Kunimitsu Kataoka

A new layered iron arsenide superconductor: (Ca,Pr)FeAs2.

A new iron-based superconductor, (Ca,Pr)FeAs2, was discovered. Plate-like crystals of the new phase were obtained, and its crystal structure was investigated by single-crystal X-ray diffraction analysis. The structure was identified as the monoclinic system with space group P2₁/m, composed of two Ca(Pr) planes, Fe2As2 layers, and As2 zigzag chain layers. Plate-like crystals of the new phase showed superconductivity, with a T(c) of ~20 K in both magnetization and resistivity measurements.

Ion-exchange synthesis, crystal structure, and physical properties of hydrogen titanium oxide H2Ti3O7.

Hydrogen titanium oxide H2Ti3O7 was prepared from Na2Ti3O7 as a parent compound via Na(+)/H(+) ion exchange in acidic solution at 333 K. It crystallizes in the monoclinic system, space group C2/m, and the lattice parameters of a = 16.0380(8) Å, b = 3.7533(1) Å, c = 9.1982(3) Å, and β = 101.414(3)°. The crystal structure of H2Ti3O7 was refined to the conventional values of Rwp = 2.60% and Rp = 1.97% with a fit indicator of GOF = Rwp/Re = 1.90 by Rietveld analysis using powder neutron diffraction data. The basic (Ti3O7)(2-) framework in H2Ti3O7 was changed from that in the parent Na2Ti3O7. The atomic coordinate of hydrogen atoms were determined by this study for the first time. The hydrogen site in the layer space was refined with a strict H1-O3 distance of 0.80(2) Å and H2-O4 distance of 0.86(2) Å in H2Ti3O7, respectively. The structural stability of H2Ti3O7 was confirmed by bond valence sums. From these results, protons were suggested as the ordered occupation in the crystal structure.

Quantitative analysis of cation mixing and local valence states in LiNixMn2−xO4 using concurrent HARECXS and HARECES measurements

Cation mixing in positive electrode materials for rechargeable lithium ion batteries, LiNixMn2-xO4 (x = 0, 0.2, 0.5) and Li0.21Ni0.7Mn1.64O4-δ (denoted as x = 0.7), is analyzed by high-angular-resolution electron-channeling X-ray/electron spectroscopy (HARECXS/HARECES) techniques, using energy-dispersive X-ray spectroscopy and electron energy-loss spectroscopy. Mixing between the tetrahedral lithium sites and the octahedral transition metal sites is quantified, and the site-dependent valence states of the transition metals are examined. In the non-doped (x = 0) sample, Mn was found to occupy only octahedral sites as either Mn(3+) or Mn(4+) For x = 0.2-0.7, some of the nickel ions (6-13% depending on x) occupy tetrahedral anti-sites. All the nickel ions are in the divalent state, regardless of the occupation site. For x = 0.2 and 0.7, manganese ions occupy both octahedral and tetrahedral sites; those in the octahedral sites are tetravalent, while the tetrahedral sites contain a mixture of divalent and trivalent ions. For x = 0.5, manganese occupies only the octahedral sites, with all ions determined to be in the tetravalent state (within experimental accuracy). All the samples substantially satisfied the local charge neutrality conditions. This study demonstrates the feasibility of using HARECXS/HARECES for quantitative analysis of the atomic configuration and valence states in lithium manganese oxide spinel materials.

Synthesis and Electrochemical Properties of Hollandite-Type KXTiO2

Single phase specimen of K0.12TiO2 with hollandite-type structure was successfully synthesized by the solid-state reaction under Ar/H2 atomosphere. K+ ions were partially extracted from the parent K0.12TiO2 specimen by the HCl treatment. The K+-extracted K0.08TiO2 specimen worked as a rechargeable electrode material. An initial capacity of 92 mAh/g (cut-off voltage: 1.0 V) could be achieved, which approximately correspond to the composition Li0.28TiO2. A reversible capacity after 50 cycles was about 50 mAh/g.

Single-crystal synthesis and structure refinement of La2Li0.5Al0.5O4 with K2NiF4-type structure

Abstract Single crystals of lanthanum lithium aluminum oxide, La2Li0.5Al0.5O4, were synthesized for the first time by a flux method at 773 K. The La2Li0.5Al0.5O4 single crystal is colorless, has an average size of 60 μm with a square platelet shape, and crystallizes in the tetragonal K2NiF4-type structure, space group I4/mmm with a = 3.7742(9) Å, c = 12.753(4) Å, V = 181.66(10) Å3, and Z = 2. The structure was determined using single-crystal X-ray diffraction data and refined to the conventional values R = 1.25% and wR = 1.28% for 524 observed reflections. The octahedral site was randomly occupied by Li and Al atoms. The corresponding polycrystalline sample was also prepared by a conventional solid-state reaction method at 1123 K.

Ion-exchange synthesis and improved Li insertion property of lithiated H2Ti12O25 as a negative electrode material for lithium-ion batteries

Abstract We successfully prepared the lithiated H2Ti12O25 sample by the H+/Li+ ion exchange synthetic technique in the molten LiNO3 at 270 °C using H2Ti12O25 as a starting compound. Chemical composition of the obtained lithiated H2Ti12O25 sample was determined to be H1.05Li0.35Ti12O25-δ having δ = 0.3 by ICP-AES and DTA-TG analyses. The H+/Li+ ion exchange was also confirmed by powder XRD, 1H-MAS NMR, and 7Li-MAS NMR measurements. Electrochemical Li insertion and extraction measurements revealed that the initial coulombic efficiency was improved from 88% in H2Ti12O25 to 93% in the lithiated H2Ti12O25 sample. In addition, superior capacity retention properties for the charge and discharge cycling performance and good charge rate capability of the present lithiated H2Ti12O25 were confirmed in the electrochemical measurements. Accordingly, the lithiated H2Ti12O25 is suggested to be one of the promising high-voltage and high-capacity oxide negative electrodes in advanced lithium-ion batteries.

Lithium-ion conducting oxide single crystal as solid electrolyte for advanced lithium battery application

Today, all-solid-state secondary lithium-ion batteries have attracted attention in research and development all over the world as a next-generation energy storage device. A key material for the all-solid-state lithium batteries is inorganic solid electrolyte, including oxide and sulfide materials. Among the oxide electrolytes, garnet-type oxide exhibits the highest lithium-ion conductivity and a wide electrochemical potential window. However, they have major problems for practical realization. One of the major problems is an internal short-circuit in charging and discharging. In the polycrystalline garnet-type oxide electrolyte, dendrites of lithium metal easily grow through the void or impurity in grain boundaries of the sintered body, which causes serious internal short-circuits in the battery system. To solve these problems, we present an all-solid-state battery system using a single-crystal oxide electrolyte. We are the first to successfully grow centimeter-sized single crystals of garnet-type by the floating zone method. The single-crystal solid electrolyte exhibits an extremely high lithium-ion conductivity of 10−3 S cm−1 at 298 K. The garnet-type single-crystal electrolyte has an advantageous bulk nature to realize the bulk conductivity without grain boundaries such as in a sintered polycrystalline body, and will be a game-changing technology for achieving highly safe advanced battery systems.

Synthesis, Crystal Structure and Physical Properties of Ba4Ti12O27

Polycrystalline sample of the reduced barium titanate Ba4Ti12O27 was prepared by solid state reaction in Ar atmosphere. The magnetic susceptibility was nearly temperature independent in the range of 75-300 K, suggesting the Van Vleck Paramagnetism. The temperature dependence of the electric conductivity does not obey any Arrhenius type behavior.

Structural Reinvestigation of Alkali Hexatitanate

Na1.614Li0.386Ti6O13 Single crystal was synthesized by a Li+ ion-exchange method from Na2Ti6O13 single crystal in a molten salt of LiNO3. The obtained Na1.614Li0.386Ti6O13 single-crystal is colorless and has the shape of a rod. Na1.614Li0.386Ti6O13 crystallizes in the monoclinic tunnel type structure, space group C2/m, and lattice parameters a = 15.144(2) Å, b = 3.7492(5) Å, c = 9.162(2) Å and β = 99.0131(9)º. The structure was determined by a single-crystal X-ray study and refined to the conventional values of R = 0.0247 and wR = 0.0451 for 969 independent observed reflections.

Structural Study of Trehalose Dihydrate by Neutron and X-ray Diffraction Experiments

The structure of α,α-trehalose dihydrate was studied by neutron and X-ray diffraction experiments to understand the relation between its superior biological protective function and the role of the hydrogen bond connecting the trehalose molecules. By direct observation of hydrogen using the neutron diffraction method, the nuclear positions and anisotropic thermal parameters of hydrogen atoms are determined accurately. The nuclear positions show clear discrepancies from the centers of the electron cloud of hydrogen determined from the X-ray data. The result is interpreted in terms of a local electric dipole moment in the hydrogen atoms. The magnitude of the dipole moment is markedly large for the hydrogen atoms participating in the hydrogen bond. The detailed electron density distribution has been determined by using X-ray data obtained at 150 K. It clearly shows the electron cloud of hydrogen spreading over the chain of hydrogen bonds. It was found that there is a part where the electron density is very lo...