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Domain walls in ferroelectric materials have tantalizing potential in disruptive memory and reconfigurable nanoelectronics technologies. Here, we demonstrate a ferroelectric domain wall switch with three distinct addressable resistance states. The device operation hinges on fully-controllable and reversible conformational changes of the domain wall. As validated by atomistic simulations consistent with the experiments, using electric field, we alter the shape -and hence the charge state -of the domain wall, and ultimately its conduction. Sequential nanoscale transitions of the walls are visualized directly using stroboscopic-piezoresponse force microscopy and Kelvin probe microscopy. Anisotropic head-to-head domain wall injection, stabilized by the majority carrier type of the ferroelectric, BiFeO 3 , is identified as the key factor that enables conformational control.

In light of directives around the world to eliminate toxic materials in various technologies, finding lead-free materials with high piezoelectric responses constitutes an important current scientific goal. As such, the recent discovery of a large electromechanical conversion near room temperature in (1−x)Ba(Zr0.2Ti0.8)O3−x(Ba0.7Ca0.3)TiO3 compounds has directed attention to understanding its origin. Here, we report the development of a large-scale atomistic scheme providing a microscopic insight into this technologically promising material. We find that its high piezoelectricity originates from the existence of large fluctuations of polarization in the orthorhombic state arising from the combination of a flat free-energy landscape, a fragmented local structure, and the narrow temperature window around room temperature at which this orthorhombic phase is the equilibrium state. In addition to deepening the current knowledge on piezoelectricity, these findings have the potential to guide the design of other lead-free materials with large electromechanical responses.

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Atomistic effective Hamiltonian simulations are used to investigate electrocaloric (EC) effects in the lead-free Ba(Zr0.5Ti0.5)O3 (BZT) relaxor ferroelectric. We find that the EC coefficient varies non-monotonically with the field at any temperature, presenting a maximum that can be traced back to the behavior of BZT's polar nanoregions. We also introduce a simple Landau-based model that reproduces the EC behavior of BZT as a function of field and temperature, and which is directly applicable to other compounds. Finally, we confirm that, for low temperatures (i.e., in non-ergodic conditions), the usual indirect approach to measure the EC response provides an estimate that differs quantitatively from a direct evaluation of the field-induced temperature change.

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The change of shape under illumination by visible light, called photostriction, is investigated in the classical ferroelectrics barium titanate and lead titanate. By means of the ∆SCF method, the use of first-principle calculations confirms that the converse piezoelectric effect is the main driving force of the photostriction of the polar axis in those materials. As a result, when compared to barium titanate and bismuth ferrite, lead titanate is a better photostrictive material in the direction of the polar axis, due to its larger longitudinal piezoelectric constant. On the other hand, in directions transverse to the polar axis, photo-induced electronic pressure can also become a sizable contribution that can either compete or cooperate with the piezoelectric effect, depending on the transitions involved. A simple Landau model is further developed and shows reasonable qualitative agreement with results from ∆SCF calculations, which is promising for a fast screening of materials with high photostrictive effects.

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