Giant shape memory and domain memory effects in antiferroelectric single crystals

We report irreversible and reversible antiferroelectric-ferroelectric phase transitions, shape memory and domain memory effects in (Pb,La)(Zr,Sn,Ti)O3 antiferroelectric tetragonal single crystals. We find that electric-field-induced antiferroelectric to ferroelectric phase transition is an irreversible process at temperatures below depolarization temperature (~50 °C), and achieve a giant shape memory strain value of 0.70% at room temperature. With the help of in situ polarized light microscope, we discover a reversible phase transition between antiferroelectric and ferroelectric phases and domain memory effect during the electric field cycling above the depolarization temperature. In addition, single crystal X-ray diffraction results reveal that field-induced transformations between incommensurate antiferroelectric and commensurate ferroelectric modulations enable the emergence of shape and domain memory effects. A physical picture is proposed to explain these phase-transformation-mediated memory effects. Their discovery is expected to stimulate a systematic research upsurge of domain engineering and structure–property relationships in the quest for developing new antiferroelectric or ferroelectric devices.

Challenges and Applications to Operando and In Situ TEM Imaging and Spectroscopic Capabilities in a Cryogenic Temperature Range

Martial Duchamp, Reinis Ignatans, Vasiliki Tileli

In this Account, we describe the challenges and promising applications of transmission electron microscopy (TEM) imaging and spectroscopy at cryogenic temperatures. Our work focuses on two areas of application: the delay of electron-beam-induced degradation and following low-temperature phenomena in a continuous and variable temperature range. For the former, we present a study of LiMn1.5Ni0.5O4 lithium ion battery cathode material that undergoes electron beam-induced degradation when studied at room temperature by TEM. Cryogenic imaging reveals the true structure of LiMn1.5Ni0.5O4 nanoparticles in their discharged state. Improved stability under electron beam irradiation was confirmed by following the evolution of the O K-edge fine structure by electron energy-loss spectroscopy. Our results demonstrate that the effect of radiation damage on discharged LiMn1.5Ni0.5O4 was previously underestimated and that atomic-resolution imaging at cryogenic temperature has a potential to be generalized to most of the Li-based materials and beyond. For the latter, we present two studies in the imaging of low-temperature phenomena on the local scale, namely, the evolution of ferroelectric and ferromagnetic domains walls, in BaTiO3 and Y3Fe5O12 systems, respectively, in a continuous and variable temperature range. Continuous imaging of the phase transition in BaTiO3, a prototypical ferroelectric system, from the low-temperature orthorhombic phase continuously up to the centrosymmetric high-temperature phase is shown to be possible inside a TEM. Similarly, the propagation of domain walls in Y3Fe5O12, a magnetic insulator, is studied from similar to 120 to similar to 400 K and combined with the application of a magnetic field and electrical current pulses to mimic the operando conditions as in domain wall memory and logic devices for information technology. Such studies are promising for studying the pinning of the ferroelectric and magnetic domains versus temperature, spin-polarized current, and externally applied magnetic field to better manipulate the domain walls. The capability of combining operando TEM stimuli such as current, voltage, and/or magnetic field with in situ TEM imaging in a continuous cryogenic temperature range will allow the uncovering of fundamental phenomena on the nanometer scale These studies were made possible using a MEMS-based TEM holder that allowed an electron-transparent sample to be transferred and electrically contacted on a MEMS chip. The six-contact double-tilt holder allows the alignment of the specimen into its zone axis while simultaneously using four electrical contacts to regulate the temperature and two contacts to apply the electrical stimuli, i.e., operando TEM imaging. This Account leads to the demonstration of (i) the high-resolution imaging and spectroscopy of nanoparticles oriented in the desired [110] zone-axis direction at cryogenic temperatures to mitigate the electron beam degradation, (ii) imaging of low-temperature transitions with accurate and continuous control of the temperature that allowed single-frame observation of the presence of both the orthorhombic and tetragonal phases in the BaTiO3 system, and (iii) magnetic domain wall propagation as a function of temperature, magnetic field, and current pulses (100 ns with a 100 kHz repetition rate) in the Y3Fe5O12 system.

AMER CHEMICAL SOC2021

Ferroelectric Effects Probed by in situ Transmission Electron Microscopy

Reinis Ignatans

Ferroelectric perovskite oxides are widely used in sensors, actuators and optical modulators and, at the same time, they show promise for implementation in future applications such as energy storage, memory and cooling devices. At the infancy of the discovery of polycrystalline ferroelectrics, BaTiO3 was considered as a candidate for various applications, but the breakthrough for the commercialization of ferroelectrics came in 1952 when the ferroelectric PbZrO3 - PbTiO3 (PZT) solid solution system was discovered. PZT and other lead containing perovskite oxides remained on the forefront of scientific and industrial interest. However, concerns over the environmental and health hazards posed by these toxic materials in the electronic and electrical equipment created a legislation change. Nowadays, all applicable products in the EU must pass RoHS (Restriction of Hazardous Substances also known as Directive 2002/95/EC) compliance. This change in the industrial standards fueled research in lead-free ferroelectric materials once again. Attention is currently placed on materials based on BaTiO3, BaZrO3, (Na/K)NbO3, Na1/2Bi1/2TiO3. Their widespread application is yet to be realized as their physical properties pale in comparison to PZT. Substantial challenges also lie in the miniaturization of polar perovskites where effects at nano and microscopic scales become dominant, which inhibit their performance.In this thesis one of the most prominent lead-free ferroelectric perovskites BaTiO3 is studied by transmission electron microscopy (TEM) techniques in combination with in situ temperature and electric field control. In detail, a reliable focused ion beam sample preparation for in situ TEM electrical biasing for ferroelectrics is established. The method is tested on BaTiO3 at room temperature, where the electrical response is observed at the expected field values, further confirmed by finite element calculations. Domain area (polarization) - applied bias loops are directly measured revealing strong pinning at lower fields, while higher fields depin domain walls allowing for more free movement following Rayleigh's law.Upon increasing the temperature, BaTiO3 exhibits morphological transformations in its domain structure from ferroelectric 180° (close to room temperature) to ferroelastic 90° state (below the Curie temperature). Electrical biasing experiments of ferroelastic needle domains show two different forward growth mechanisms. In one case, the needle domains are shown to move freely, influenced mainly by Peierls-like potentials, while when perpendicular domains meet, their movement is hindered by strong domain domain pinning mediated by electromechanical fields. The latter one produces square shaped P-E like loops with distinct steps, indicative of Barkhausen pulses.Finally, the effect of ferroelectric phase transition on the electronic structure is studied with in situ electron energy loss spectroscopy (EELS) and density functional theory (DFT) calculations. Core-loss EELS measurements show that changes in the Ti 3d states qualitatively agree with DFT calculations. Off-axis low-loss EELS allows precise bandgap measurements in both phases. Custom experimental and data treatment methodologies are developed to retrieve momentum resolved dielectric functions, which show excellent fit with DFT calculated ones at small momentum transfer. A pathway to study oxygen deficient perovskites with EELS is theoretically demonstrated with DFT.

EPFL2022

Pulsed laser deposition of (SrBa)Nb2O6 thin films and their properties

Anna Infortuna

Strontium barium niobate (SrxBa1-xNb2O6, shortly SBN) is a solid solution system with tetragonal tungsten bronze crystal structure. It exhibits a ferroelectric phase with only one polar axis and a transition temperature depending on the Sr/Ba ratio. This thesis studies the growth of SBN thin films, in order to obtain a ferroelectric thin film model system for the study of 180° domain switching close to the ferroelectric phase transition. SBN has been chosen not only because uniaxial, but because it is possible to tune the temperature of phase transition varying the Sr/Ba ratio. It is thus possible to study electric properties of the phase transition close to room temperature, where conduction phenomena related to defects are normally lower. Thin films of SBN were grown by Pulsed Laser Deposition in a vacuum system constructed during this thesis work. For the formulation of a model it is desirable to have a system as close as possible to a single crystal. Therefore deposition parameters have been optimized to grow SBN thin films with the polar axis oriented perpendicular to the film plane. The films are integrated in parallel plate capacitor structures, in order to measure dielectric properties. The stability of the ferroelectric phase is found to be sensitive to impurities; contaminants with concentration below 1% induce a lowering of the transition temperature of about 100°C. Strain as well has influence on the phase transition; in the case of substrate with thermal expansion coefficients different from the SBN ones, the SBN film is under stress. A 0.6% tensile strain induces a lowering of the transition temperature of about 100°C, this is the case of silicon single crystal substrate. Grown on strontium titanate single crystals (STO), whose thermo-mechanical properties are close to the one of SBN, the films exhibit a phase transition temperature compatible with the one measured on single crystals. The obtained films suffer from leakage that is reduced by thermal treatment in oxygen rich atmosphere. The oxygen vacancies, created during the deposition process, are considered to be responsible of leakage. Vacancies recovery upon annealing does not eliminate the problem, and measurement of polarization at room temperature is difficult. However, piezoelectric measurements performed on individual grains, with atomic force microscope, prove that these have ferroelectric properties; by these experiment has been proved that it is possible to switch the polarization. Films with a disordered structure of grains are much less leaky than the ones with columnar grains spanning the whole cross section. These observations lead to the conclusion that conduction goes trough the grain boundaries. On STO single crystals, SBN grows epitaxial in Volmer-Weber mode. With a suitable preparation of the substrate surface, it is possible to induce the growth of films with different orientations of the polar axis: in the film plane, perpendicular to it or inclined of 45°. A model is proposed, explaining the epitaxial relation with the substrate on the basis of the perovskite kernel contained in the unit cell of the tetragonal tungsten bronze structure. The film grows from the nucleation of the perovskite structure that rules the orientation of the film; the high temperature at which the substrate is kept during the deposition, assure the necessary mobility for the arriving atoms to organize in the SBN structure around these nuclei. The study of literature data allows to state that such a model is valid not only for substrates with perovskite structure, but can be extended to substrates with a cubic crystal structure, like magnesium oxide.

EPFL2005