Cation size effects in oxygen ion dynamics of highly disordered pyrochlore-type ionic conductors

Abstract

In this work we evaluate the effect of cation size on the dc activation energy needed for oxygen ion migration, ${E}_{\text{dc}}$, in highly disordered pyrochlore-type ionic conductors ${A}_{2}{B}_{2}{\text{O}}_{7}$. Twenty six compositions with the general formula, ${\text{Ln}}_{2}{\text{Zr}}_{2\ensuremath{-}y}{\text{Ti}}_{y}{\text{O}}_{7}$, ${\text{Ln}}_{1.7}{\text{Mg}}_{0.3}{\text{Zr}}_{2}{\text{O}}_{7}$ ($\text{Ln}=\text{Y}$, Dy, and Gd), and ${\text{Gd}}_{2\ensuremath{-}y}{\text{La}}_{y}{\text{Zr}}_{2}{\text{O}}_{7}$, were prepared by mechanical milling, and their electrical properties were measured by using impedance spectroscopy as a function of frequency and temperature. By using the coupling model we also examine the effect of cation radii ${R}_{A}$ and ${R}_{B}$ on the microscopic potential-energy barrier, ${E}_{a}$, which oxygen ions encounter when hopping into neighboring vacant sites. We find that, for a fixed $B$-site-cation radius ${R}_{B}$, both activation energies decrease with increasing $A$-site-cation size, ${R}_{A}$, as a consequence of the increase in the unit-cell volume. In contrast, for a given ${R}_{A}$ size, the ${E}_{\text{dc}}$ of the ${\text{Ln}}_{2}{\text{Zr}}_{2\ensuremath{-}y}{\text{Ti}}_{y}{\text{O}}_{7}$ series increases when the average ${R}_{B}$ size increases. This behavior is associated with enhanced interactions among mobile oxygen ions as the structural disorder increases with ${R}_{B}$.