Ionic Conductivity of Tantalum Oxide at Very High Fields

The abnormally long jump distance $\lambda$ which characterizes the ionic conduction of glasses as a function of electric field, $\sigma ={\sigma }_0\exp (-\frac{q\lambda E}{\mathrm{kT}})$, and the fact that for tantalum oxide this distance changes by a factor of two at a critical field are explained in terms of a model in which ionic conduction is a two-stage process involving the creation and motion of interstitial metallic ions. Below a critical field both the number of interstitial ions and their mobility are rapidly varying functions of the field and the jump distance decreases discontinuously as the field increases. Above the critical field the number of interstitials depends much less strongly or not at all on the field, the field acts only to increase the ionic mobility, and the jump distance is of the order of the interatomic spacing. It is suggested that in tantalum oxide electrostatic repulsion between tantalum ions may provide the principle barrier to ionic motion.