  High-energy x-ray diffraction experiments peformed on rhombohedral
(1-x)Pb(Zn1/3Nb2/3)O3-xPbTiO3(PZN-xPT) crystals with x= 4.5 and 8% show that an
electric field applied along the [001] direction induces the tetragonal phase,
as proposed by Park and Shrout. Our experiments reveal that in 4.5PT such phase
change occurs via a third phase with monoclinic symmetry, MA, which is observed
at intermediate field values. This is in agreement with first-principles
calculations by Fu and Cohen predicting the rotation of the polarization
between the rhombohedral and tetragonal phases in this material. A different
polarization path between the rhombohedral and tetragonal phases, through a
second monoclinic phase, MC, has been previously reported in 8PT. The
microscopic characterization of these crystals allows us to explain the
ultra-high macroscopic strain observed in PZN-x%PT under an electric field.
Moreover, some unusual scattering profiles displayed by exceptionally good
crystals, are experimental evidence of the high anharmonicities and
near-degeneracy of the different phases in these extremely deformable
materials.
