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Superionic UO2: A Model Anharmonic Crystalline Material

Published

Author(s)

Jack F. Douglas, Hao Zhang, Xinyi Wang, Alexandros Chremos

Abstract

Crystalline materials at elevated temperatures and pressures can exhibit properties more reminiscent of simple liquids than ideal crystalline materials, an attribute making these materials useful in diverse applications. Superionic crystalline materials having a liquid-like conductivity  are particularly interesting for battery, fuel cell, and other energy applications, and we study UO2 since this material is widely studied given its commercial importance as reactor fuel, and as a prototypical superionic material. We first investigate basic thermodynamic and structural properties (static structure factor, thermal expansion, specific heat, internal pressure, degree of long-range order, etc.) based on an embedded atom method potential optimized to reproduce basic thermodynamic properties over a large temperature (T) range. Based on this foundation, we quantify structural relaxation through the computation of the self- and collective intermediate scattering functions, dynamic heterogeneity through the calculation of the non-Gaussian parameter and the quantification of string-like collective hopping motion of O ions within the crystal lattice, and average molecular mobility through the calculation of the self-diffusion coefficient of O ions in the bulk and the interfacial region and the calculation of the collective ion diffusion coefficient through the computation of the conductivity. We find that the non-Arrhenius diffusion and structural relaxation time of this prototypical superionic material can be quantitatively described in terms of a generalized activated transport model (‘string model’) in which the activation energy varies in direct proportion to the average string length
Citation
The Journal of Chemical Physics

Keywords

superionic material, conductivity, string-like collective motion, melting, anharmoic motion, solid electrolytes
Created May 7, 2019, Updated April 24, 2020