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Strontium aluminate, with suitable lattice parameters and environmentally friendly water solubility, has been strongly sought for use as a sacrificial layer in the preparation of freestanding perovskite oxide thin films in recent years. However, due to this material's inherent water solubility, the methods used for the preparation of epitaxial films have mainly been limited to high-vacuum techniques, which greatly limits these films' development. In this study, we prepared freestanding single-crystal perovskite oxide thin films on strontium aluminate using a simple, easy-to-develop, and low-cost chemical full-solution deposition technique. We demonstrate that a reasonable choice of solvent molecules can effectively reduce the damage to the strontium aluminate layer, allowing successful epitaxy of perovskite oxide thin films, such as 2-methoxyethanol and acetic acid. Molecular dynamics simulations further demonstrated that this is because of their stronger adsorption capacity on the strontium aluminate surface, which enables them to form an effective protective layer to inhibit the hydration reaction of strontium aluminate. Moreover, the freestanding film can still maintain stable ferroelectricity after release from the substrate, which provides an idea for the development of single-crystal perovskite oxide films and creates an opportunity for their development in the field of flexible electronic devices. [ABSTRACT FROM AUTHOR]

Strain engineering is an important method to control the structure and properties of functional thin films. Here, a new method to induce chemical strain through controllable substrate strain is proposed, which was first applied to double-perovskite thin films. We significantly improved the ferroelectricity of BiSmFe2O6-δ double-perovskite thin films to ∼4.80 μC/cm2, approximately improved six times. The value is more excellent than that of the orthorhombic double-perovskite ferroelectric systems. Synchrotron-based x-ray diffraction and spherical aberration-corrected scanning transmission electron microscopy show that tensile strain can change the epitaxial growth mode and increase the lattice volume. Meanwhile, first-principles density functional theory calculations show that the tensile strain reduces the formation energy of oxygen vacancy. The increased oxygen vacancies can induce a large negative chemical pressure of −7.69 GPa imposed on the thin films on SrTiO3 substrates. The existence of more oxygen vacancies in the Fe-O octahedra of the thin films drives Fe ions away from their high-symmetrical central position, leading to the improvement of ferroelectricity. In addition, the large polarization and oxygen vacancy migration promote the improved functional properties of the thin films, such as large resistive switching (103 times). This strategy and approach will effectively promote the further application of the novel orthorhombic rare-earth double-perovskite devices. [ABSTRACT FROM AUTHOR]

Double perovskite ferroelectric thin films are completely new material systems derived from single perovskite. Their diversity of composition and structure and the tendency for spontaneous atomic ordering broaden the path for the development of ferroelectric thin films. The ordered double perovskite ferroelectric thin films lead to excellent ferroelectric, dielectric, magnetic, and optical properties, promising further applications in photovoltaic cells, information memory, and spintronic and photoelectric devices, where the intrinsic coupling and tuning of multiple properties could also push it into multifunctional intersecting devices. However, complex internal physical mechanisms and difficult preparation conditions have prevented its further development. Based on ordered/disordered ferroelectric thin films of double perovskites, this paper first discusses ordered characterization methods such as superstructure reflection/diffraction peaks, especially for epitaxial thin films, saturation magnetization (macroscopic), and scanning transmission electron microscopy (microscopic). In response to the generally poor ordering of present systems, the paper also reviews the internal structure of the material and the external synthesis conditions that affect the ordering, including the valence and radii of the cations, preparation methods, element substitution and strain engineering, in the hope of triggering further research into ordered double perovskite ferroelectrics. Combined with the current state of research on existing double perovskite ferroelectricity thin film systems, advances in the fields of ferroelectric photovoltaics, magnetoelectric coupling, dielectric tunability, resistive switching, and photoelectric coupling have been presented. Finally, the challenges facing the material system are discussed and an outlook is provided for the development of the field. [ABSTRACT FROM AUTHOR]

Strain is an effective means of tuning the crystal structure to obtain a variety of fascinating properties, but how to apply flexible strain to meet the different needs of the film at each location has rarely been reported. In this study, a novel approach for designing strain‐damping structures that facilitate the imposition of flexible strain is introduced. A wide range of strain modulation is demonstrated in SmCoO3 films (a‐axis:+4.5%–+1.7%, b‐axis: +3.2%–+0.4%, c‐axis:+2.2%–+1.4%) under positive pressure by introducing Sm2O3 as a dopant. When SmCoO3 films are subjected to triaxial tensile strain, they exhibit a ferroelectric polarization of 7.12 µC cm−2. Through positive pressure modulation, resulting in a further increase in the ferroelectric polarization (up to 11.62 µC cm−2, which represents the maximum performance of the orthogonal rare earth transition metal oxide family). Moreover, the electron spin order can be effectively controlled, and the film's saturation magnetization increases to 14.83 emu cm−3 (+94.1%). This damping structure allows for flexible modulation of chemical strain in epitaxial film, achieving a delicate balance between film strain and structure, which provides valuable insights for all ferroelectrics based on structural distortion. [ABSTRACT FROM AUTHOR]

Double perovskite films have been extensively studied for ferroelectric order, ferromagnetic order, and photovoltaic effects. The customized ion combinations and ordered ionic arrangements provide unique opportunities for bandgap engineering. Here, a synergistic strategy to induce chemical strain and charge compensation through inequivalent element substitution is proposed. A-site substitution of the barium ion is used to modify the chemical valence and defect density of the two B-site elements in Bi 2 FeMnO 6 double perovskite epitaxial thin films. We dramatically increased the ferroelectric photovoltaic effect to ∼135.67 μA/cm 2 from 30.62 μA/cm 2 , which is the highest in ferroelectric thin films with a thickness of less than 100 nm under white-light LED irradiation. More importantly, the ferroelectric polarization can effectively improve the photovoltaic efficiency of more than 5 times. High-resolution HAADF-STEM, synchrotron-based X-ray diffraction and absorption spectroscopy, and DFT calculations collectively demonstrate that inequivalent ion plays a dual role of chemical strain (+1.92 and -1.04 GPa) and charge balance, thereby introducing lattice distortion effects. The reduction of the oxygen vacancy density and the competing Jahn-Teller distortion of the oxygen octahedron are the main phenomena of the change in electron-orbital hybridization, which also leads to enhanced ferroelectric polarization values and optical absorption. The inequivalent strategy can be extended to other double perovskite systems and applied to other functional materials, such as photocatalysis for efficient defect control.

Polarization rotation caused by various strains, such as substrate and/or chemical strain, is essential to control the electronic structure and properties of ferroelectric materials. This study proposes anion-induced polarization rotation with chemical strain, which effectively improves ferroelectricity. A method for the sulfurization of BiFeO 3 thin films by introducing sulfur anions is presented. The sulfurized films exhibited substantial enhancement in room-temperature ferroelectric polarization through polarization rotation and distortion, with a 170% increase in the remnant polarization from 58 to 100.7 μC cm -2 . According to first-principles calculations and the results of X-ray absorption spectroscopy and high-angle annular dark-field scanning transmission electron microscopy, this enhancement arose from the introduction of S atoms driving the re-distribution of the lone-pair electrons of Bi, resulting in the rotation of the polarization state from the [001] direction to the [110] or [111] one. The presented method of anion-driven polarization rotation might enable the improvement of the properties of oxide materials.