Session: 08-02: Micro/Nanoscale Heat Conduction

Paper Number: 131241

131241 - Phonon-Mediated Ionic Transport in Fluorite-Structured Solids 

Abstract: 

Ionic hopping in solids with a fluorite crystal structure is commonly recognized as a crucial mechanism for facilitating liquid-like ionic transport. Additionally, it contributes to the scattering of lattice vibrational energy transport, resulting in noticeably low phonon lattice thermal conductivity [1,2]. This simultaneous effect of low lattice thermal conductivity and high ionic diffusivity is usually observed in fluorite-structured materials, such as Cu₂Se, Ag₂Te, and Li₂S. These fluorite behaviors induced by ionic hopping are of great importance in a variety of applications based on high ionic diffusivity or low thermal conductivity. For instance, on the one side, the rapid movement of lithium ions in energy materials such as lithium thiophosphate, garnet, and other lithium conductors at room temperature opens up new possibilities for advancing all-solid-state batteries [3]. On the other side, the ultralow lattice thermal conductivity resulting from phonon-ion scatterings contributes to the remarkable thermoelectric performance observed in materials like Cu₂S when the Cu+ ions exhibit liquid-like mobility within the material [4]. Up to now, extensive studies have shown that ionic hopping within and between the lattice sites is the origin of the high mobility of ionic diffusivity, and their scatterings to the lattice vibrations lead to a large reduction in thermal conductivity [5]. However, the effects of phonons on ionic hopping and diffusion in materials have received less attention and are not as thoroughly studied compared to the ionic-phonon scatterings. Here, we investigate the phonon effects on ionic hopping inside the Ag₂Te crystals by using nudged elastic-band calculations and lattice dynamics calculations. We found that Ag⁺ ion diffusion is determined by only a small subset of vibrational modes, e.g., more than 80% of the Ag⁺ ion diffusion originates from less than 5% of the vibrational modes between 0.8 and 2.0 THz, in which the single vibrational mode can contribute more than 10% to Ag⁺ ion diffusion. Our molecular dynamics simulations show that the diffusivity of Ag⁺ ions and the corresponding ionic conductivity are determined by these subset phonon modes. Therefore, by leveraging phonon engineering techniques, it becomes possible to enhance diffusivity and conductivity in fluorite-structured solids without making substantial changes to their chemical composition. This approach offers a pathway for improving the performance of various systems, including solid-state batteries, thermoelectrics, and other ion-based technologies.

[1]       Q. Ren et al., Nat. Mater. 1 (2023).

[2]       M. K. Gupta et al., Energy Environ. Sci. 14, 6554 (2021).

[3]       T. Famprikis et al., Nat. Mater. 18, 1278 (2019).

[4]       H. Liu et al., Nat. Mater. 11, 422 (2012).

[5]       Y. Zhou et al., Nat Commun 9, 4712 (2018).

Presenting Author: Yixin Xu The Hong Kong University of Science and Technology

Presenting Author Biography: Mr. Yixin Xu is now a Ph.D. Candidate at Hong Kong University of Science and Technology, majoring in mechanical engineering. His research interests focus on nanoscale thermal transport, including phonon transport across solid/solid interface and phase-change heat transfer, e.g., boiling and condensation. Mr. Xu is now investigating the thermal transport of complex systems with multi-heterostructures to advance the understanding and optimization of thermal transport within these systems.

Authors:

Yixin Xu The Hong Kong University of Science and Technology
Yanguang Zhou The Hong Kong University of Science and Technology