Giant electromechanical coupling of ferroelectric relaxors controlled by local-structure vibrations

Michael Manley (Materials Science and Technology Division, ORNL)

Understanding how inhomogeneous nanoregions form and enhance functional properties is an outstanding scientific challenge for a broad class of materials. Ferroelectric relaxors, in particular, have electromechanical responses tenfold larger than industry-standard piezoelectric ceramic materials and have dramatically improved technologies ranging from medical 3D ultrasonography to sonar. The giant coupling is widely associated with the presence of polar nanoregions (PNRs) in these materials, but a microscopic explanation for how PNRs form or enhance coupling has remained elusive for decades. Using neutron scattering to characterize the lattice dynamics of ferroelectric relaxor materials, we discovered an Anderson-type phonon localization mechanism that explains the PNRs in terms of localized vibrations [1]. More recently, using neutron scattering measurements of the lattice dynamics and local structure, we find that localized vibrations also enable the giant coupling by softening the underlying macro-domain polarization rotations. The mechanism involves an avoided crossing, where the collective motion of the PNR modes with acoustic phonons results in a softer shear mode. Furthermore, the local vibrations and a component of the local structure align in an electric field and this focuses softening for particular shear modes, revealing a way to tune the ultrahigh piezoelectric response by engineering the elastic shear softening in these technologically important materials.

[1] M. E. Manley, et al. Nature Commun. 5:3683 doi: 10.1038/ncomms4683 (2014).

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