Microstructural evolution of charged defects in the fatigue process of polycrystalline BiFeO3 thin films

Fatigue failure in ferroelectrics has been intensively investigated in the past few decades. Most of the mechanisms discussed for ferroelectric fatigue have been built on the hypothesis of variation in charged defects, but these are rarely evidenced by experimental observation. Here, using a combination of complex impedance spectra techniques, piezoresponse force microscopy and first-principles theory, we examine the microscopic evolution and redistribution of charged defects during the electrical cycling in BiFeO3 thin films. The dynamic formation and melting behaviors of oxygen vacancy (V-O) order are identified during the fatigue process. It reveals that the isolated V-O tends to self-order along grain boundaries to form a planar-aligned structure, which blocks the domain reversals. Upon further electrical cycling, migration of V-O within vacancy clusters is accommodated with a lower energy barrier (similar to 0.2 eV) and facilitates the formation of a nearby-electrode layer incorporated with highly concentrated V-O. The interplay between the macroscopic fatigue and microscopic evolution of charged defects clearly demonstrates the role of ordered V-O clusters in the fatigue failure of BiFeO3 thin films. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.