Showing papers by "Masaki Takata published in 2002"
TL;DR: The direct observation of dioxygen molecules physisorbed in the nanochannels of a microporous copper coordination polymer by the MEM (maximum entropy method)/Rietveld method, using in situ high-resolution synchrotron x-ray powder diffraction measurements is reported.
Abstract: We report the direct observation of dioxygen molecules physisorbed in the nanochannels of a microporous copper coordination polymer by the MEM (maximum entropy method)/Rietveld method, using in situ high-resolution synchrotron x-ray powder diffraction measurements. The obtained MEM electron density revealed that van der Waals dimers of physisorbed O2 locate in the middle of nanochannels and form a one-dimensional ladder structure aligned to the host channel structure. The observed O–O stretching Raman band and magnetic susceptibilities are characteristic of the confined O2 molecules in one-dimensional nanochannels of CPL-1 (coordination polymer 1 with pillared layer structure).
547 citations
TL;DR: In this paper, it was found from x-ray diffraction measurements that the ferroelectric polarization originates from the tilting of polyhedra and buckling of R layers, which persists up to 1000 K.
Abstract: A ferroelectricity-magnetism-coexisting system, hexagonal $R{\mathrm{MnO}}_{3}(R=\mathrm{Y},$ Lu, and Sc), has been investigated by synchrotron x-ray and neutron powder diffraction measurements. It is found from x-ray diffraction measurements that the ferroelectric polarization originates from the tilting of ${\mathrm{MnO}}_{5}$ polyhedra and the buckling of R layers, which persists up to 1000 K. Neutron diffraction measurements have revealed the reduction of ordered moments from the expected value, as well as strong magnetic diffuse scattering existing far above ${T}_{\mathrm{N}},$ both of which are caused by geometrical frustration of the triangular lattice of Mn ions. We have also investigated the effects of Zr doping into the R site and have found that Zr doping drastically suppresses both ferroelectric distortion and magnetic ordering.
240 citations
TL;DR: A triclinic perovskite, BiNiO3, has been synthesized at a high pressure of 6 GPa in an oxidizing atmosphere as mentioned in this paper, which showed insulating behavior with localized spins of S = 1.
Abstract: A new triclinic perovskite, BiNiO3, has been synthesized at a high pressure of 6 GPa in an oxidizing atmosphere. Structure refinements based on synchrotron X-ray powder diffraction and bond-valence calculations revealed a disproportionation of the Bi ions into Bi3+ and Bi5+, both being located in distorted oxygen octahedra. Because of the presence of Bi5+, the oxidation state of Ni was +2, rather than +3 as had been expected; BiNiO3 thus showed insulating behavior with localized spins of S
= 1.
130 citations
TL;DR: In this article, the bonding nature of hydrogen in MgH2 was fully uncovered by examining the charge density distribution of this substance obtained by the maximum entropy method from the synchrotron radiation powder data.
Abstract: MgH2 is one of the promising base materials for hydrogen storage, which is a key technology of clean energy source. In this study, the bonding nature of hydrogen in MgH2 was fully uncovered by examining the charge density distribution of this substance obtained by the maximum entropy method from the synchrotron radiation powder data. MgH2 can be expressed as Mg1.91+ H0.26−, which is much weaker ionicity than the theoretical expectations. It also shows weak covalence between Mg and H. Though the bonding nature of hydrogen in MgH2 is rather complex, i.e., the mixture of ionic and covalent bonding, it is certain that hydrogen is weakly bonded to Mg, which must be a big advantage of hydrogenation–dehydrogenation of this substance.
120 citations
TL;DR: The MEM analysis by ENIGMA becomes applicable to huge systems, such as proteins and polymers, when the phased structure factors are provided and the fast Fourier transformation (FFT) technique is implemented.
Abstract: ENIGMA (Electron and Nuclear Image Generator by Max-ent Analysis) is a program package to evaluate three-dimensional electron and nuclear density from X-ray and neutron diffraction data by using the maximum-entropy method (MEM). Compared with the previous program package MEED, ENIGMA saves computing time and frees memory space at the same time by employing parallel data processing. The fast Fourier transformation (FFT) technique is also implemented. As a consequence of these improvements, the MEM analysis by ENIGMA becomes applicable to huge systems, such as proteins and polymers, when the phased structure factors are provided. The package is transferable to a wide variety of parallel computers, because it is written in Fortran 90 and a standard message-passing interface (MPI).
106 citations
TL;DR: In this article, a structural transition from the cubic (F\(\overline{4}\)3m; Z = 4) to the tetragonal (I\(overline {4}µm2, Z = 2) phase was observed.
Abstract: Temperature dependence of atomic coordinates is determined for RbMn[Fe(CN) 6 ] by means of synchrotron-radiation (SR) X-ray powder structural analysis. We observed a structural transition from the cubic (F\(\overline{4}\)3m; Z = 4) to the tetragonal (I\(\overline{4}\)m2; Z = 2) phase at ≈ 210 K in the cooling run and at ≈ 300 K in the warming run. In the low-temperature tetragonal phase, we found Jahn–Tellar type distortion of the MnN 6 octahedra and compression of the averaged Fe–C bond distance. These structural data suggest that the structural transition is triggered by the inter-metallic charge-transfer from the Mn(II) site to the Fe(III) site.
53 citations
TL;DR: In cubic PbZrO 3, which undergoes an antiferroelectric phase transition, the distinct disorder of Pb at twelve sites toward the neighboring O is detected for the first time by analyzing high-energies.
Abstract: In cubic PbZrO 3 , which undergoes an antiferroelectric phase transition, the distinct disorder of Pb at twelve sites toward the neighboring O is detected for the first time by analyzing high-energ...
52 citations
TL;DR: In this paper, electron-density distributions of paraelectric and ferroelectric materials were analyzed by means of synchrotron x-ray diffraction, and the obtained electron density distributions well reproduced the atomic arrangements of the material.
Abstract: Electron-density distributions of paraelectric and ferroelectric ${\mathrm{SrBi}}_{2}{\mathrm{Nb}}_{2}{\mathrm{O}}_{9}$ were analyzed by means of synchrotron x-ray diffraction. The obtained electron-density distributions well reproduced the atomic arrangements of the material. Bonding electrons due to hybridization of orbitals were clearly seen in the Nb-O bonds in the octahedron and in the Bi-O bonds in the ${\mathrm{Bi}}_{2}{\mathrm{O}}_{2}$ layer. The hybridization between $\mathrm{Nb}(4d)$ and $\mathrm{O}(2p)$ orbitals was enhanced in the ferroelectric phase, and one of the Nb-O bonds showed a strong covalent character below the ferroelectric Curie temperature. Weak hybridization of the $\mathrm{Sr}(5s)$ with $\mathrm{O}(2p)$ orbitals appears to play an important role in structural distortion and ferroelectric polarization. The changes in the electron-density distribution through the ferroelectric phase transition are similar to those observed in ferroelectric ${\mathrm{PbTiO}}_{3}$ perovskite.
28 citations
TL;DR: In this article, a spin-crossover complex with a photo-excitation with a continuous-wave (CW) green (532 nm) laser at 91 K was studied.
Abstract: We have performed in situ synchrotron-radiation X-ray powder structural analysis in a spin-crossover complex ([Fe(ptz) 6 )](BF 4 ) 2 : ptz = propyltetrazole) under a photo-excitation with a continuous-wave (CW) green (532 nm) laser at 91 K. The Fe 2+ ions in the Fe(ptz) 6 molecules take the low-spin (LS: S = 0) state at the ground state, and are photo-excited selectively into the high-spin (HS: S = 2) state. When the excitation power P exceeds the critical value, we observed a novel secondary phase which can be ascribed to the condensation of the photo-excited HS ions.
23 citations
TL;DR: In this article, the temperature dependence of atomic coordinates is precisely determined for an iron (II) complex, Fe(ptz) 6 ](BF 4 ) 2 (ptz = propyltetrazole), by means of synchrotron-radiation (SR) x-ray powder diffraction measurements.
Abstract: Temperature dependence of atomic coordinates is precisely determined for an iron (II) complex, [Fe(ptz) 6 ](BF 4 ) 2 (ptz = propyltetrazole), by means of synchrotron-radiation (SR) x-ray powder diffraction measurements. The spin state of the Fe 2+ ions is known to change from high-spin (HS; S = 2) to low-spin (LS; S = 0) with a decrease in temperature to below T c ≈130 K. We have found that the spin state transition accompanies a significant contraction of the Fe–N4 bond distance from ≈ 2.20 A (HS) to ≈ 2.00 A (LS).
19 citations
TL;DR: Dept. of Crystalline Materials Science at Nagoya University, Nagoya 464-8603 as mentioned in this paper, 1.1.1 Koto, Mikazuki-cho, Sayo-gun, Hyogo 679-5198
Abstract: Dept. of Crystalline Materials Science, Nagoya University, Nagoya 464-8603 Dept. of Applied Physics, Nagoya University, Nagoya 464-8603 SPring-8/JASRI, 1-1-1 Koto, Mikazuki-cho, Sayo-gun, Hyogo 679-5198 Faculty of Science and Technology, Science University of Tokyo, Chiba 278-8510 ISSP, University of Tokyo, Kashiwa 277-8581 Dept. of Physics, Saitama University, Saitama 338-8570 SPring-8/JAERI, 1-1-1 Koto, Mikazuki-cho, Sayo-gun, Hyogo 679-5198
TL;DR: In this article, the precise crystal structures of the hole-doped Mott insulator system, Y 1- x Ca x TiO 3 ( x = 0.37, 0.39 and 0.41), are studied for the temperature range between 20 K and 300 K by synchrotron radiation (SR) X-ray powder diffraction.
Abstract: In order to explore the origin of the metal–insulator (M–I) transition, the precise crystal structures of the hole-doped Mott insulator system, Y 1- x Ca x TiO 3 ( x =0.37, 0.39 and 0.41), are studied for the temperature range between 20 K and 300 K by synchrotron radiation (SR) X-ray powder diffraction. For both Y 0.63 Ca 0.37 TiO 3 and Y 0.61 Ca 0.39 TiO 3 compositions, the orthorhombic ( P b n m ) – monoclinic ( P 2 1 / n ) structural phase transition occurs at approximately 230 K, which is much higher than their individual M–I transition temperatures, i.e., 60 K and 130 K, respectively. For these compositions, significant phase separation (low-temperature orthorhombic + monoclinic) is also found in the vicinity of the M–I transition temperature. On the other hand, Y 0.59 Ca 0.41 TiO 3 , which does not exhibit M–I transition and preserves a metallic behavior down to 1.5 K, is in a two phase state from 20 K to 300 K. It is concluded that the existence of the phase separation causes the M–I transition in...
TL;DR: In this article, the spin density distribution of the B 6 octahedron with CaB 6 was deduced by the maximum entropy method (MEM) using polarized neutron diffraction (PND) data.
Abstract: Here, we present, for the first time, the spin density distribution of CeB 6 , isostructural with CaB 6 , deduced by the maximum entropy method (MEM) using polarized neutron diffraction (PND) data. Surprisingly, it reveals the presence of significant amounts of localized spin moment at non atomic sites between the nearest-neighbor boron atoms and the center of the triangle surface in the B 6 octahedron network, in addition to the negative spin moment in the center of the B 6 octahedron.
TL;DR: In this article, the coexistence of the magnetic clusters in terms of intrinsic change separation was investigated by means of small-angle neutron scattering (SANS) and Lorentz micrograph experiments, and it was found that ferromagnetic microclusters of the order of ∼10 nm are invaded in the antiferromagnetic matrix with the G-type spin structure.
Abstract: Microscopic magnetic structures of Tb 1- x Ca x MnO 3 ( x ∼1) have been investigated by means of small-angle neutron scattering (SANS) and Lorentz micrograph experiments. We have found that ferromagnetic microclusters of the order of ∼10 nm are invaded in the antiferromagnetic matrix with the G-type spin structure. We have interpreted the coexistence of the magnetic clusters in terms of intrinsic change separation.
TL;DR: In this article, a half-doped manganites (Nd 0 1 2 5 Sm 0 8 7 5 ) 1 /2 Sr 1 / 2 MnO 3, by means of the synchrotron-radiation x-ray powder diffraction measurement was investigated.
Abstract: Pressure effects on the structural parameters are investigated in a half-doped manganites (Nd 0 1 2 5 Sm 0 8 7 5 ) 1 / 2 Sr 1 / 2 MnO 3 , by means of the synchrotron-radiation x-ray powder diffraction measurement The compound shows a pressure-induced metal-insulator transition due to the subtle competition between the double-exchange interaction and the charge-ordering instability [Y Tokura et al, Phys Rev Lett 76, 3184 (1996)] We have found that application of pressure induces the four-long and two-short deformation of the MnO 6 octahedra We present a possible scenario for the pressure-induced charge-ordering
TL;DR: In this paper, the electron density distribution for nominal rare earth metals and fullerene molecules was calculated by a maximum entropy method based on the Rietveld analysis of synchrotron X-ray diffraction pattern, unambiguously demonstrated a covalent SmC bond, which is almost as strong as the interatomic bonding of crystal Si.
TL;DR: In this paper, the magnetic moments are located between the Borons and the center of triangle bonding in the B 6 octahedral, which is the first evidence that the magnetic moment is located between Borons.
Abstract: We performed the polarized neutron diffraction measurements in phase II of CeB 6 to observe the magnetic form factors. By the Maximum Entropy Method (MEM), we obtained the unexpected spin density distribution in CeB 6 . We clearly observed the existence not only at Ce site but also at between B 6 octahedral and at the center of triangle bonding in the B 6 octahedral. This is the first evidence that the magnetic moments are located between the Borons.
TL;DR: In this paper, a large Debye-Scherrer camera with an imaging plate as a detector has been designed and installed at BL02B2 in SPring-8 for the charge density study, which has succeeded in observing significant electron-orbital hybridizations revealed in the bonding electron distributions of PbTiO 3 and BaTiO3, which are closely related to the ferroelectric properties.
Abstract: The advent of third-generation synchrotron-radiation (SR) provides a new opportunity of the supreme powder diffractometry, which allows one to determine accurate charge-densities, when the collected data are analyzed by the MEM/Rietveld method. The Large Debye-Scherrer Camera with an Imaging Plate as a detector has been designed and installed at BL02B2 in SPring-8 for the charge-density study. We have succeeded in observing the significant electron-orbital hybridizations revealed in the bonding electron distributions of PbTiO 3 and BaTiO 3 , which are closely related to the ferroelectric properties.
TL;DR: In this article, the physical properties relating to 4f electrons in cerium phosphide, especially the temperature dependence and the isomorphous transition that occurs at around 10 GPa, were studied by means of x-ray powder diffraction and charge density distribution maps derived by the maximum-entropy method.
Abstract: The physical properties relating to 4f electrons in cerium phosphide, especially the temperature dependence and the isomorphous transition that occurs at around 10 GPa, were studied by means of x-ray powder diffraction and charge density distribution maps derived by the maximum-entropy method. The compressibility of CeP was exactly determined using a helium pressure medium and the anomaly that indicated the isomorphous transition was observed in the compressibility. We also discuss the anisotropic charge density distribution of Ce ions and its temperature dependence.
TL;DR: The recent progress of the accurate charge density studies of fullerides by the maximum entropy method (MEM) utilizing X-ray powder diffraction is reviewed in this article, where the results for C60 superconductors (Rb2CsC60, K2RbC60 Na2RbbC60) and endohedral metallo-fullerenes (Y@C82, La@C 82, Sc@c82, Sc2@C84, Sc 2@C 84, Sc 3@C83, Sc 4.6, Sc
Abstract: The recent progress of the accurate charge density studies of fullerides by the Maximum Entropy Method (MEM) utilizing X-ray powder diffraction is reviewed. Results for C60 superconductors (Rb2CsC60, K2RbC60 Na2RbC60) and endohedral metallofullerenes (Y@C82, La@C82, Sc@C82, Sc2@C84, Sc3@C82, Sc2@Cfi6 and Sc2C2@C84) are given. The MEM charge densities of the fullerene superconductors, Rb2CsC60, K2RbC60 and Na2RbC60, show distinct structural differences from that of non-superconductors, C60 and Li2CsC60, reflecting the superconducting properties. The charge deficiencies of the doped metal atoms, which should be associated with charge transfer from the metal atoms to the C60 molecule seems to have strong correlation to the superconducting transition temperature, Tc. As the bigger the charge transfer, the higher the Tc. The accurate MEM charge densities of metallofullerenes presents the direct imaging of endohedral nature of metallofullerenes indicating the charge transfer from metal atoms to carbon cage, which governs the stability of the unique endohedral structures. The MEM/Rietveld method, which is the crucial method for the structure determination of fullerides, is also mentioned in some detail.
TL;DR: In this paper, a simple scheme is introduced which extracts the valence electron density from X-ray diffraction data by employing the maximum entropy method, and reveals in detail the bonding nature.
Abstract: A simple scheme is introduced which extracts the valence electron density from X-ray diffraction data by employing the maximum entropy method. It is applied to silicon, and reveals in detail the bonding nature. A spool-shaped charge distribution is observed between Si atoms corresponding to the bond, and the charge density along the bonding direction has a two-peak structure. Furthermore, concentric sphere-shaped reduction in charge density is observed around the nuclei, which corresponds to the nodes in wavefunctions. These features agree quantitatively with those obtained by first principles calculation.
31 Oct 2002