Field-induced relaxation of bulk composition due to internal boundaries

The establishment of compositional variations in solids caused by an external electric field usually requires electrodes that block either electronic or ionic charge carriers. Here we present unambiguous experimental and theoretical evidence that the compositional variations can occur even in the case of completely nonblocking electrodes if grain boundaries are present. In addition to the proper grain boundary impedance arc characterized by the dielectric relaxation time, tau, a second arc occurs at much lower frequencies (tau(delta)). It is shown that this low-frequency response refers to a chemical relaxation (Warburg diffusion) induced by the selectively blocking grain boundary. The effect is the analogue to the chemical relaxation induced by selectively blocking electrodes (cf. Wagner-Hebb polarization). (C) 2003 American Institute of Physics.