Origin of diffuse scattering in relaxor ferroelectrics

Abstract

High-pressure and variable temperature single crystal synchrotron X-ray measurements combined with first-principles based molecular dynamics simulations study diffuse scattering in the relaxor ferroelectric system PSN (PbSc 1/2 Nb 1/2 O3). Constant temperature experiments show pressure induced transition to the relaxor phase at different temperatures characterized by butterfly and rod shaped diffuse scattering around the {h00} and {hh0} Bragg spots, respectively. The simulations reproduce the observed diffuse scattering features as well as their pressure-temperature behavior, and show that they arise from polarization correlations between chemically-ordered regions, which in previous simulations were shown to behave as polar nanoregions. Simulations also exhibit radial diffuse scattering (elongated towards and away from Q=(000)), that persists even in the paraelectric phase, consistent with previous neutron experiments on (PbMg 1/3 Nb 2/3 O3) (PMN).Single crystal relaxors have huge electromechanical coupling, and show much promise for ultrasonic transducer applications [1]. They have broad frequency and temperature dependent dielectric maxima, which in the special case of relaxor-ferroelectrics [2,3] drops to a much lower value below the ferroelectric transition temperature (T F E ). The origin of the relaxor phase has been a topic of intense research for over a decade. From refractive index measurements, Burns et al. [4] suggested that formation of polar clusters below a characteristic temperature, now called the Burns temperature (T d ) [4], gives rise to dielectric dispersion, which was confirmed experimentally [5]. These clusters thought to be a few nanometers in size are called polar nanoregions (PNRs).Recent X-ray and neutron experiments show characteristic shapes of diffuse scattering in the relaxor phase of several lead based relaxors [6,7,8,9] absent in their paraelectric or ferroelectric phases. The main observed feature is anisotropic diffuse scattering around the Bragg peaks along <110> directions. The ferroelectric phase shows weak streaks similar to BaTiO 3 and KNbO 3 , but rotated by 45-degrees. The paraelectric phase only shows radial diffuse scattering.Several hypotheses previously advanced to explain the characteristic shapes [6,10,11,12], invoke some type of artificial anisotropic features such as anisotropic strain, correlated atomic shifts, or domain boundaries to generate the experimentally observed anisotropic diffuse scattering features. Fitting the shapes of diffuse scattering features is not sufficient to uniquely determine the nature of the microstructural feature that give rise to them. Incorporating realistic polarization fluctuations via first-principles derived models provide a basis for clarifying the nature of the PNR in relaxors and experimental diffuse scattering observations provide a critical test for any theoretical model. In this Letter, we clarify the microstructural origin of diffuse scattering features in the relaxor-and the paraelectric-phases of PSN, and related materi...

Anharmonic atomic vibrations in the relaxor ferroelectric Pb(Mg1/3Nb2/3)OYamanaka1, Ahart2, Nakamoto3 et al. 2012Phys. Rev. BSelf Cite8050View full textAdd to dashboardCite

“…These results are consistent with first-principles MD simulations [7]. Heights of the positive and negative peaks decrease in magnitude with increasing pressure and the peak positions are displaced from the centers of the atoms.…”

“…In the present study, higher rank tensors were introduced to constrain the atomic displacement due to thermal vibrations of the atoms We found a hydrostatic compression of the unit cell for experimental pressures up to 7.3 GPa, without any structural transition, as shown in Fig. 1 Diffuse streaks found along the <110> direction in the (100) reciprocal lattice plane can be interpreted with respect to the correlations between the displacements of the Pb ion within the PNR clusters [7]. One may assume that the chemical disorder between Nb and Mg in the B site would produce a larger temperature factor than that in the A site, because heavy atoms such as Pb generally show a small temperature factor.…”

“…It was observed that the polarization was large in the chemically ordered regions (CORs), and almost close to zero in the CDRs, suggesting that the PNRs could be chemically ordered regions. Further studies based on the same MD model [7] revealed that the diffuse scattering, another feature of the relaxors, could be attributed to strong correlations between the displacement of the Pb ion within the PNR clusters, and such correlations between Pb ions prefer a particular orientation within the crystal structure. A number of studies have addressed the phenomenon of diffuse scattering [13].…”

“…In addition, their high permittivity and high piezoelectric constants make them suitable for applications in devices for sonar or medical imaging [1]. While extensive theoretical [3][4][5][6][7] and experimental studies [1][2][3][7][8][9][10][11][12] have advanced the fundamental understanding of relaxor behavior, their properties are still poorly understood. The difficulties stem from the complexity of these materials, which have a high degree of compositional, structural and polar disorder.…”

Strain Mechanisms in Lead-Free Ferroelectrics for Actuators

“…64 These PNRs can be regarded as clusters on the order of 10 nm composed of thermally activated dipolar entities that originate from local deviations in crystal structure and/or chemical heterogeneities in the A-and/or B-site cation sublattices. [80][81][82][83][84] Several attempts have been made to rationalize relaxors within one theoretical model. However, so far no general agreement in their microscopic picture has been achieved.…”