An atomic interaction is identified in all perovskite compounds, such as ABO 3 oxides, that can potentially result in unconventional structures. The term is harmonic in nature and couples the motions of the A cations with the rotations of the oxygen octahedra in the perovskite lattice.When strong enough, this coupling leads to hybrid normal modes that present both (anti)polar and rotational characters, which are keys to understand a variety of exotic phases. For example, we show that not only does this new coupling explain the long-period soft phonons characterizing prototype antiferroelectric PbZrO 3 , but it also provides us with an unified description of the complex antipolar structures of a variety of perovskites, including the possible occurrence of incommensurate phases.This coupling is further demonstrated to result, in the continuum limit, in a energy invariant adopting an analytical form that has been previously overlooked, to the best of our knowledge.
Materials possessing antipolar cation motions are currently receiving a lot of attention because they are fundamentally intriguing while being technologically promising. Most studies devoted to these complex materials have focused on their static properties or on their zone-center phonons. As a result, some important dynamics of antipolar cation distortions, such as the temperature behavior of their phonon frequencies, have been much less investigated, despite the possibility to exhibit unusual features. Here, we report the results and analysis of atomistic simulations revealing and explaining such dynamics for BiFeO 3 bulks being subject to hydrostatic pressure. It is first predicted that cooling such material yields the following phase transition sequence: the cubic paraelectric Pm3m state at high temperature, followed by an intermediate phase possessing long-range-ordered in-phase oxygen octahedral tiltings, and then the Pnma state that is known to possess antipolar cation motions in addition to in-phase and antiphase oxygen octahedral tiltings. Antipolar cation modes are found to all have high phonon frequencies that are independent of temperature in the paraelectric phase. On the other hand and in addition to antipolar cation modes increasing in number, some phonons possessing antipolar cation character are rather soft in the intermediate and Pnma states. Analysis of our data combined with the development of a simple model reveals that such features originate from a dynamical mixing between pure antipolar cation phonons and fluctuations of oxygen octahedral tiltings, as a result of a specific trilinear energetic coupling. The developed model can also be easily applied to predict dynamics of antipolar cation motions for other possible structural paths bringing Pm3m to Pnma states.
Despite extensive studies on size effects in ferroelectrics, how structures and properties evolve in antiferroelectrics with reduced dimensions still remains elusive. Given the enormous potential of utilizing antiferroelectrics for high‐energy‐density storage applications, understanding their size effects will provide key information for optimizing device performances at small scales. Here, the fundamental intrinsic size dependence of antiferroelectricity in lead‐free NaNbO3 membranes is investigated. Via a wide range of experimental and theoretical approaches, an intriguing antiferroelectric‐to‐ferroelectric transition upon reducing membrane thickness is probed. This size effect leads to a ferroelectric single‐phase below 40 nm, as well as a mixed‐phase state with ferroelectric and antiferroelectric orders coexisting above this critical thickness. Furthermore, it is shown that the antiferroelectric and ferroelectric orders are electrically switchable. First‐principle calculations further reveal that the observed transition is driven by the structural distortion arising from the membrane surface. This work provides direct experimental evidence for intrinsic size‐driven scaling in antiferroelectrics and demonstrates enormous potential of utilizing size effects to drive emergent properties in environmentally benign lead‐free oxides with the membrane platform.
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