  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", which
however 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 (VO)
order are identified during the fatigue process. It reveals that the isolated
VO tend to self-order along grain boundaries to form a planar-aligned
structure, which blocks the domain reversals. Upon further electrical cycling,
migration of VO within vacancy clusters is accommodated with a lower energy
barrier (~0.2 eV) and facilitates the formation of nearby-electrode layer
incorporated with highly concentrated VO. The interplay between the macroscopic
fatigue and microscopic evolution of charged defects clearly demonstrates the
role of ordered VO cluster in the fatigue failure of BiFeO3 thin films.
