A New Growth Mechanism of Polar Nanoregions by Phonon-Relaxation Mode-Coupling in a Relaxor Ferroelectric

Abstract

We have performed inelastic neutron scattering measurements to elucidate the interplay between soft phonons and polar nanoregions (PNRs) in a relaxor ferroelectric 0.7Pb(Mg 1/3 Nb 2/3 )O 3 –0.3PbTiO 3 . In the thermal evolution of transverse phonons, we observed successive damping for both optical (TO) and acoustic (TA) modes, which can be described by a mode-coupling between phonons and relaxation motion of polarization in PNRs. The TO phonon damping at higher temperatures is induced by PNRs which drag surrounding soft mode regions into relaxation mode ones: Pb ions shift along a polarization of each PNR from ideal cubic positions. On the other hand, the TA phonon damping at lower temperatures corresponds to the transverse relaxation motion of polarization within PNRs. Such new growth mechanism of PNRs not only explains the relaxor properties over a wide temperature range but also unifies different approaches based on order–disorder-type and displacive-type lattice dynamics.