Local atomic order and hierarchical polar nanoregions in a classical relaxor ferroelectric (original) (raw)
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Polar nanoregions and diffuse scattering in the relaxor ferroelectric PbMg_{1/3}Nb_{2/3}O_{3}
Physical Review B, 2012
We present a fully atomistic model of polar nanoregions (PNRs) in the relaxor ferroelectric PbMg 1/3 Nb 2/3 O 3. Our molecular dynamics calculations reproduce both the characteristic form of the neutron diffuse scattering distribution and its temperature dependence. A shell model was used with a modified version of a published interatomic potential that was based on ab initio calculations. The parameters of this potential were optimized for the present work to provide a better description of the O atom interactions, as these are particularly important for neutron scattering. At high temperatures the Pb ions are displaced from their mean site positions in a direction that has an isotropic distribution, but at low temperatures the distribution condenses into eight localized sites displaced from the average position along each of the eight possible 1 1 1 directions. At intermediate temperatures (300 K) the distribution is cuboidal with some preference for 1 1 1 displacements but with all possible displacement directions present. Longitudinal correlations between the displacements of Pb-Mg/Nb and Pb-O increase monotonically in magnitude as the temperature changes from 700 K to 10 K with the sign of the Pb-O correlation being negative. At low temperatures this increase in correlation results in polar nanoregions that are clearly visible in plots of the local structure, although the exact form of these domains is more difficult to visualize. We show that the form of these PNRs can be revealed by an examination of conditional displacement distributions at low temperatures. Therein a strongly anisotropic cooperative displacement behavior is found. Remnants of this correlation pattern persist at much higher temperatures, but progressively a relatively smaller proportion of the Pb ions appears to be taking part and there is a substantially larger random component. It seems likely that the onset of the characteristic structured diffuse scattering at around 400 K coincides with the appearance of this cooperative displacement behavior.
Physical Review B, 2005
Large scale molecular dynamics simulations of a first-principles Hamiltonian for the model relaxor ferroelectric, P b(Sc 1/2 N b 1/2)O3, were used to determine the nature of correlations between shortrange chemical-and polar nano-regions that are thought to be essential to the glassy low-T behavior exhibited by some relaxors. Relative to chemically disordered regions (CDR), chemically ordered regions (COR) exhibit enhanced polarization, and polarization-fluctuations at all temperatures. Magnitudes of pairwise cluster-cluster polarization correlations follow the trend: COR-COR-¿ COR-CDR-¿ CDR-CDR-correlations. This result implies that the characteristic length-scale for polar nano-regions is the same as that for chemical short-range order.
2011
The complex structure of relaxor ferroelectrics comprises polar nanoregions (PNRs) which appear upon cooling below the Burns temperature and quenched compositional (chemical) disorder. The relation between the polar nanostructure and compositionally ordered regions (CORs) often observed in relaxors has been the subject of extensive theoretical investigations; however, the experimental data, especially concerning Pb(B0 1=3B00 2=3)O3-type complex perovskite relaxors, are rather
The relation of local order to material properties in relaxor ferroelectrics
Nature materials, 2018
Correlating electromechanical and dielectric properties with nanometre-scale order is the defining challenge for the development of piezoelectric oxides. Current lead (Pb)-based relaxor ferroelectrics can serve as model systems with which to unravel these correlations, but the nature of the local order and its relation to material properties remains controversial. Here we employ recent advances in diffuse scattering instrumentation to investigate crystals that span the phase diagram of PbMgNbO-xPbTiO (PMN-xPT) and identify four forms of local order. From the compositional dependence, we resolve the coupling of each form to the dielectric and electromechanical properties observed. We show that relaxor behaviour does not correlate simply with ferroic diffuse scattering; instead, it results from a competition between local antiferroelectric correlations, seeded by chemical short-range order, and local ferroic order. The ferroic diffuse scattering is strongest where piezoelectricity is ...
Phonon localization drives polar nanoregions in a relaxor ferroelectric
Nature Communications, 2014
Relaxor ferroelectrics exemplify a class of functional materials where interplay between disorder and phase instability results in inhomogeneous nanoregions. Although known about for 30 years, there is no definitive explanation for polar nanoregions (PNRs). We show that ferroelectric-phonon localization drives PNRs in relaxor ferroelectric (Pb(Mg1/3Nb2/3)O3)0.7-(PbTiO3)0.3 (PMN-30%PT) using neutron scattering. At the frequency of a preexisting resonance mode, nanoregions of standing ferroelectric phonons develop with a coherence length equal to one wavelength and the PNR size. Anderson-localization of ferroelectric phonons by resonance modes explain our observations and, with nonlinear slowing, the PNRs and relaxor properties. Phonon localization at additional resonances near the zone edges explains competing antiferroelectric distortions known to occur at the zone edges. Our results indicate the size and shape of PNRs are not dictated by complex structural details, as always assumed, but by phonon resonance wavevectors. This discovery could guide the design of next generation relaxor ferroelectrics.
Mode coupling and polar nanoregions in the relaxor ferroelectric Pb (Mg 1/3 Nb 2/3) O 3
We present a quantitative analysis of the phonon lineshapes obtained by neutron inelastic scattering methods in the relaxor ferroelectric Pb(Mg 1/3 Nb 2/3 )O3 (PMN). Differences in the shapes and apparent positions of the transverse acoustic (TA) and transverse optic (TO) phonon peaks measured in the and Brillouin zones at 690 K are well described by a simple model that couples the TA and soft TO modes in which the primary parameter is the wave vector and temperature-dependent TO linewidth Γ(q, T ). This mode-coupling picture provides a natural explanation for the uniform displacements of the polar nanoregions, discovered by Hirota et al., as the PNR result from the condensation of a soft TO mode that also contains a large acoustic component.
Local Lattice Dynamics and the Origin of the Relaxor Ferroelectric Behavior
Physical Review Letters, 2008
Relaxor ferroelectricity is observed in many strongly disordered ferroelectric solids. However, the atomistic mechanism of the phenomenon, particularly at high temperatures, is not well understood. In this Letter we show the local lattice dynamics as the origin of relaxor ferroelectricity through the first use of the dynamic pair-density function determined by pulsed neutron inelastic scattering. For a prototypical relaxor ferroelectric, PbMg 1=3 Nb 2=3 O 3 , we demonstrate that the dynamic local polarization sets in around the so-called Burns temperature through the interaction of off-centered Pb ions with soft phonons, and the slowing down of local polarization with decreasing temperature produces the polar nanoregions and the relaxor behavior below room temperature.