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 [1]. 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 [2]. 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 [3]. 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.

[1] Cowley, R. A. et al. Adv. Phys. 60, 229 (2011).
[2] Swainson, I. P. et al. Phys. Rev. B 79, 224301 (2009).
[3] Manley, M. E., et al. Nature Commun. 5:3683 doi: 10.1038/ncomms4683 (2014).