Strain Effect on Structural Transition in SrRuO₃ Epitaxial Thin Films

We carried out detailed structural characterizations across the structural transition in SrRuO3 epitaxial thin films grown on SrTiO3 (001)pc substrate. The fabricated films undergo a structural transition from the low-temperature orthorhombic phase to the high-temperature pseudocubic phase at 280 °C. We find that, for films thinner than 20 nm, the transition becomes broader while thicker films display a sharp transition. Detailed X-ray diffraction measurements including reciprocal space mappings at various temperatures also reveal that the thinner films have a distorted orthorhombic unit cell resulting from the strain-induced additional rotation in the RuO6 octahedra, while, for the high-temperature pseudocubic phase, the film structure remains the same irrespective of film thickness. The results strongly suggest that the substrate-induced strain has a strong influence on the RuO6 rotation pattern in the epitaxial thin film

Octahedral Tilt Propagation Controlled by A‑Site Cation Size at Perovskite Oxide Heterointerfaces

A clear correlation between the A-site cation size and the octahedral tilt propagation from the substrates into the ATiO3 (A = Ba2+, Sr2+, Sr2+0.7Ca2+0.3, and Sr2+0.5Ca2+0.5) epitaxial thin films was found from the observations of ATiO3/GdScO3 heterostructures using high-resolution annular bright-field scanning transmission electron microscopy. The in-plane oxygen displacements at the interface increase with decreasing the A-site cation size and facilitate the TiO6 octahedral tilt propagation across the interface. The results highlight the significance of the A-site cation size as a controlling factor for structural distortions at oxide-based heterointerfaces

Oxygen Release and Incorporation Behaviors in BaFeO₃ Polymorphs with Unusually High-Valence Fe⁴⁺

Fully oxygenated perovskite BaFeO3 containing unusually high-valence Fe4+ shows three crystal polymorphs with the same chemical composition. The 3C-type BaFeO3 has a simple cubic perovskite structure consisting of corner-sharing FeO6 octahedra, while the 6H- and 12R-type BaFeO3 have hexagonal perovskite structures consisting of both corner-sharing and face-sharing FeO6 octahedra. The compounds readily release oxygen into the air to reduce the high-valence state of the Fe ions, but the oxygen release behaviors strongly depend on the crystal structure. The 3C-type BaFeO3 releases oxygen topotactically from the corner-shared sites of the FeO6 octahedra at a temperature as low as 130 °C. In contrast, the 6H- and 12R-type BaFeO3 preferentially release oxygen from the face-shared sites above 320 and 460 °C, respectively, although they include the corner-shared sites in the crystal structures. The resultant oxygen-deficient 3C-type BaFeO2.5 does not incorporate back oxygen in air, whereas the 12R-type hexagonal structure shows completely reversible oxygen release and incorporation in air. Once the 12R-type structure is established, unusually high-valence states such as Fe4+ can be stabilized without extreme conditions

Oxygen Release and Incorporation Behaviors in BaFeO₃ Polymorphs with Unusually High-Valence Fe⁴⁺

Fully oxygenated perovskite BaFeO3 containing unusually high-valence Fe4+ shows three crystal polymorphs with the same chemical composition. The 3C-type BaFeO3 has a simple cubic perovskite structure consisting of corner-sharing FeO6 octahedra, while the 6H- and 12R-type BaFeO3 have hexagonal perovskite structures consisting of both corner-sharing and face-sharing FeO6 octahedra. The compounds readily release oxygen into the air to reduce the high-valence state of the Fe ions, but the oxygen release behaviors strongly depend on the crystal structure. The 3C-type BaFeO3 releases oxygen topotactically from the corner-shared sites of the FeO6 octahedra at a temperature as low as 130 °C. In contrast, the 6H- and 12R-type BaFeO3 preferentially release oxygen from the face-shared sites above 320 and 460 °C, respectively, although they include the corner-shared sites in the crystal structures. The resultant oxygen-deficient 3C-type BaFeO2.5 does not incorporate back oxygen in air, whereas the 12R-type hexagonal structure shows completely reversible oxygen release and incorporation in air. Once the 12R-type structure is established, unusually high-valence states such as Fe4+ can be stabilized without extreme conditions

Nanoscale Structural and Chemical Properties of Antipolar Clusters in Sm-Doped BiFeO₃ Ferroelectric Epitaxial Thin Films

The local atomic structure and nanoscale chemistry of an antipolar phase in Bi0.9Sm0.1FeO3 epitaxial thin films are examined by an array of transmission electron microscopy (TEM) coupled with electron diffraction and electron energy-loss spectroscopy methods. The observations are tied to macroscopic properties of the films, namely, polarization-electric field hysteresis loops, dielectric constant-electric field hysteresis loops, and the dielectric loss. At room temperature, the local Sm deficiency was determined to destabilize the long-range ferroelectric state, resulting in the formation of local antipolar clusters with the appearance of PbZrO3-like antiparallel cation displacements, which give rise to 1/4{011} and 1/4{211} reflections as well as 1/2{321}, because of in-phase oxygen octahedral tilts. Aberration-corrected TEM analysis reveals that the antipolar structure is actually a lamellar of highly dense ferroelectric domains with alternating polarizations. With increasing temperature, a phase transition was observed at 150 °C, which is attributed to the reduction of the antiparallel displacements, giving way to cell-doubling structural transition

Nanoscale Structural and Chemical Properties of Antipolar Clusters in Sm-Doped BiFeO₃ Ferroelectric Epitaxial Thin Films

The local atomic structure and nanoscale chemistry of an antipolar phase in Bi0.9Sm0.1FeO3 epitaxial thin films are examined by an array of transmission electron microscopy (TEM) coupled with electron diffraction and electron energy-loss spectroscopy methods. The observations are tied to macroscopic properties of the films, namely, polarization-electric field hysteresis loops, dielectric constant-electric field hysteresis loops, and the dielectric loss. At room temperature, the local Sm deficiency was determined to destabilize the long-range ferroelectric state, resulting in the formation of local antipolar clusters with the appearance of PbZrO3-like antiparallel cation displacements, which give rise to 1/4{011} and 1/4{211} reflections as well as 1/2{321}, because of in-phase oxygen octahedral tilts. Aberration-corrected TEM analysis reveals that the antipolar structure is actually a lamellar of highly dense ferroelectric domains with alternating polarizations. With increasing temperature, a phase transition was observed at 150 °C, which is attributed to the reduction of the antiparallel displacements, giving way to cell-doubling structural transition

Ultralong Distance Hydrogen Spillover Enabled by Valence Changes in a Metal Oxide Surface

Hydrogen spillover is a phenomenon in which hydrogen atoms generated on metal catalysts diffuse onto catalyst supports. This phenomenon offers reaction routes for functional materials. However, due to difficulties in visualizing hydrogen, the fundamental nature of the phenomenon, such as how far hydrogen diffuses, has not been well understood. Here, in this study, we fabricated catalytic model systems based on Pd-loaded SrFeOx (x ∼ 2.8) epitaxial films and investigated hydrogen spillover. We show that hydrogen spillover on the SrFeOx support extends over long distances (∼600 μm). Furthermore, the hydrogen-spillover-induced reduction of Fe4+ in the support yields large energies (as large as 200 kJ/mol), leading to the spontaneous hydrogen transfer and driving the surprisingly ultralong hydrogen diffusion. These results show that the valence changes in the supports’ surfaces are the primary factor determining the hydrogen spillover distance. Our study leads to a deeper understanding of the long-debated issue of hydrogen spillover and provides insight into designing catalyst systems with enhanced properties

Oxygen Release and Incorporation Behaviors Influenced by A‑Site Cation Order/Disorder in LaCa₂Fe₃O₉ with Unusually High Valence Fe^3.67+

Fully oxygenated perovskites with the same chemical composition crystallize in different polymorphs, o-LaCa2Fe3O9 with A-site triple-layer order and d-(LaCa2)­Fe3O9 with A-site disorder. Both compounds contain unusually high valence Fe3.67+ and, when they are heated, release oxygen to reduce such an unusual valence state. Thermogravimetry analysis revealed that the ordered/disordered arrangements of the A-site cations in the perovskite structures strongly influence the stability of unusually high valence Fe ions at the B-site. The A-site-ordered o-LaCa2Fe3O9 releases its oxygen topotactically and selectively from the FeO6 octahedra between the Ca layers above 400 °C, and the released oxygen is not incorporated back on cooling in air, Ar, or O2 atmospheres. On the other hand, oxygen in d-(LaCa2)­Fe3O9 is released from and incorporated into the rigid octahedra reversibly when the compound is heated in air or O2. More importantly, the oxygen-deficient d-(LaCa2)­Fe3O8 obtained by heating d-(LaCa2)­Fe3O9 in Ar incorporates extra oxygen to increase the valence state of Fe in an unusually high value even under ambient conditions. Once the A-site cation disorder structure framework is established, unusually high valence states, which usually require extreme conditions, can be stabilized by incorporating extra oxygen into the structure even under ambient conditions

Partially Reversible Anionic Redox for Lithium-Excess Cobalt Oxides with Cation-Disordered Rocksalt Structure

Li-excess electrode materials potentially boost the energy density of Li-ion batteries, but the origin of the instability of anionic redox in cation-disordered rocksalt material is still under debate. In this study, a binary system of Li3NbO4–CoO is targeted as electrode materials for lithium storage applications. In this binary system, stoichiometric LiCo2/3Nb1/3O2 crystallizes into a rocksalt-type structure with partial ordering of Nb ions. Upon increase of the Li3NbO4 fraction, cation ordering is lost, forming a cation-disordered rocksalt structure in Li-excess phases. Although Li-excess Li4/3Co2/9Nb4/9O2 delivers a large reversible capacity as electrode materials, inferior cyclability and large voltage hysteresis for charge/discharge curves are noted. Irreversible structural changes in electrochemical cycles are also evidenced from results of in situ XRD measurements, suggesting that anionic redox is destabilized for Li4/3Co2/9Nb4/9O2. X-ray absorption spectroscopy reveals that partial stabilization of ligand holes as observed in SrCoO3 is achieved for these oxides. Ligand holes are more effectively stabilized for Li7/6Co4/9Nb7/18O2 with less Li-excess and Co-rich composition. Through systematic study of the binary system of Li3NbO4–CoO with different chemical compositions, factors affecting reversibility and irreversibility of anionic redox are further discussed

Overpotential-Induced Introduction of Oxygen Vacancy in La_0.67Sr_0.33MnO₃ Surface and Its Impact on Oxygen Reduction Reaction Catalytic Activity in Alkaline Solution

Oxygen reduction reaction (ORR) catalytic activity of La_0.67Sr_0.33MnO₃ epitaxial thin films was investigated in a KOH solution by using a rotating-disk electrode. We found that while the films exhibit ORR current, the current is not limited by oxygen transport resulting from the film electrode rotation and shows the large hysteresis against the potential sweep direction. This behavior is in stark contrast to the oxygen reduction reaction activity of an electrode ink made from LSMO bulk powder, whose ORR current is oxygen-transport limited. <i>In situ</i> synchrotron X-ray absorption spectroscopy also reveals that the valence state of Mn in the LSMO film surface is lowered under the reducing atmosphere caused by the overpotential. This indicates the overpotential-induced introduction of oxygen vacancies in the film surface. We also show that the ORR current of the LSMO films exposed to the reducing atmosphere is lowered than that of the original surface. These results indicate that the ORR catalytic activity of LSMO surfaces is strongly influenced by oxygen vacancies