  Bulk single-crystal relaxor-ferroelectrics, like Pb(Mg1/3Nb2/3)O3-PbTiO3
(PMN-PT), are widely known for their large piezoelectricity. This is attributed
to polarization rotation which is facilitated by the presence of various
crystal symmetries for compositions near a morphotropic phase boundary (MPB).
Relaxor-ferroelectric thin films, which are necessary for low-voltage
applications, suffer a reduction in their piezoelectric response due to
clamping by the passive substrate. To understand the microscopic behavior of
this adverse phenomenon, we employ AC electric field driven in-operando
synchrotron x-ray diffraction (XRD) on patterned device structures to
investigate the piezoelectric domain behavior under an electric field for both
a clamped (001) PMN-PT thin film on Si and a (001) PMN-PT membrane released
from its substrate. In the clamped film, the substrate inhibits the field
induced rhombohedral (R) to tetragonal (T) phase transition resulting in a
reversible R to Monoclinic (M) transition with a reduced longitudinal
piezoelectric coefficient d33 < 100 pm/V. Releasing the film from the substrate
results in recovery of the R to T transition and results in a d33 > 1000 pm/V.
Using diffraction with spatial mapping, we find that lateral constraints
imposed by the boundary between active and inactive material also inhibits the
R to T transition. Phase-field calculations on both clamped and released PMN-PT
thin films simulate our experimental findings. Resolving the suppression of
thin film piezoelectric response is critical to their application in
piezo-driven technologies.
