Study of Slow Lattice Dynamics in Relaxor Ferroelectric PMN–30%PT by Neutron Spin Echo Technique

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We have used the neutron spin echo technique to characterize the dynamics of polar nanoregions (PNRs) in the relaxor ferroelectric ($1-x$)Pb(Mg1/3Nb2/3)O3–$x$PbTiO3 (PMN–$x$PT) with $x=30$% ($T_{\text{C}}\sim 400$ K). We measured the normalized intermediate scattering function $I(Q,t)/I(Q,0)$ for a butterfly-shaped diffuse scattering around the (100) Bragg point, which is associated with atomic shifts, i.e., polarizations in PNRs. For $T\geq 500$ K, $I(Q,t)/I(Q,0)$ shows a peak structure at 0.5 ns, which is ascribed to the coexistence of echo signals from a relaxation mode and those from a low-frequency vibrational mode with $\omega\sim 26$ μeV. Such a low-frequency vibrational mode for $\langle 110\rangle$ directions indicate a flat free-energy surface between the rhombohedral and tetragonal phases. On cooling below 450 K, the peak structure in $I(Q,t)/I(Q,0)$ disappears and diffuse scattering intensity increases, which indicates that the condensed or pinned low-frequency mode for $\langle 110\rangle$ directions is the origin of PNRs.
2010-03-15