperatures deviate severely from the center of cages yielding blue-shifted hybridized modes. These blue-shifted hybridized modes are responsible for the reduction of the phonon relaxation time in a wide range of frequencies. Based on these findings, the authors propose guidelines for reducing k(L) in schwarzites by properly tuning the frequency of one hybridized mode. Further reduction of k(L) can be achieved by simultaneously introducing multiple hybridized modes with different characteristic frequencies. This study provides insights to the controllable thermal transport properties in schwarzites by engineering the hybridized phonon modes. (C) 2018 Elsevier Ltd. All rights reserved.