Phonon localization in relaxor ferroelectrics
Michael E. Manley,
Materials Science and Technology Division, Oak Ridge National Laboratory
Relaxor ferroelectrics are disorder-frustrated ferroelectric materials
with technologically important properties that are not very well
understood. Although it has been argued for 30 years that the
properties of relaxor ferroelectrics originate with polar nanoregions
(PNRs), there has been no agreement on how exactly PNRs form. Possible
explanations range from analogs with frustration in magnetic spin
glasses, to short-range chemical order, to intrinsic localized modes
. However, none of these models explain the size, shape, and
distributions of the PNRs, nor do they explain the appearance of
antiferroelectric nanoregions with zone-edge type displacements
. Using neutron scattering to characterize the lattice dynamics of
relaxor PMN-30%PT, we discovered a phonon localization mechanism that
explains the size and shape of the PNRs as well as the zone-edge
antiferroelectric nanoregions in terms of the coherent trapping of the
transverse optic phonons by randomly distributed localized resonance
modes . More recently, we have also found evidence that the
delocalization of the PNR modes below the Curie temperature induces
instability via an avoided crossing with acoustic phonons. In the
avoided crossing the lower symmetric branch is deflected towards zero
frequency, driving instability. This instability is likely behind the
giant electromechanical coupling near the morphotropic phase boundary,
which has allowed these materials to transform a range of
technologies, including medical ultrasound and sonar.
 Cowley, R. A. et al. Adv. Phys. 60, 229 (2011).
 Swainson, I. P. et al. Phys. Rev. B 79, 224301 (2009).
 Manley, M. E., et al. Nature Commun. 5:3683 doi: 10.1038/ncomms4683 (2014).