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 A2B2O7. Twenty compositions of general formula Ln₂Zr_2-yTi_yO₇ (Ln = Y, Dy and Gd) and Gd_2-yLa_yZr₂O₇, 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 RB, 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 Ln₂Zr_2-yTi_yO 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.