Probing local polar structures in PZN-xPT and PMN-xPT relaxor ferroelectrics with neutron and x-ray scattering

Abstract

"Polar nano-regions" (PNR) are nanometer sized local polar structures found in relaxor ferroelectric materials. They play key roles in understanding many relaxor properties. Here I will discuss some of our recent work using neutron scattering and x-ray diffraction to probe the properties of the PNR in two prototypical lead based perovskite relaxor systems Pb(Zn1/3Nb2/3)O3 (PZN), Pb(Mg1/3Nb2/3)O3 (PMN), and their solid solutions with PbTiO3 (PT). The local atomic structures in the PNR are rather complicated, consisting of polarizations/atomic shifts along both ⟨110⟩ and ⟨100⟩ directions. These different (components of) local polarizations respond differently to external electric fields along [001] or [111] directions; and are associated with phonons of different polarizations (T1 and T2 phonon modes). Our work also indicates that the PNR are dynamic at high temperatures well above the Curie temperature TC, and gradually freeze with cooling. A significant portion of the PNR has already become static before a long-range polar order can be established in the bulk. The short-range polar order in the PNR is quite robust, and persists when the system is cooled (with or without external electric field) into the ferroelectric phase. Because of the frustration between charge neutrality and lattice strain in relaxor systems, the low temperature phase of these relaxor materials usually does not have perfect long-range order, but rather a phase where both short-range and long-range polar orders coexist and compete.