Dynamics of mobile oxygen ions in disordered pyrochlore-type oxide-ion conductors

We investigate the effect of cation size in the dc activation energy needed for oxygen ion conductivity, E-dc in highly disordered pyrochlore-type ionic conductors A(2)B(2)O(7). Twenty compositions of general formula Ln(2)Zr(2-y)Ti(y)O(7) (Ln = Y, Dy and Gd) and Gd2-yLayZr2O7, were prepared by mechanical milling and their electrical properties measured by using impedance spectroscopy at different temperatures. We also evaluate, by using Ngai's Coupling Model, the effect of cation radii R-A and R-B, on the microscopic potential-energy barrier, E-a, that oxygen ions encounter when jumping 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 increment in the unit cell volume. In contrast, and for a given R-A size, the dc activation energy E-dc of the Ln(2)Zr(2-y)Ti(y)O(7) series increases when the average R-B size increases. The latter behavior is explained in terms of the enhanced interactions among mobile oxygen ions as the structural disorder increases when R-B approaches R-A.