Programme

I.6 -- Invited, VCTP-50

Date: Tuesday, 5 August 2025

Time: 10:30 - 11:10

Exploring structures and dynamics of materials with mobile atoms using machine-learning interatomic potential

Ching-Ming Wei

Institute of Atomic and Molecular Sciences, Academia Sinica, Taiwan

Various materials, such as superionic materials, lithium-ion batteries, and metal-organic frameworks, possess mobile atoms with liquid-like behavior in the rigid frameworks of other atoms. Experimentally, to reveal the detailed structures of mobile atoms is typically probed by a try-and-error fitting approach of X-ray Powder Diffraction (XPD) using the concept of partial occupancy. Since the mobile atoms diffuse very fast inside and between different crystal unit cells, limited numbers of partial occupancy cannot describe the details of the structures. To rescue this deficiency, we extend the concept of partial occupancy used at X-ray Powder Diffraction into continuous mode to define and obtain the three-dimensional atomic density distribution function (3D-ADDF) [1] of mobile atoms, which is easily obtained by performing reliable large-scale molecular dynamics simulations. Conversely, the diffusion behavior of mobile atoms is usually probed by ab initio molecular dynamics simulations, where enormous computing resources are required for a complete thorough study. Thus, only limited cases are investigated without providing the most critical quantity, such as the diffusion constants and barriers. To address this shortcoming, we perform large-scale molecular dynamics simulations (up to 10 ns simulation times) based on machine-learning interatomic potentials, fitted from ab initio molecular dynamics simulations [2], to have complete studies for various materials: (i) superionic materials including Ag2S, Ag8SiTe6, Cu2S, Zn3.6+xSb3, and Cu7+xPS6; (ii) lithium-ion battery including LixCoO2, and Li6+xPS5Cl; (iii) metal-organic framework: H2O, CO2, N2, CH4 diffusion on [Ca(C4O4)(H2O)] (termed UTSA-280). Our results indicate that the Arrhenius equation can describe the diffusion behaviors of the various mobile atoms in the rigid framework very well, where the activation barriers range from 0.1 to 0.3 eV. The small diffusion barriers provide the fundamental origin for the liquid behaviors of mobile atoms. References [1] C.-R. Hsing, D.-L. Nguyen, and C.-M. Wei, Phys. Rev. Mater. 6, 083601 (2022). [2] C.-R. Hsing, D.-L. Nguyen, and C.-M. Wei, Phys. Rev. Mater. 8, 043806 (2024).

Presenter: Ching-Ming Wei