Koji Kimura

Low-energy anharmonic phonons in rutile TiO ₂ were investigated by inelastic x-ray and neutron scattering (IXS and INS) techniques. Owing to the different sensitivities of IXS and INS to phonon modes, the transverse optical branches corresponding to the B _1u and A _2u modes at the Γ point were selectively observed in the respective spectra. It was found that the lowest-energy B _1u mode exhibits pronounced softening with decreasing temperature, while the A _2u mode also shows softening, as previously reported. The observed temperature-dependent softening was well reproduced by density functional theory calculations based on the stochastic self-consistent harmonic approximation, although the magnitude of the softening was quantitatively underestimated. The pronounced softening of the B _1u mode, as well as its finite linewidth, is discussed in terms of possible higher-order phonon interactions between the B _1u and A _2u modes. The implications of this strong anharmonicity for lattice-related physicochemical properties, including dielectric response, thermal transport, Li-ion insertion, and polaron-related photocatalytic phenomena, are also addressed.

X-ray fluorescence holography (XFH) and extended X-ray absorption fine structure (EXAFS) measurements were used to investigate local structure around Fe ions in Fe-doped BaTiO₃ crystal. XFH-reconstructed atomic images exhibited two notable features: the Ti images showed higher intensities than simulations, whereas the Ba images were significantly blurred. These features were qualitatively reproduced by introducing positional fluctuations of Ba atoms into the simulations, which blur the Ba atomic images and suppress artifacts in the B-site plane. However, the experimental results could not be fully reproduced even with large Ba positional fluctuations, suggesting additional factors such as partial Fe substitution at the Ba site or nanoscale aggregation of Fe. The EXAFS analysis further supports this picture by suggesting a minor contribution from Fe at the Ba site and a weak Fe–Ba correlation. This study provides fundamental insights into how dilute dopants modify local structure and functionality of BaTiO₃.

Valence transitions in europium compounds, such as EuO and EuNi₂(Si_1-xGe_x)₂, usually induce substantial structural distortions because the ionic radius difference between Eu²⁺ (1.25 Å) and Eu³⁺ (1.06 Å) exceeds the Hume-Rothery tolerance. Here, we conducted valence-specific structural analysis of strained EuVO₂H thin films using X-ray fluorescence holography (XFH), where intersite charge transfer between Eu²⁺ and V³⁺ converts ∼30% of Eu²⁺ into Eu³⁺ (and V³⁺ into V²⁺). XFH measurements at the Eu L-edge revealed that a 15 nm single-crystalline film maintains remarkable structural coherence, with Eu²⁺ and Eu³⁺ randomly distributed and showing minimal local distortion despite the >15% size mismatch. This unusual stability is attributed to the size flexibility of H^- anions in the EuH layers, which act as structural buffers. The ability to accommodate mixed-valence states without introducing structural disorder likely underlies the enhanced magnetic anisotropy in EuVO₂H.

This work presents a combined theoretical and experimental investigation of optimized crystal structural and electronic properties of Ba-S compounds (BaS, BaS₂, BaS₃). Films were synthesized via e-beam evaporation followed by...

The domain structure of Sn-doped BaTiO ₃ was examined with piezoresponse force microscopy and spatially resolved Raman spectroscopy, and derived reversible and irreversible contributions from Rayleigh analysis of the dielectric response.

We applied x-ray fluorescence holography (XFH) to a Li0.15La0.85/3NbO3 Li ion conductor to investigate the La3+ ionic arrangement. Thus far, this compound has been believed to have the A-site-deficient perovskite structure with the alternately arranged La-/Li-rich and La-/Li-empty layers (A1 and A2 layers, respectively) along the c axis. However, our XFH results revealed that A sites in the A2 layers are partially occupied by La3+ ions. From the detailed analysis of the obtained atomic images, it was indicated that the La3+ ions are not randomly distributed but form a column-like arrangement, where the La3+ ions in the A2 layers (La2 ions) are aligned in the c-axis direction and those in the A1 layers are aggregated near these La2 ions. We discuss the reason why the La3+ ions are located in the A2 layers in terms of crystal growth processes, and we also examine the structural origin of the enhanced ionic conductivity of the present sample.

To meet the growing demand for energy, multimodal energy conversion systems, such as photoferroelectrics, are receiving increased attention. Among these, oxide perovskites like Sn substituted barium titanate exhibit enhanced piezoelectric coefficient and dielectric permittivity at room temperature compared to the parent composition. This study investigates whether these enhancements originate from intrinsic Sn incorporation or extrinsic contributions. Additionally, the impact of Sn on the electronic bandgap is examined, which is critical for multifunctional applications. A combined experimental and first-principles approach is employed to systematically analyze different Sn concentrations (0–12.5 mol. % Sn), focusing on B-site ordering in BaTiO3. The experimental analysis includes dielectric and ferroelectric measurements, diffuse reflectance spectroscopy, and photoconductivity measurements for bandgap estimations. Computational screening reveals that Sn ions repel each other and preferentially adopt second neighbor B-site positions along the Ti–O–Ti bond. While the bandgap shows minimal changes (meV range), spontaneous polarization and polarization hysteresis decrease significantly with increasing Sn content. The experimentally observed increase in piezoelectric response is thus attributed primarily to extrinsic effects on the meso- and macroscales from the domain structure and the coexistence of phases around 11 mol. % Sn, rather than to intrinsic incorporation of Sn into the perovskite lattice.