Phonon anomalies induced by polar nano-regions in a relaxor Ferroelectric

Guangyong Xu (BNL, New York)

Inelastic neutron scattering was used to measure both acoustic and optic phonons polarized along <110> (T2 mode) in the relaxor ferroelectric compound PZN-4.5PT. In the low temperature rhombohedral phase, a single domain state was achieved by cooling the single crystal sample under an external electric field of 2 kV/cm along the [111] direction. Phonon measurements were performed near the (2,2,0) and (2,-2,0) Bragg peaks, which are equivalent in the high temperature cubic phase, but not so any more when the symmetry is lowered with the ferroelectric phase transition. We have found that the TA2 phonon couples closely to the diffuse scattering, which arises from polar nano-regions in the system. With the redistribution of diffuse scattering under the external field (see Ref. 1), a clear hardening of TA2 mode was observed near the (2,2,0) Bragg peak, while the TA2 mode near (2,-2,0) Bragg peak softens significantly and becomes over-damped. On the other hand, the TO2 mode was not significantly affected. Our results indicate local inhomogeneities such as the PNR can have direct and significant effects on the lattice dynamics and stability of the whole system in a macroscopic manner.

Ref. 1: ``Electric-field-induced redistribution of polar nano-regions in a relaxor ferroelectric'', Guangyong Xu, Z. Zhong, Y. Bing, Z.-G. Ye, and G. Shirane, Nature Materials 5, 134, (2006).

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