Takashi Kamiyama

Synthesis, structure, and phase relationship in lithium manganese oxide spinel

Phase relationships, structures, magnetic properties, and phase transitions in the lithium manganese oxide spinel were studied based on a wide variety of samples synthesized at temperatures over a range of 750 ≤ n t n ≤ 900 °C with Li/Mn ratios between 0.5 and 0.55 and various starting materials. The system was divided into three categories: oxygen deficient spinels, LiMn2O4−δ; lithium-substituted spinels, Li1+xMn2−xO4−δ; and the stoichiometric spinel, LiMn2O4. Their structures were discussed based on the structure data determined by TOF neutron and synchrotron X-ray Rietveld analysis. The amount of oxygen vacancy was determined to be δ n ≈ 0.14(3) in LiMn2O4−δ, for example, for the spinel synthesized at 900 °C, and the nearly stoichiometric composition was obtained at 750 °C followed by heating at 470 °C for several times from lithium hydroxide and manganese oxides as starting materials. The oxygen vacancy was dependent upon the synthesis conditions and starting materials. It decreased as the Li/Mn ratio varied from 0.5 to 0.55, and as the synthesis temperature decreased from 900 to 750 °C. Based on the synthesis and structure results, the phase relationship in the ternary diagram, Li–Mn–O, is discussed.

Praseodymium-modified SrBi2Ta2O9 with improved polarization properties at low electric field

Effects of praseodymium modification on the crystal structure and polarization properties were investigated for layered ferroelectric SrBi2Ta2O9 (SBT). Neutron powder diffraction and x-ray photoelectron analyses showed that praseodymium ions are substituted at the Sr site as Pr3+ and that Sr vacancies are created for compensating the charge difference between Sr2+ and Pr3+. Polarization measurements using dense ceramics indicated that low-field polarization properties were significantly improved by the praseodymium substitution. It is demonstrated that Pr–SBT has an advantage over SBT and Bi-modified SBT for a low-voltage operating ferroelectric-memory material.

Structural Distortion on Metal–Insulator Transition in Ordered Double Perovskite Ca2FeReO6

The crystal and magnetic structures of an ordered double perovskite, Ca 2 FeReO 6 , were studied by high-resolution neutron powder diffraction as a function of the temperature from 7 K to 550 K. All of the diffraction data were precisely refined by the Rietveld method, and we confirmed a structural phase transition at around 140 K where the metal–insulator transition occurs from ferrimagnetic metal (FM) to ferrimagnetic insulator (FI) phases. At this temperature, there exists a change in the distortion direction of [ReO 6 ] octahedra together with a spin reorientation, which strongly supports the occurrence of orbital ordering of the t 2 g electrons. FM and FI phases coexist in a narrow temperature range at around 140 K, which is typically seen in the first-order phase transition. A phase separation was not detected in our well-characterized sample.

Prebending effects in bronze route Nb3Sn wires

Control and optimization of the residual strain are one of the most important issues in the development of Nb3Sn superconducting magnets. We found that the repeated bending loads at room temperature change the prestrain of Nb3Sn wires and result in the enhancement of Jc, Bc 2 and Tc. We call this repeated bending strain prebending strain. In order to understand the prebending effect, superconducting properties were measured as functions of temperature, field, axial tensile stress and strain. In addition, the three-dimensional strain state was also evaluated by the neutron diffraction. Those obtained results strongly suggest that the prebending process changes the radial residual strain as well as the axial one independently. Hence, it is considered that the prebending effect is effective for the control and optimization of the three-dimensional strain state on the react and wind process.

High-temperature phase transition in lanthanum titanate perovskite La0.64(Ti0.92,Nb0.08)O3

Abstract High-temperature phase transition of lanthanum niobium titanate La 0.64 (Ti 0.92 ,Nb 0.08 )O 3 has been studied using neutron powder diffraction and the Rietveld method. The material is orthorhombic ( Cmmm ) between 295 and 534 K, while it is tetragonal ( P 4/ mmm ) between 637 and 740 K. The anti-phase tilt along the b axis of the oxygen octahedron around Ti and Nb atoms was found to induce the tetragonal-to-orthorhombic phase transition. The angle of the tilt and the b / a ratio of cell parameters decrease with an increase of temperature and become 0° and unity, respectively, at a transition temperature between 534 and 637 K.

Structural phase transitions in the ferroelectric oxides Ba1−xPbxBi2Nb2O9 (x = 0.375,0.625)

Temperature induced structural phase transitions in the lead doped Aurivillius oxides Ba1−xPbxBi2Nb2O9 (x = 0.375,0.625) are explored using powder neutron diffraction methods in the context of the pure parent compounds, PbBi2Nb2O9 and BaBi2Nb2O9. At both lead concentrations the system is found to exhibit behaviour similar to that found in PbBi2Nb2O9. That is, at room temperature the oxides are orthorhombic and the Pb rich compound transforms to a tetragonal structure via a second orthorhombic form. The presence of Ba lowers the phase transition temperatures relative to the undoped lead compound but is not sufficient to induce a change in the general mode of structural transition.

Acoustic phonon dynamics in liquid CCl4

The dynamic scattering factor S ( Q ,ω) was measured by high-resolution inelastic X-ray scattering using intense X-rays from a third-generation synchrotron radiation facility. The observed spectra demonstrate the existence of longitudinal propagating modes at small Q values, although the collective excitations are highly damped (not overdamped) as in the classical van der Waals liquid Ar. The Q –ω relation of the excitation shows a positive dispersion of about 37%, much larger than in liquid metals but similar to that in liquid Ar. The collective dynamics of liquid CCl 4 at small Q values can be interpreted in the framework of classical dense liquids.

Structural Studies of Hydrated Tricalcium Silicate by Neutron Powder Diffraction

Structure refinements have been carried out on the pure tricalcium silicate (C3S) and the hydrated C3S with the D2O–C3S mass ratio, which is 0.5, using neutron powder diffraction (NPD). The multi-phase Rietveld analysis of the hydrated C3S revealed the coexistence with the Ca(OD)2 (trigonal phase) and the unhydrated C3S (triclinic one). The Ca(OD)2 phase was hardly observed on the NPD patterns in the first ∼6u2009h of hydration, while the several Bragg reflections of Ca(OD)2 appeared drastically from ∼6 to ∼24u2009h, and then the hydration reaction rate was gradually suppressed. We could apply the Avrami-model to the initial hydration reaction process of C3S.

Structure of New Superionic Glasses (CuI)x(AgPO3)1−x

New superionic glasses (CuI)x(AgPO3)1−x with x=0.1, 0.2 and 0.3 have been prepared by rapid quenching method. Neutron diffraction experiments have been performed by a HIT-II spectrometer at the Neutron Spallation Source, KEK, Japan. A small first diffraction peak at low Q around 0.8u2009A was observed in the structure factor for x=0.2 and x=0.3, but for x=0.1 it shows only a shoulder that is similar to the undoped glass AgPO3. Three new peaks appear at the radial distribution function of (CuI)x(AgPO3)1−x at ∼ 1.9, 2.7 and 3.4u2009A, that may correspond to the Cu-O, Cu-I and Cu-Cu pair correlations, respectively. Those peaks increase with the amount of CuI. These results indicate that there are two different kinds of Cu atoms. Cu-I bond in the glass is very similar to that in crystalline α-CuI, and Cu-O bond is very similar to that in Cu2O insulating crystal.

Ordering of Oxygen and Nitrogen in J-Phase Lu4Si2O7N2

The crystal structure of cuspidine-type Lu 4Si2O7N2, the so-called J-phase, has been refined by Rietveld analysis from time-of-flight neutron powder diffraction data in order to confirm the ordering of oxygen and nitrogen atoms. This compound crystal lizes in a monoclinic cell, space group P21/c (No. 14-1) with a = 7.4243(1), b = 10.2728(1), c = 10.6628(1) Å, and β = 109.773(1)°. The diffraction peaks are also indexable with monoclinic lattice parameters of a = 7.4243(1), b = 10.2728(1), c = 10.7356(1) Å, and β = 110.828(1)°, space group P21/n (No. 14-2). The nitrogen atoms in Lu 4Si2O7N2 are connected to Si atoms and form Si 2(O,N)7 silicon oxynitride unit of composition Si 2O5N2. One N atom occupies the bridging site between Si atoms, and the other N atom is statisti c lly situated at the terminal sites. It is estimated that this unit comprises SiO 3N and SiO2N2 tetrahedra. The four Lu sites correspond to 6-fold, 7-fold (x2) and 8-fold coordination with Lu-(O,N) interatom ic distances of less than 2.9 Å. Introduction One of the interests in M–Si–O–N compounds is the configuration of oxyge n and nitrogen (O/N) in the structures. A cuspidine-type silicon-oxynitride with a general formula of RE4Si2O7N2 (RE = rare earth), the so-called RE-J-phase, has been known in sys tem of the type Si 3N4-SiO2-RE2O3 [1-9]. Recently, it has been found that Si 3N4 ceramics can have improved bending strength at high-temperatures by using Lu 2O3 as the additive instead of others such as Sc 2O3, La2O3, and Yb2O3 [10]. In Lu2O3-doped sintered Si 3N4 ceramics, the LuJ-phase, Lu 4Si2O7N2, was found at the grain boundary [11]. Unfortunately, only the cell constants and X-ray intensity data have been reported for Lu-J-phase [ 12]. The structure of LuJ-phase with respect to O/N configuration has not been investigated by other researc her . It is difficult to detect and distinguish light elements such as O and N by X-ray Key Engineering Materials Online: 2003-04-15 ISSN: 1662-9795, Vol. 237, pp 53-58 doi:10.4028/www.scientific.net/KEM.237.53