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- Htet, Cho Sandar, et al.
(författare)
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Atomic structural mechanism for ferroelectric-antiferroelectric transformation in perovskite NaNbO3
- 2022
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Ingår i: Physical Review B. - 2469-9950. ; 105:17
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Tidskriftsartikel (refereegranskat)abstract
- Sodium niobate (NaNbO3 or NN) is described as "the most complex perovskite system,"which exhibits transitions between, as well as coexistence of, several ferroelectrics (FE) and antiferroelectric (AFE) phases at different temperatures. Recently, solid solutions of NN with stabilized AFE phases(s) have gained attention for energy-related applications, such as high-density energy storage and electrocaloric cooling. A better understanding of the atomic mechanisms responsible for AFE/FE phase transitions in NaNbO3 can enable a more rational design of its solid-solution systems with tunable functional properties. Here, we have investigated changes in the average and local atomic structure of NN using a combination of x-ray/neutron diffraction and neutron pair-distribution function (PDF) analyses. The Rietveld refinement of the x-ray/neutron-diffraction patterns indicates a coexistence of the FE Q (P21ma) and AFE P (Pbma) phases in the temperature range of 300K≤T≤615K, while PDF analysis indicated that the local structure (r<8Å) is better described by a P21ma symmetry. Above 615 K, the average structure transitions to an AFE R phase (Pmmn or Pnma), while PDF analysis shows an increased disordering of the octahedral distortions and Na displacements at the local scale. These results indicate that the average P/Q/R phase transitions in NN can be described as a result of complex ordering of distorted octahedral tilts at the nanoscale and off-centered displacements of the Na atoms.
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| 2. |
- Htet, Cho Sandar, et al.
(författare)
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Effect of Local Structural Distortions on Antiferroelectric-Ferroelectric Phase Transition in Dilute Solid Solutions of K xNa1- xNbO3
- 2022
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Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 61:50, s. 20277-20287
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Tidskriftsartikel (refereegranskat)abstract
- The fundamental principles that govern antiferroelectric (AFE)-ferroelectric (FE) transitions are not well understood for many solid solutions of perovskite compounds. For example, crystal chemical considerations based on the average Goldschmidt tolerance factor or ionic polarizability do not precisely predict the boundary between the AFE and FE phases in dilute solid solutions of alkali niobates, such as KxNa1-xNbO3(x ≤ 0.02). Here, based on detailed structural analysis from neutron total scattering experiments, we provide insights about how the relative local distortions around the A- and B-sites of the ABO3perovskite structure affect the AFE/FE order of the average crystallographic phases in KxNa1-xNbO3. We show that a higher (lower) ratio of B-site-centered distortions over A-site-centered distortions drives transition toward a long-range FE (AFE) phase, which is based on a competition between the long-range polarizing field of the Nb-O dipoles and the disordering effect of local distortions around the A-site. Our study provides a predictive tool for designing complex solid-solution perovskites with tunable (anti)ferroelectric polarization properties, which can be of interest for various energy-related applications such as high-density energy storage and solid-state cooling.
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| 3. |
- Htet, Cho Sandar, et al.
(författare)
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Local structural mechanism for enhanced energy storage properties in heterovalent doped NaNbO3 ceramics
- 2024
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Ingår i: Journal of the European Ceramic Society. - 0955-2219. ; 44:3, s. 1597-1609
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Tidskriftsartikel (refereegranskat)abstract
- In recent years, there is a growing interest for new lead-free oxides with reversible antiferroelectric (AFE)-ferroelectric (FE) phase transition for high-power energy-storage applications. NaNbO3-based ceramics are particularly attractive due to their easy synthesis and cost-effectiveness. In order to stabilize reversible AFE-FE phase transition, NaNbO3 is doped with a combination of heterovalent substitutions, although the underlying structural mechanism for the same is poorly understood. Here, we investigated local and average structures of Ca/Zr doped NaNbO3 using neutron total scattering. We show that Ca/Zr doping increases the average AFE phase (Pbma) fraction, however, the material remains as a composite of both FE (P21ma) and AFE regions. Analysis of local structure suggests that increase in the long-range AFE phase results from more extensive twinning of local FE regions, due to introduced charge disorder. We propose that enhanced energy-storage properties of Ca/Zr-doped NaNbO3 arises from localized twin boundary motion between the defect-induced pinning centers.
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