Hajime Hojo

Atomic Structure of a CeO2 Grain Boundary: The Role of Oxygen Vacancies

Determining both cation and oxygen sublattices of grain boundaries is essential to understand the properties of oxides. Here, with scanning transmission electron microscopy, electron energy-loss spectroscopy, and first-principles calculations, both the Ce and oxygen sublattices of a (210)Σ5 CeO(2) grain boundary were determined. Oxygen vacancies are shown to play a crucial role in the stable grain boundary structure. This finding paves the way for a comprehensive understanding of grain boundaries through the atomic scale determination of atom and defect locations.

Suppression of temperature hysteresis in negative thermal expansion compound BiNi1−xFexO3 and zero-thermal expansion composite

Negative thermal expansion (NTE) of BiNi1−xFexO3 is investigated. All x = 0.05, 0.075, 0.10, and 0.15 samples shows large NTE with the coefficient of linear thermal expansion (CTE) αL exceeding −150 ppm K−1 induced by charge transfer between Bi5+ and Ni2+ in the controlled temperature range near room temperature. Compared with Bi1−xLnxNiO3 (Ln: rare-earth elements), the thermal hysteresis that causes a problem for practical application is suppressed because random distribution of Fe in the Ni site changes the first order transition to second order-like transition. The CTE of BiNi0.85Fe0.15O3 reaches −187 ppm K−1 and it is demonstrated that 18 vol. % addition of the present compound compensates for the thermal expansion of epoxy resin.

Epitaxial growth of room-temperature ferrimagnetic semiconductor thin films based on the ilmenite-hematite solid solution

Epitaxial thin films composed of 0.7FeTiO3∙0.3Fe2O3 solid solution have been prepared on α-Al2O3 (0001) substrates by a pulsed laser deposition method, and their electrical and magnetic properties have been examined. A single phase of the ordered phase can be obtained under limited deposition conditions: oxygen partial pressure of 1.0×10−3Pa and substrate temperature of 600–700°C. The as-deposited film is semiconducting and ferrimagnetic below room temperature, while subsequent annealing in vacuum leads to the Curie temperature above room temperature. On the other hand, the thin films with the disordered phase appear to be antiferromagnetic and also insulating.

Room-temperature ferrimagnetic semiconductor 0.6FeTiO3∙0.4Fe2O3 solid solution thin films

The authors report on the fabrication of room-temperature ferrimagnetic semiconductor thin films composed of 0.6FeTiO3∙0.4Fe2O3 solid solution on α-Al2O3 (0001) substrates using a pulsed laser deposition method. A single ordered phase (R3¯ symmetry) is obtained under very limited deposition conditions including oxygen partial pressure of 2.0×10−3Pa and substrate temperature of 700°C. The thin film with the ordered phase is an n-type semiconductor and ferrimagnetic with the Curie temperature >400K. The Hall effect measurements at room temperature suggest that the carrier spins are polarized.

Atomic structure and strain field of threading dislocations in CeO2 thin films on yttria-stabilized ZrO2

Threading dislocations in CeO2 thin films grown on yttria-stabilized ZrO2 substrates were investigated by transmission electron microscopy (TEM), high-resolution TEM, and scanning TEM. It is revealed that there are two kinds of threading dislocations with the Burgers vector of b=1/2⟨110⟩: one is pure edge-type and the other is mixed-type. Comparing the strain field of the mixed-type dislocations with that of the Peierls–Nabarro and the Foreman dislocation models, we find that the Foreman model better describes it in CeO2.

Atomic structures and oxygen dynamics of CeO2 grain boundaries

Material performance is significantly governed by grain boundaries (GBs), a typical crystal defects inside, which often exhibit unique properties due to the structural and chemical inhomogeneity. Here, it is reported direct atomic scale evidence that oxygen vacancies formed in the GBs can modify the local surface oxygen dynamics in CeO2, a key material for fuel cells. The atomic structures and oxygen vacancy concentrations in individual GBs are obtained by electron microscopy and theoretical calculations at atomic scale. Meanwhile, local GB oxygen reduction reactivity is measured by electrochemical strain microscopy. By combining these techniques, it is demonstrated that the GB electrochemical activities are affected by the oxygen vacancy concentrations, which is, on the other hand, determined by the local structural distortions at the GB core region. These results provide critical understanding of GB properties down to atomic scale, and new perspectives on the development strategies of high performance electrochemical devices for solid oxide fuel cells.

Enhanced Piezoelectric Response due to Polarization Rotation in Cobalt‐Substituted BiFeO3 Epitaxial Thin Films

Polarization rotation induced by an external electric field in piezoelectric materials such as PbZr1-x Tix O3 is generally regarded as the origin of their large piezoelectric responses. Here, the piezoelectric responses of high-quality cobalt-substituted BiFeO3 epitaxial thin films with monoclinic distortions are systematically examined. It is demonstrated that polarization rotation plays a crucial role in improving the piezoelectric responses in this material.

A-Site and B-Site Charge Orderings in an s–d Level Controlled Perovskite Oxide PbCoO3

Perovskite PbCoO3 synthesized at 12 GPa was found to have an unusual charge distribution of Pb2+Pb4+3Co2+2Co3+2O12 with charge orderings in both the A and B sites of perovskite ABO3. Comprehensive studies using density functional theory (DFT) calculation, electron diffraction (ED), synchrotron X-ray diffraction (SXRD), neutron powder diffraction (NPD), hard X-ray photoemission spectroscopy (HAXPES), soft X-ray absorption spectroscopy (XAS), and measurements of specific heat as well as magnetic and electrical properties provide evidence of lead ion and cobalt ion charge ordering leading to Pb2+Pb4+3Co2+2Co3+2O12 quadruple perovskite structure. It is shown that the average valence distribution of Pb3.5+Co2.5+O3 between Pb3+Cr3+O3 and Pb4+Ni2+O3 can be stabilized by tuning the energy levels of Pb 6s and transition metal 3d orbitals.

Atomic structure of a Σ3 [110]/(111) grain boundary in CeO2

The atomic structure of a Σ3 [110]/(111) grain boundary in CeO2 was studied by scanning transmission electron microscopy, electron energy loss spectroscopy, and the first-principles calculations. It was revealed that this grain boundary does not promote the formation of oxygen vacancies and keeps oxygen stoichiometry, which is different from that of Σ5 CeO2 grain boundary studied previously [H. Hojo, T. Mizoguchi, H. Ohta, S. D. Findlay, N. Shibata, T. Yamamoto, and Y. Ikuhara, Nano Lett. 10, 4668 (2010)]. It was found that the difference in grain boundary oxygen stoichiometry is correlated with the grain boundary atomic structure.

Superconductivity in Noncentrosymmetric Iridium Silicide Li2IrSi3

The effects of lithium absorption on the crystal structure and electronic properties of IrSi3, a binary silicide with a noncentrosymmetric crystal structure, were studied. X-ray and neutron diffraction experiments revealed that hexagonal IrSi3 (space group P6_3mc) transforms into trigonal Li2IrSi3 (space group P31c) upon lithium absorption. The structure of Li2IrSi3 is found to consist of a planar kagome network of silicon atoms with Li and Ir spaced at unequal distances between the kagome layers, resulting in a polar structure along the c-axis. Li2IrSi3 exhibited type-II superconductivity with a transition temperature Tc of 3.8 K, displaying a structure type that no previous superconductors have been reported to have.