Atomic structural mechanism for ferroelectric-antiferroelectric transformation in perovskite NaNbO3

Sodium niobate (${\mathrm{NaNbO}}_{3}$ 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 ${\mathrm{NaNbO}}_{3}$ 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 $Q$ ($P{2}_{1}ma$) and AFE $P$ ($Pbma$) phases in the temperature range of $300\phantom{\rule{0.16em}{0ex}}\mathrm{K}\ensuremath{\le}T\ensuremath{\le}615\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, while PDF analysis indicated that the local structure $(r<8\phantom{\rule{0.16em}{0ex}}\AA{})$ is better described by a $P{2}_{1}ma$ 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.