Giant Electromechanical Coupling of Relaxor Ferroelectric Controlled by Polar Nanoregion Dynamics
Michael E. Manley, Douglas L. Abernathy, Raffi Sahul, Daniel E Parshall, Jeffrey W. Lynn, Andrew D. Christianson, Paul J. Stonaha, Eliot D. Specht, John D. Budai
Ferroelectric relaxors are inhomogeneous materials prized for their giant electromechanical coupling, which has revolutionized sensor and ultrasound applications, but is not well understood. It has been attributed to critical points in the phase diagram and argued that phonon softening induced by polar nanoregions (PNRs) underlies instability. It remains unclear, however, how PNR structures soften phonons and it has been argued that PNRs may not be necessary to obtain ultrahigh piezoelectric responses. Here we show that PNR dynamical modes induce phonon softening directly through an avoided crossing with acoustic phonons in canonical PZN-PT and PMN-PT ferroelectric relaxors using neutron scattering. Furthermore, we demonstrate that poling in an electric field aligns the PNR modes and that this alignment drives a large softening enables the polarization rotations responsible for the giant electromechanical coupling. Our work establishes that PNR dynamics play a crucial role in the high performance of ferroelectrics relaxors as actuators and sensors, and reveals a new strategy for engineering better materials.