Ferroelectric memories have attracted much attention because of the invaluable advantage of nonvolatile and high-speed read-write functions. The current status of ferroelectric memory materials is briefly reviewed, and the effects of oxygen vacancies analyzed by first-principles band-structure calculations are described. We show that the defect engineering of cation vacancies as well as oxygen vacancies is effective in controlling the polarization properties of layered ferroelectrics.
New lithium ionic conductor, thio-LISICON (LIthium Super Ionic CONductor) is reviewed. The thio-LISICON has a general composition of Li4-xM1-yM'yS4, where the M and M' are Ge, Si or P, Al, Ga, respectively. In the thio-LISICON family, Li3.25Ge0.25P0.75S4 showed the highest conductivity of 2.2 × 10-3 Scm-1 at 25 °C among the materials synthesized, and the conductivity value is comparable to or even higher than liquid-electrolytes. The thio-LISICON has a structure similar to the γ-Li3PO4 type. Materials relationships to the oxides LISICON family is indicated. The advantages of the solid electrolyte to all solid-state lithium battery are discussed.
We investigated the relation between crystal structure, electronic states and electrode performance of LiMn2-xMxO4 (M = Zn, Ni etc.) as cathode materials for lithium ion secondary batteries. The cycle performance is improved by substitution of Mn with M. The crystal structure was determined by Rietveld analysis using powder neutron diffraction. The electron density distribution was obtained by XRD using MEM/Rietveld method. From the results, the stability of the host structure is important for cycle performance.
Crystal structure of the thermoelectric compound known as “Ca3Co4O9” has been determined using a 3+1 dimensional superspace group approach. The cobaltite has been revealed to be a misfit layer compound which consists of two monoclinic subsystems of [CoO2] and [Ca2CoO3] . The [CoO2] subsystem has a CdI2- type triangular lattice of cobalt, while the [Ca2CoO3] subsystem compises a three-layered ordered rock salt-type slab. Since the both subsystems have incommensurate b-axis lengths, the structure formula is expressed as [Ca2CoO3] pCoO2, wherep=bCoO2/bCa2CoO3. Due to the potential interaction between the subsystems, atomic displacive modulation is observed for each atom. The most significant modulation has been found in the Co-O layer of the [Ca2CoO3] subsystem.
The layered cobalt oxides show large thermopower, low resistivity, and low thermal conductivity, and are a promising material for thermoelectric power generation. They share the CdI2-type hexagonal CoO2 layer as a common unit, where large entropy of kBlog6 is stored on Co4+ ion. They show large anisotropy between the in-and out-of-plane directions depending on the block layers. Lattice thermal conductivity is smaller for heavier ions. Some of them show a characteristic misfit structure, which induces a significant in-plane anisotropy.
Recently mechanical alloying (MA) has been employed to synthesize various kinds of materials. The MA of graphite under Ar gas atmosphere transformed the hexagonal graphite into nanostructured graphite which has a lot of dangling bonds. In case of the mechanical alloying of the graphite under D2 gas atmosphere, deuterium was absorbed by the solid-gas reaction. The radial distribution function RDF (r) derived by neutron diffraction experiments informed us the location of deuterium in the nano-structured graphite. One is the C-D covalent bond and the other is deuterium located between layers of the graphite.
Metal hydride is one of key materials for developing fuel-cell vehicles, which is most demanded for reduction of CO2 emission. In this report, our recent work relating to crystal structure and nano-structure of metal hydrides studied by ‘in situ’ X-ray and neutron diffraction for clarifying the mechanism of hydriding reaction of metal hydride will be described.
Silicon nitride-based ceramics having high functional mechanical properties are described with respect to phase relationships between Si3N4 and the grain boundary phases of rare-earth silicon-oxynitrides. The crystal structures of La4Si2O7N2 (La-J-phase) and Lu4Si2O7N2 (Lu-J-phase) have been determined by time-of-flight neutron powder diffractometry. The local structures of Si2 (O, N) 7 ditetrahedra including O and N atom distribution in the rare-earth silicon-oxynitrides are revealed.
Realization of novel functions in transparent oxides utilizing intrinsic nanostructures in their crystal structures are discussed by taking layered compounds, InGaO3 (ZnO) m and LnCuOS/Se as examples. The layered structure forms“natural superlattice”or“multi-quantum well”, providing distinct features due to the electron confinement to the nano-sized layers including efficient blue emission and large nonlinear optical susceptibility associated with“room temperature exciton”and degenerated p-type conduction by“modulation doping”. These features allow for realizing emerging devices such as all optical switches and high mobility transparent field effect transistor.
Coordination Polymers (CPs) are opening up a new field in porous materials. A large number of CPs have been so far synthesized and characterized because of their versatile applicability to gas storage, molecular sieves, size- or shape-selective catalysis, and ion exchange. Moreover, CPs, in spite of their crystalline form, are suitable for creating flexible and dynamic porous frameworks, which are hardly realized by inorganic zeolites and carbon materials. We described here the several unique structures and functions of CPs.
Recently the time-resolved X-ray analysis has been applied to organic crystals. Two-dimensional detector and low temperature technique made it possible to analyze the metastable species produced in the crystal by photoirradation. The structures of the diplatinum complexes at the excited state have been analyzed with the same technique. The time-resolved analysis using the combination of the pulsed X-rays from synchrotron radiation and the exciting pulsed photons from laser has been developed. The structural changes in the processes of photoexcitation and the photo-induced phase transition in the time scale less than 1 ns have been made clear.
We have reviewed recent studies on atomic short-range order (SRO) in Al-Ni-Co decagonal quasicrystals by anomalous X-ray scattering. Anisotropic distributions of diffuse scattering around Bragg reflections are analyzed in context of phasonic diffuse scattering. Phason strains also cause peak broadening of Bragg reflections. Both chemical and topological distortions are responsible for the order-disorder phase transitions. In addition, a decrease of the Bragg intensity at high temperatures and the anomalous Debye-Waller factor are explained as consequences of the phason flip.
Review is given on recent neutron scattering studies of the prototypical relaxor Pb (Mg1/3Nb2/3) O3 (PMN), particularly focusing on diffuse scattering from polar nanoregions (PNRs) . Inconsistency between the relative intensities of diffuse scattering and those of the lowest-energy transverseoptic phonons in PMN has remained unsolved for a decade. A simple model consisting of two components δcm and δshift, which has been proposed to solve the contradiction, is introduced in this article.
The relation between structures and electromagnetic properties is reviewed on vanadium oxides with the trellis lattice and the A-site ordered perovskite manganites. These compounds show novel phase transitions in which the charge, orbital and spin degrees of freedom are coupled with the lattices. For example, a quarter filled ladder system α'-NaV2O5 undergoes a charge order transition at 35 K, accompanied by lattice dimerization and spin gap formation, and under high pressure it shows various stacking sequences of the zigzag-type in-plane charge-order along the c-axis, so-called “Devils Flowers”. On the other hand, the novel properties of the A-site ordered perovskite manganites are closely related to the most remarkable structural property that the MnO2 layer is sandwiched by two types of rock salt layers with very different lattice sizes.
Since the discovery of high-Tc superconductor MgB2 by our group, a large number of experimental and theoretical works has been performed. The superconductivity in this simple binary alloy is essentially explained within a framework of BCS theory from both sides. It is considered that the high-Tc superconductivity at 40 K in MgB2 is caused by the metallic nature of the two dimensional sheets formed by B atoms and the presence of strong electron-phonon interactions which together with the high vibrational frequencies of the light B atoms ensure a high transition temperature. The main problems in this stage are the possibility in application of MgB2 and whether high-Tc superconductors exist in boride and carbide systems. In this paper, we review the physical properties of MgB2 which has been cleared until now.
A structural change has been found in PrRu4P12 accompanied by a metal-insulator transition at about 60 K by electron diffraction. At room temperature the structure of Filled-Skutterudite type compound PrRu4P12 belongs to space group Im3 (No.204), and is a metal. At about transition temperature new and weak reflections appear at h + k + 1= 2n + 1 positions. Space group and structure of the low temperature phase is discussed to be P23 (No. 195) or Pm3 (No.200) . The origin of the transition was discussed.
Pyrochlore oxides have attracted much interest because their crystal structures include the corner-sharing tetrahedra. This represents an ideal system for studying the effect of geometrical frustration in the limit of low disorder. Recently, a new class of magnetic state known as“spin ice”is discovered in the pyrochlore rare-earth oxides. Recent our study reveals a new macroscopically degenerate ground state in the spin ice induced by a magnetic field. This is called“kagomé ice”.
The crystal structure, magnetic and electrical properties of A-site-ordered perovskite manganites Ln1/2Ba1/2MnO3 (Ln=rare earth) were investigated. In the ordered Ln1/2Ba1/2MnO3 (Ln = Sm-Y), a new charge/orbital ordering pattern was found. Electron diffraction studies revealed a series of superlattice reflections with modulation vectors at q2 = (1/2, 1/2, 1/2) as well as at q1= (1/4, 1/4, 0) in the tetragonal setting (ap × ap × 2ap, ap being the cubic perovskite lattice parameter) . Together with the results of the resonant X-ray scattering, a model for the three-dimensional charge/orbital ordering is proposed.