  Relaxor ferroelectric perovskites are highly polarizable and can exhibit
giant coupling between elastic strain and an applied electric field. Here, we
report an in situ extended X-ray absorption fine structure (EXAFS) study of a
PbZn1/3Nb2/3O3 (PZN) single crystal as a function of the electric field. We
show that the strong dipoles in the NbO6 octahedra bonds are aligned along the
four \textless{} 011 \textgreater{} directions close to the orientation of the
electric field, while a small reversible polar shift occurs for Zn in the
direction of the electric field, i.e., positive or negative. This reversible
Zn-O polar shift is proposed to play an important role in both the "easy"
switching of the ferroelectric polarization and the giant piezoelectric effect
in PZN.
