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, E-dc, in highly disordered pyrochlore-type ionic conductors A(2)B(2)O(7). Twenty six compositions with the general formula, Ln(2)Zr(2-y)Ti(y)O(7), Ln(1.7)Mg(0.3)Zr(2)O(7) (Ln=Y, Dy, and Gd), and Gd2-yLayZr2O7, 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-dc of the Ln(2)Zr(2-y)Ti(y)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.