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

In this work we evaluate the effect of cation size on the dc activation energy needed for oxygen ion migration, Edc, in highly disordered pyrochlore-type ionic conductors A2 B2 O7. Twenty six compositions with the general formula, Ln2 Zr2-y Tiy O7, Ln1.7 Mg0.3 Zr2 O7 (Ln=Y, Dy, and Gd), and Gd2-y Lay Zr2 O7, 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 RA and RB on the microscopic potential-energy barrier, Ea, which oxygen ions encounter when hopping into neighboring vacant sites. We find that, for a fixed B -site-cation radius RB, both activation energies decrease with increasing A -site-cation size, RA, as a consequence of the increase in the unit-cell volume. In contrast, for a given RA size, the Edc of the Ln2 Zr2-y Tiy O7 series increases when the average RB size increases. This behavior is associated with enhanced interactions among mobile oxygen ions as the structural disorder increases with RB.