A Computational Study of Yttria-Stabilized Zirconia: II. Cation Diffusion

TL;DR

This study uses ab initio calculations to explain why cation diffusion in yttria-stabilized zirconia is slow, revealing defect structures and migration paths that cause significant atomic disturbances, contrasting with fast oxygen diffusion.

Contribution

It provides the first accurate computational prediction of cation diffusivity and elucidates the atomic mechanisms behind slow cation diffusion in cubic zirconia.

Findings

Schottky pairs dominate cation vacancies

Cation migration involves passing through interstitial sites

Cation diffusion causes significant local atomic disturbances

Abstract

Cubic yttria-stabilized zirconia is widely used in industrial electrochemical devices. While its fast oxygen ion diffusion is well understood, why cation diffusion is much slower-its activation energy (~5 eV) is 10 times that of anion diffusion-remains a mystery. Indeed, all previous computational studies predicted more than 5 eV is needed for forming a cation defect, and another 5 eV for moving one. In contrast, our ab initio calculations have correctly predicted the experimentally observed cation diffusivity. We found Schottky pairs are the dominant defects that provide cation vacancies, and their local environments and migrating path are dictated by packing preferences. As a cation exchanges position with a neighboring vacancy, it passes by an empty interstitial site and severely displaces two oxygen neighbors with shortened Zr-O distances. This causes a short-range repulsion against the migrating cation and a long-range disturbance of the surrounding, which explains why cation diffusion is relatively difficult. In comparison, cubic zirconia's migrating oxygen only minimally disturbs neighboring Zr, which explains why it is a fast oxygen conductor.