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Coupling of emergent octahedral rotations to polarization in (K,Na)NbO3 ferroelectrics

DOI: 10.1038/s41598-017-15937-x DOI Help

Authors: I. Levin (National Institute of Standards and Technology) , V. Krayzman (National Institute of Standards and Technology) , G. Cibin (Diamond Light Source) , M. G. Tucker (Spallation Neutron Source) , M. Eremenko (National Institute of Standards and Technology) , K. Chapman (Advanced Photon Source) , R. L. Paul (National Institute of Standards and Technology)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Scientific Reports , VOL 7

State: Published (Approved)
Published: November 2017
Diamond Proposal Number(s): 13430

Open Access Open Access

Abstract: Perovskite potassium sodium niobates, K1−xNaxNbO3, are promising lead-free piezoelectrics. Their dielectric and piezoelectric characteristics peak near x = 0.5, but the reasons for such property enhancement remain unclear. We addressed this uncertainty by analyzing changes in the local and average structures across the x = 0.5 composition, which have been determined using simultaneous Reverse Monte Carlo fitting of neutron and X-ray total-scattering data, potassium EXAFS, and diffuse-scattering patterns in electron diffraction. Within the A-sites, Na cations are found to be strongly off-centered along the polar axis as a result of oversized cube-octahedral cages determined by the larger K ions. These Na displacements promote off-centering of the neighboring Nb ions, so that the Curie temperature and spontaneous polarization remain largely unchanged with increasing x, despite the shrinking octahedral volumes. The enhancement of the properties near x = 0.5 is attributed to an abrupt increase in the magnitude and probability of the short-range ordered octahedral rotations, which resembles the pre-transition behavior. These rotations reduce the bond tension around Na and effectively soften the short Na-O bond along the polar axis – an effect that is proposed to facilitate reorientation of the polarization as external electric field is applied.

Subject Areas: Materials

Instruments: B18-Core EXAFS

Other Facilities: Advanced Photon Source