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

Preparation of Monodisperse and Spherical Rutile VO₂ Fine Particles

Preparation of Monodisperse and Spherical Rutile VO2 Fine Particle

Geometrical Spin Frustration and Monoclinic-Distortion-Induced Spin Canting in the Double Perovskites Ln₂LiFeO₆ (Ln = La, Nd, Sm, and Eu) with Unusually High Valence Fe⁵⁺

We discovered new B-site-ordered double perovskites Ln2LiFeO6 (Ln = La, Nd, Sm, and Eu) with most likely unusually high valence Fe5+, which was stabilized by strong oxidizing high-pressure synthesis. Despite large antiferromagnetic interactions between Fe5+ spins in these compounds, the magnetic ordering is strongly suppressed due to the geometrical frustration of Fe5+ located in a face-centered cubic lattice. In addition, canted magnetic structures are stabilized only in those with Ln = Sm and Eu, which is most likely due to significant Dzyaloshinskii Moriya interaction caused by large monoclinic structural distortion. These results provide a deep understanding of the structure–property relationships in geometrically frustrated B-site-ordered double perovskites

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

Solid Solutions of Pauli-Paramagnetic CaCu₃V₄O₁₂ and Antiferromagnetic CaMn₃V₄O₁₂

Solid solutions of Pauli-paramagnetic CaCu₃V₄O₁₂ and antiferromagnetic CaMn₃V₄O₁₂ were prepared by a high-pressure synthesis technique. All samples crystallized in the A-site-ordered perovskite structure with isovalent Cu²⁺ and Mn²⁺ ions at the square-planar A′ site. The V ion at the B site kept a charge state close to +4 in all of the solid solutions, and the electrons of V were delocalized and contributed to the metallic properties. The substitution of Mn²⁺ for Cu²⁺ in CaCu₃V₄O₁₂, where both Cu and V electrons were delocalized, produced the <i>S</i> = ⁵/₂ localized moments, and the spins at the Mn site interacted antiferromagnetically. Spin-glass-like magnetic behaviors due to the random distribution of Cu/Mn ions at the A′ site were observed at intermediate compositions of the solid solution, whereas the antiferromagnetic transition was observed at the end composition CaMn₃V₄O₁₂

Extraction of Anisotropic Thermal Vibration Factors for Oxygen from the Ti <i>L</i>_2,3-Edge in SrTiO₃

The atomic vibration factor is commonly measured by diffraction experiments. However, it is difficult to estimate the accurate value from the powdered sample, particularly in anisotropic vibration cases. Here, we demonstrate an alternate method to extract anisotropic atomic vibration factors for oxygen in SrTiO3 using the monochromated Ti L2,3-edge electron energy-loss spectrum measured by scanning transmission electron microscopy combined with crystal field multiplet calculation including anisotropic vibration effects. First, we theoretically investigated the effects of individual elemental thermal vibration on the Ti L2,3-edge spectrum. This reveals that only atomic vibration of O mainly affects the spectral shape with sensitivity while the effects of that of Ti and Sr are very small. Thus, we extract the anisotropic atomic vibration factor of oxygen from experimental L2,3-edge spectra by estimating with calculated ones. As a result, this demonstrates that the estimated temperature-dependent anisotropic atomic vibration factors for oxygen in SrTiO3 show good agreement with previously reported experiments and theoretical values. The investigation of atomic vibration at the atomic scale would be useful for further understanding of thermal properties of functional materials