Charge carrier defect-dopant complexes in ZrO₂ co-doped with yttria and scandia

DOI: 10.62564/M4-OV1602

Oleksandr Vasiliev, Valerii Bekenev, Vladyslav Bilyi, Yehor Brodnikovskyi

Frantsevich Institute for Problems of Materials Science National Academy of Science of Ukraine


Co-doping ZrO₂ with yttria and scandia is an attractive way to produce stabilized cubic zirconia with maximized conductivity and fine-tuned thermal stability for specific application conditions/temperatures. As co-doping may lead to formation of defects with similar dopant atoms (homogeneous) or with Y and Sc atoms (heterogeneous), which can impact ionic conductivity, the aim of this work was to clarify which configuration is a more probable case in the mixed system using DFT modeling. The dopant-vacancy complexes were modeled with oxygen vacancy residing closer to one of the dopant atoms (dipoles) and equidistant to two atoms[1] (tripoles) at 3.2 mol. % and as tripoles with vacancies arranged along the [111] direction in cubic zirconia at 10.3 mol. % of added oxide. The configurations were fully relaxed with Quantum Espresso software package with calculation parameters ensuring energy and atomic forces convergence within 1.0·10⁻⁸ eV and 2.7·10⁻² eV/Å respectively. The total energies of the relaxed configurations were used to analyze their relative stability. The dipole configurations showed instability with respect to tripoles for all cases, with homogeneous yttrium configurations having the smallest energy difference with the more stable tripoles of 3.9 meV/atom and the largest of 20.2 meV/atom for heterogeneous configuration with the vacancy near the yttrium atom; for the scandium case the difference was 13.1 meV/atom. Among the tripoles, heterogeneous case has better stability at low concentrations with mixing energy of -7.8 meV/atom, while homogeneous seem to be more stable at high concentrations (mixing energy is 4.2 meV/atom), suggesting a transition at some concentration threshold. As stabilization of cubic zirconia typically requires more than 8 mol. % of dopant, a simple linear dependence of ionic conductivity on the concentration of one of the dopants should be expected in practical co-doped systems.

Keywords
stabilized cubic zirconia, defects, DFT

Acknowledgments
This work was supported by the Ministry of Education and Science of Ukraine, project "Development of electrolyte with improved operational properties for solid oxide fuel cell application" (0123U102769, under contract RN/22-2023 dated 05/24/2023)

References
[1] Parkes MA, Tompsett DA, d’Avezac M, Offer GJ, Brandon NP, Harrison NM. The atomistic structure of yttria stabilised zirconia at 6.7 mol%: an ab initio study, Phys Chem Chem Phys, 2016. No. 18. P. 31277–85.