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Local structure, pseudosymmetry, and phase transitions in Na_{1/2}Bi_{1/2}TiO_{3}–K_{1/2}Bi_{1/2}TiO_{3} ceramics

Levin, I. and Reaney, I. M. and Anton, Eva-Maria and Jo, Wook and Rödel, Jürgen and Pokorny, J. and Schmitt, L. A. and Kleebe, H.-J. and Hinterstein, Manuel and Jones, J. L. (2013):
Local structure, pseudosymmetry, and phase transitions in Na_{1/2}Bi_{1/2}TiO_{3}–K_{1/2}Bi_{1/2}TiO_{3} ceramics.
87, In: Physical Review B, (2), APS, pp. 024113(1-11), ISSN 1098-0121, [Online-Edition: http://dx.doi.org/10.1103/PhysRevB.87.024113],
[Article]

Abstract

The structural behavior of ceramic solid solutions (1−x)Na1/2Bi1/2TiO3–xK1/2Bi1/2TiO3 (NBT-KBT) was studied using high-resolution powder diffraction and transmission electron microscopy. A temperature-independent morphotropic phase boundary (MPB) separating NBT-like pseudorhombohedral (R) and KBT-like pseudotetragonal (T) phases was observed at x ≈ 0.2. For x<0.2, both local and average room-temperature structures are similar to those in NBT. Simultaneous long-range antiphase and short-range in-phase octahedral rotations average, resulting in effective antiphase a−a−c− tilting, which yields monoclinic symmetry when probed by x-ray diffraction (XRD). For these compositions, polar ordering is coupled to antiphase octahedral rotations so that tilting and ferroelectric (FE) domains coincide. Compositions with x>0.2 exhibit a tetragonal-like distortion; however, complex splitting of reflections in XRD patterns suggests that the actual symmetry is lower than tetragonal. For 0.2≤x≤0.5, in-phase octahedral tilting a0b+a0 (or a+b0b0) is present but confined to the nanoscale, while for x>0.5 the structure becomes untilted. In-phase tilting evolves above the ferroelectric transition and occurs around a nonpolar (a or b) axis of the average T structure. The onset of polar order has no significant effect on the coherence length of in-phase tilting, which suggests only weak coupling between the two phenomena. The average symmetry of the T phase is determined by the effective symmetry (Imm2) of assemblages of coherent in-phase tilted nanodomains. Near the MPB, the coexistence of extended R- and T-like regions is observed, but lattice distortions within each phase are small, yielding narrow peaks with a pseudocubic appearance in XRD. The temperature of the FE phase transition exhibits a minimum at the MPB. The structured diffuse scattering observed in electron diffraction patterns for all the compositions suggests that polar order in NBT-KBT solid solutions is modulated away from the average displacements refined using powder diffraction data.

Item Type: Article
Erschienen: 2013
Creators: Levin, I. and Reaney, I. M. and Anton, Eva-Maria and Jo, Wook and Rödel, Jürgen and Pokorny, J. and Schmitt, L. A. and Kleebe, H.-J. and Hinterstein, Manuel and Jones, J. L.
Title: Local structure, pseudosymmetry, and phase transitions in Na_{1/2}Bi_{1/2}TiO_{3}–K_{1/2}Bi_{1/2}TiO_{3} ceramics
Language: English
Abstract:

The structural behavior of ceramic solid solutions (1−x)Na1/2Bi1/2TiO3–xK1/2Bi1/2TiO3 (NBT-KBT) was studied using high-resolution powder diffraction and transmission electron microscopy. A temperature-independent morphotropic phase boundary (MPB) separating NBT-like pseudorhombohedral (R) and KBT-like pseudotetragonal (T) phases was observed at x ≈ 0.2. For x<0.2, both local and average room-temperature structures are similar to those in NBT. Simultaneous long-range antiphase and short-range in-phase octahedral rotations average, resulting in effective antiphase a−a−c− tilting, which yields monoclinic symmetry when probed by x-ray diffraction (XRD). For these compositions, polar ordering is coupled to antiphase octahedral rotations so that tilting and ferroelectric (FE) domains coincide. Compositions with x>0.2 exhibit a tetragonal-like distortion; however, complex splitting of reflections in XRD patterns suggests that the actual symmetry is lower than tetragonal. For 0.2≤x≤0.5, in-phase octahedral tilting a0b+a0 (or a+b0b0) is present but confined to the nanoscale, while for x>0.5 the structure becomes untilted. In-phase tilting evolves above the ferroelectric transition and occurs around a nonpolar (a or b) axis of the average T structure. The onset of polar order has no significant effect on the coherence length of in-phase tilting, which suggests only weak coupling between the two phenomena. The average symmetry of the T phase is determined by the effective symmetry (Imm2) of assemblages of coherent in-phase tilted nanodomains. Near the MPB, the coexistence of extended R- and T-like regions is observed, but lattice distortions within each phase are small, yielding narrow peaks with a pseudocubic appearance in XRD. The temperature of the FE phase transition exhibits a minimum at the MPB. The structured diffuse scattering observed in electron diffraction patterns for all the compositions suggests that polar order in NBT-KBT solid solutions is modulated away from the average displacements refined using powder diffraction data.

Journal or Publication Title: Physical Review B
Volume: 87
Number: 2
Publisher: APS
Divisions: 11 Department of Materials and Earth Sciences > Earth Science > Geo-Material-Science
11 Department of Materials and Earth Sciences > Material Science > Nonmetallic-Inorganic Materials
11 Department of Materials and Earth Sciences > Earth Science
11 Department of Materials and Earth Sciences > Material Science
11 Department of Materials and Earth Sciences
Date Deposited: 04 Feb 2013 08:39
Official URL: http://dx.doi.org/10.1103/PhysRevB.87.024113
Identification Number: doi:10.1103/PhysRevB.87.024113
Funders: This work benefited from the use of the Advanced Photon Source supported by the Department of Energy Office of Basic Energy Sciences under Contract No DE-AC02-06CH11357., J.J. acknowledges funding support from the US National Science Foundation under Award No. DMR-0746902 and the US Department of the Army under Contract No. W911NF-09-1-0435., Contributions by the Institute of Materials Science at Darmstadt were supported through the state center ADRIA.
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