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

I Levin,M G Tucker,G Cibin,K Chapman,R L Paul,M Eremenko,V Krayzman

doi:10.1038/s41598-017-15937-x

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.

Highlights

Perovskite-like potassium sodium niobates, K1−xNaxNbO3 (KNN), are on the short list of commercially viable lead-free piezoelectrics[1,2]
The K- and Na-rich parts of the diagram are thought to be divided by a morphotropic phase boundary (MPB) at x ≈ 0.5, which separates the ferroelectric K-rich O and Na-rich monoclinic M phase fields
According to the published phase diagram[2,3,4], the untilted M structure is stable over a narrow compositional range between x ≈ 0.5 and x ≈ 0.6, while for higher Na content it transforms to another monoclinic phase, which combines ferroelectric displacements with in-phase octahedral rotations about the pseudo-cubic axis perpendicular to the polarization direction

Summary

Introduction

Perovskite-like potassium sodium niobates, K1−xNaxNbO3 (KNN), are on the short list of commercially viable lead-free piezoelectrics[1,2]. The birefringence measurements on single crystals of KNN13 suggested a different monoclinic structure (MA/MB) for x = 0.5, in which the polarization vector deviates from the mirror plane normal to the a-axis in the Amm[2] setting. None of these monoclinic models have been validated by structural refinements, presumably because of the vanishingly small, if real, distortions from the orthorhombic cell

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Conclusion