Realizing the potential of perovskite ferroelectrics in electronics demands a comprehensive understanding of their structure down to atomic level. This involves exploring various structural imperfections, including point defects, dislocations, and domain walls, which interact and significantly impact the material's behavior. Dynamic insights into these imperfections are crucial for comprehending the macroscopic response of ferroelectric materials.
Utilizing advanced atomic-scale scanning transmission electron microscopy with specialized holders for in situ analysis and a 4D STEM pixelated detector, we can directly observe and analyse defect types, strain, electric field distribution, and dynamic responses to external stimuli.
The presentation focuses on the static and dynamic local structural characteristics of lead-free ferroelectrics, specifically bismuth ferrite and potassium sodium niobate. Examining different types of domain walls in potassium sodium niobate-based materials and various domain walls in bismuth ferrite-based materials, such as needle-like, zigzag, lamellar, charged, and uncharged walls, provides valuable insights. In the case of potassium sodium niobate, our research digs into studying domain growth, coalescence, and the interaction of different types of domain walls under voltage, directly observed within the microscope. For bismuth ferrite, we concentrate on domain walls with distinct mechanical and electrical properties compared to the matrix material, exploring their interaction with charged point defects under applied voltage.
The presentation emphasizes that the dynamics of domain walls, particularly in the presence of defects, reveal unique and complex phenomena at the atomic level. This involves variations in defect distribution, changes in bound charge distribution, distortions in the unit cell, and subsequent strain redistribution at domain walls under electrical stimuli.

Fluorite-based crystalline materials have the general chemical formula AX2 (A = Ca, Hf, Zr, Ce, while X = F, O) and find wide application in fuel cells, electroceramics, oxygen sensors and exhaust reduction systems. Despite their rather simple and prototypical crystallographic structure, “fluorites” remain the subject of much academic research and unexpected scientific discoveries [1], such as ferroelectricity in HfO2-based thin films or large electromechanical coupling in polycrystalline Gd-doped CeO2-x (CGO) ceramics/films [2]. Moreover, a recent discovery has demonstrated the potential to induce significant piezoelectric effects in centrosymmetric CGO by breaking symmetry through the controlled introduction of ionic defects, specifically oxygen vacancies (VO) [2]. Similarly, it was found that the motion and redistribution of defects induced by an electric field result in notable changes in the dielectric permittivity of the system, which are closely associated with the magnitude of the electromechanical effects [3]. Hence, controlling the presence of VO in the CGO and similar oxides is key for achieving significant permittivity and electromechanical effects. Our previous findings show that the control of VO contents in the epitaxial CGO(001) films grown on Nb-doped SrTiO3(001) single crystal using pulsed laser deposition (PLD) is highly limited, regardless of the oxygen environment (oxygen partial pressure, PO), during high-temperature growth (e.g., T = 700 °C) [4]. In our current study, we overcome this limitation by introducing a new, effective method to adjust the VO amounts in these films by employing a two-step growth process. The strong variation in the VO content observed in these engineered CGO films illustrates the emergence of significant apparent dielectric permittivity, thus enabling the generation of substantial electromechanical coupling and piezoelectric effect [4].

Reference
[1] U. Schroeder, et al., Nat. Rev. Mater., 7(8), 653–669 (2022).
[2] D.-S. Park, et al., Science, 375, 653–657 (2022).
[3] D. Damjanovic, Rep. Prog. Phys., 61(9), 1267–1324 (1998).
[4] A. Palliotto, JPhys Energy, In Press (2024).

Helium implantation – or more generally ion implantation – has been used for a long time as a means to tune the structure and properties of materials. Implantation typically impacts the material via two different mechanisms: the creation of defects, which can even cause amorphization at high ion doses, and a “negative pressure” effect, whereby helium is inserted interstitially in the crystal structure and causes an inflation of the unit cell. In ferroic oxides, both mechanisms are a priori relevant for property tuning. Defect engineering by helium implantation has notably been used successfully on PZT thin films. The negative pressure effect has also been demonstrated for example in BiFeO3 epitaxial thin films. In epitaxial films, the in-plane strain remains fixed by the substrate even upon implantation so that the effect is an elongation of the out-of-plane lattice constant. In general, negative pressure is particularly appealing as a way to induce structural phase transitions, particularly to induce polar phases in non-polar materials.

In this work, we demonstrate the use of helium implantation on polycristalline thin films synthesized by the sol-gel route. Two types of films are investigated: ferroelectric pure and co-doped BiFeO3 and the antiferroelectric PbZrO3. We demonstrate that helium implantation also leads to signficant modification of the films structure and properties associated to an out-of-plane elongation of the crystal structure. Phase transitions are observed in both cases above a critical Helium dose. The respective roles of defect creation vs. negative pressure will be discussed.