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.
Original language | English |
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Article number | 174113 |
Journal | Physical Review B |
Volume | 105 |
Issue number | 17 |
DOIs | |
State | Published - May 1 2022 |
Funding
The work described in this paper was partially supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. CityU 11306720). Funding support from City University of Hong Kong (Project No. CityU 7005644) is gratefully acknowledged. F.M., D.R.S., and M.R.V.J. thank the Danish Agency for Science, Technology, and Innovation for funding the instrument center DanScatt. Affiliation with the Center for Integrated Materials Research (iMAT) at Aarhus University is gratefully acknowledged. We acknowledge MAX IV Laboratory for time on DanMAX. Research conducted at MAX IV, a Swedish national user facility, is supported by the Swedish Research council under Contract 2018–07152, the Swedish Governmental Agency for Innovation Systems under Contract No. 2018–04969, and Formas under Contract No. 2019–02496. DanMAX is funded by the NUFI Grant No. 4059-00009B. A portion of this research used resources at the Spallation Neutron Source, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory (Grant No. IPTS-27376 for POWGEN and NOMAD experiments).