Showing papers by "Masaki Takata published in 2001"
TL;DR: Clear evidence of the Pb-O hybridization in tetragonal PbTiO3, which has been theoretically predicted as a key factor of much larger ferroelectricity of this substance than that of Ba TiO3 is obtained.
Abstract: Accurate charge-density distributions of cubic and tetragonal ${\mathrm{PbTiO}}_{3}$ and ${\mathrm{BaTiO}}_{3}$ have been obtained by the MEM(maximum entropy method)/Rietveld analysis using synchrotron-radiation powder data. The Pb-O bonds in tetragonal ${\mathrm{PbTiO}}_{3}$ show rather strong covalency, while those in cubic ${\mathrm{PbTiO}}_{3}$ are ionic. This is the clear evidence of the Pb-O hybridization in tetragonal ${\mathrm{PbTiO}}_{3}$, which has been theoretically predicted as a key factor of much larger ferroelectricity of this substance than that of ${\mathrm{BaTiO}}_{3}$. Tetragonal ${\mathrm{PbTiO}}_{3}$ forms a layered structure of a two-dimensional covalent-bonding network consisting of the $\mathrm{Ti}\ensuremath{-}{O}_{5}$ pyramid.
392 citations
21 Jul 2001-Nuclear Instruments & Methods in Physics Research Section A-accelerators Spectrometers Detectors and Associated Equipment
TL;DR: In this paper, the design and performance of a large Debye-scherrer camera with imaging plate (IP) as a detector is reported, which was very recently installed at SPring-8, BL02B2, and provides a rapid and accurate powder diffraction system utilizing third generation SR.
Abstract: The design and performance of a large Debye–Scherrer Camera with imaging plate (IP) as a detector, which was very recently installed at SPring-8, BL02B2, is reported. By taking advantage of high beam quality of SPring-8, the camera enables one a rapid collection of a high counting statistics and high angular resolution powder pattern, which can lead to accurate structure analyses. The camera also provides easy access to structural changes at varied temperatures between 15–1000 K. The camera provides a rapid and accurate powder diffraction system utilizing third generation SR.
307 citations
TL;DR: Not Sc 2 @C86 but Sc2 C2 @C84 : that is the conclusion of 13 C NMR spectroscopic and synchrotron X-ray powder diffraction studies on a new metallofullerene obtained by arc discharge of Sc2 O3 /graphite rods.
Abstract: Not Sc2 @C86 but Sc2 C2 @C84 : that is the conclusion of 13 C NMR spectroscopic and synchrotron X-ray powder diffraction studies on a new metallofullerene obtained by arc discharge of Sc2 O3 /graphite rods. The picture shows a schematic representation of the deduced structure, in which the novel scandium carbide cluster Sc2 C2 is encaged in D2d -C84 .
260 citations
TL;DR: In this article, the thermal expansion coefficient in single-walled carbon nanotube bundles was determined as $(\ensuremath{-}0.15\ifmmode\pm\else\textpm\fi{} 0.20)\ifmmodes\times\else \texttimes\fa{}{10}^{--}5} (1/\mathrm{K})$ for the tube diameter.
Abstract: Thermal expansion coefficient in single-walled carbon nanotube bundles was determined as $(\ensuremath{-}0.15\ifmmode\pm\else\textpm\fi{}0.20)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5} (1/\mathrm{K})$ for the tube diameter and $(0.75\ifmmode\pm\else\textpm\fi{}0.25)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5} (1/\mathrm{K})$ for the triangular lattice constant by means of x-ray scattering between 300 K to 950 K. The value for the intertube gap was $(4.2\ifmmode\pm\else\textpm\fi{}1.4)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5} (1/\mathrm{K}),$ which is larger than $2.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5} (1/\mathrm{K})$ for the c-axis thermal expansion in graphite. The results reveal the presence of a remarkably larger lattice anharmonicity in nanotube bundles than that of graphite. The small value for the tube diameter is consistent with the seamless tube structure formed by a strong covalent bond between carbon atoms comparable to that in graphite.
174 citations
TL;DR: The recent progress of the accurate charge density studies by the Maximum Entropy Method (MEM) utilizing X-ray powder diffraction is reviewed with some examples in this paper, where results for PrBCO (PrBa 2 Cu 3 O 7-δ ), YBCO(YBa 2 CsC 60, K 2 RbC 60, Na 2 R bC 60 and Na 2 rbC60 ) and the layered manganite, NdSr 2 Mn 2 O 7, which is well known as colossal magnetoresistive (C
Abstract: The recent progress of the accurate charge density studies by the Maximum Entropy Method (MEM) utilizing X-ray powder diffraction is reviewed with some examples. Results for PrBCO (PrBa 2 Cu 3 O 7-δ ), YBCO (YBa 2 Cu 3 O 7-δ ), C 60 superconductors (Rb 2 CsC 60 , K 2 RbC 60 , Na 2 RbC 60 ) and the layered manganite, NdSr 2 Mn 2 O 7 , which is well known as colossal magnetoresistive (CMR)-related material, are given. For non-super conductor, PrBCO, it is found in the MEM charge density that there exist directional robes of the charge density from Pr atom toward the O atoms in the CuO 2 conduction planes. On the other hand, for a very well known high Tc super conductor, YBCO, appreciable charge densities in the interatomic region around the Y atom is not recognized in the MEM charge density. The distinct difference of the charge densities between PrBCO and YBCO presents clear experimental evidence of the hybridization between Pr(4f)-O(2pπ) orbitals which supports the idea that the hole trapping by the hybridized states suppresses the superconductivity in PrBCO. The MEM charge densities of the fullerene superconductors, Rb 2 CsC 60 , K 2 RbC 60 and Na 2 RbC 60 , show distinct structural differences from that of non-superconductors, C 60 and Li 2 CsC 60 , reflecting the superconducting properties. And the charge deficiencies of the doped metal atoms, which should be associated with charge transfer from the metal atoms to the C 60 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 density of antiferromagnetic manganite, NdSr 2 Mn 2 O 7 , presents the direct imaging of spontaneous ordering of the dx 2 - y 2 orbital indicating anisotropic exchange couplings between the local-spins on the Mn sites, which causes an unique layered-type spin order. The theoretical background of the MEM is also mentioned in some detail.
153 citations
TL;DR: In this paper, the design and performance of a large Debye-Scherrer Camera with Imaging Plate (IP) as detectors, which was very recently installed at SPring-8, BL02B2, is reported.
137 citations
TL;DR: X-ray diffraction study of multiwalled carbon nanotube (MWNT) grown by arc discharge in hydrogen atmosphere is presented in this paper, where it is found that the thermalexpansion coefficient along the radial direction of MWNT is widely distributed in a range from $1.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}
Abstract: X-ray diffraction study of multiwalled carbon nanotube (MWNT) grown by arc discharge in hydrogen atmosphere is presented. It is found that the thermal-expansion coefficient along the radial direction of MWNT is widely distributed in a range from $1.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5} {\mathrm{K}}^{\ensuremath{-}1}$ to $2.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5} {\mathrm{K}}^{\ensuremath{-}1},$ indicating the existence of both of Russian doll MWNT and highly defective MWNT. Russian doll MWNT is suggested to have the outer diameter less than $\ensuremath{\sim}100 \mathrm{}\mathrm{\AA{}}\mathrm{}.$ Thicker MWNT's are typically highly defective, and may have the jelly roll (scroll) or defective polygonal structure consisting of flat graphite domains.
115 citations
TL;DR: A double shell of Platonic solids: A trajectory that connects the six-membered rings of Ih -C80 is followed by the two La atoms of La2 @C80 according to maximum entropy/Rietveld analysis of the synchrotron powder-diffraction data produces a pentagonal-dodecahedral charge-density distribution for La2 inside the icosahedral C80 cage.
Abstract: A double shell of Platonic solids: A trajectory that connects the six-membered rings of Ih -C80 is followed by the two La atoms of La2 @C80 according to maximum entropy/Rietveld analysis of the synchrotron powder-diffraction data. The result is a pentagonal-dodecahedral charge-density distribution for La2 (brown in the picture) inside the icosahedral C80 cage (green).
90 citations
TL;DR: In this paper, the spinel AlV 2 O 4 shows a phase transition at ∼700 K with anomalies of transport and magnetic properties, which corresponds to a charge-ordering transition.
Abstract: We have found that spinel AlV 2 O 4 shows a phase transition at ∼700 K with anomalies of transport and magnetic properties. It is clarified that this corresponds to a charge-ordering transition, where the V 2.5+ ions are separated into three V 2.5-δ ions and one V 2.5+3δ ion. This three-one type of charge ordering, which is coupled with rhombohedral distortion along the (111) axis, reduces the high degeneracy of ground states inherent in this spinel structure, and thus can be understood in terms of “charge frustration.”
70 citations
TL;DR: In this article, temperature variation of the crystal structure has been investigated for Tb 2 Ba 2 Co 4 O 11, which shows a metal-insulator (MI) transition at T MI = 340 K.
Abstract: Temperature variation of the crystal structure has been investigated for Tb 2 Ba 2 Co 4 O 11 , which shows a metal-insulator (MI) transition at T MI = 340 K The compound has a layered-type double-perovskite structure along the c -axis, with alternating the CoO 6 octahedron and the CoO 5 pyramids along the a -axis With decrease of temperature below T MI , the distortion of the octahedral and pyramidal sites is fairly released, suggesting that spin state transition of the Co 3+ ions is the origin for the MI transition
TL;DR: In this article, the accurate charge density of MgB 2 was observed at room temperature (R.T.) and 15 K by the maximum entropy method (MEM)/Rietveld analysis using synchrotron radiation powder data.
Abstract: The accurate charge density of MgB 2 was observed at room temperature (R.T.) and 15 K by the maximum entropy method (MEM)/Rietveld analysis using synchrotron radiation powder data. The obtained charge density clearly revealed the covalent bonding feature of boron forming the 2D honeycomb network in the basal plane, on the other hand, Mg is found to be in divalent state. A subtle but clear charge concentration was found on boron 2D sheets at 15 K, which should be related to superconductivity.
TL;DR: In this article, the authors used Synchrotron X-ray diffraction to measure the isothermal compressibility of the binary boride system with stoichiometry MgB2, revealing a stiff tightly packed incompressible solid with only moderate bonding anisotropy between intra-and inter-layer directions.
Abstract: Considerable excitement has been caused recently by the discovery that the binary boride system with stoichiometry MgB2 is superconducting at the remarkably high temperature of 39 K (1). This potentially opens the way to even higher Tc values in a new family of superconductors with unexpectedly simple composition and structure. The simplicity in the electronic and crystal structures could allow the understanding of the physics of high-Tc superconductivity without the presence of the multitude of complicated features, associated with the cuprates. Synchrotron X-ray diffraction was used to measure the isothermal compressibility of MgB2, revealing a stiff tightly-packed incompressible solid with only moderate bonding anisotropy between intra- and inter-layer directions. These results, combined with the pressure evolution of the superconducting transition temperature, Tc establish its relation to the B and Mg bonding distances over a broad range of values.
TL;DR: In this paper, structural parameters for Mo pyrochlore were systematically investigated by means of synchrotron radiation x-ray powder diffraction, and it was shown that the Mo-O-Mo bond angle increases with the average ionic radius of rare-earth ion.
Abstract: Structural parameters are systematically investigated for Mo pyrochlore ${R}_{2}{\mathrm{Mo}}_{2}{\mathrm{O}}_{7}$ $(R=\mathrm{Dy},$ Gd, Sm, and Nd) by means of synchrotron radiation x-ray powder diffraction. This system shows a crossover from the spin-glass (SG) state to the ferromagnetic metallic (FM) state as the averaged ionic radius ${r}_{\mathrm{R}}$ of the rare-earth ion increases. We have found the Mo-O-Mo bond angle increases with ${r}_{\mathrm{R}}$ from $129.7\ifmmode^\circ\else\textdegree\fi{}$ for ${\mathrm{Dy}}_{2}{\mathrm{Mo}}_{2}{\mathrm{O}}_{7}$ (SG) to $131.5\ifmmode^\circ\else\textdegree\fi{}$ for ${\mathrm{Nd}}_{2}{\mathrm{Mo}}_{2}{\mathrm{O}}_{7}$ (FM). This behavior is opposite to the so-called Kanamori-Goodenough rule.
TL;DR: In this paper, a joint experimental and theoretical study of the stability and structural properties of Ba-doped silicon type-I clathrates Ba 8Si46 synthesized under high pressures is presented.
Abstract: We present a joint experimental and theoretical study of the stability and structural properties of Ba-doped silicon type-I clathrates Ba 8Si46 synthesized under high pressures. The thermodynamic stability of Ba8Si46 under high pressure has been discussed from the total energy calculations of some barium silicides within the local density approximation ~LDA!. We have theoretically found that pressure favors the formation of the clathrate phase as experimentally observed. We have also performed a synchrotron x-ray-diffraction experiment of Ba8Si46 prepared under high pressures. Some of the missing endohedral Ba elements in the small cage of Si~20! have been observed by x-ray crystallography, while big cages of Si~24! are found to be completely occupied by Ba elements. The stabilization energies of Ba atoms in the endohedral sites estimated within the present LDA calculation suggest that this is presumably attributed to the energetical site preference of Ba atoms between d(6) and a(2) sites. In addition, the isothermal parameter of Ba in the big cage of Si ~24! has been found to be larger than that in the small Si~20! unit, which is consistent with some theoretical predictions in earlier works.
TL;DR: In this article, the occupancy of the Fe3+ ion on one sublattice of the perovskite-B site by Rietveld refinement of the synchrotron-radiation X-ray powder patterns was evaluated.
Abstract: Heat treatment effects on the physical properties have been investigated for Sr2FeMoO6 single crystals with double perovskite structure. We have evaluated the occupancy g of the Fe3+ ion on one sublattice of the perovskite-B site by Rietveld refinement of the synchrotron-radiation X-ray powder patterns. Both the saturation magnetization Ms and critical temperature Tc for the ferrimagnetic transition decrease linearly with g in the large-g region (g ≥~ 0.84), but begin to deviate from the empirical relations for g ≤~ 0.84. We interpreted this behavior in terms of variation of the correlation length of the spin ordering.
TL;DR: In this article, an enhancement of the specific heat jump at the spin-ordering temperature was found, interpreted in terms of the intrinsic charge separation, and the authors interpreted the enhancement as an improvement of the temperature variation of the magnetic susceptibility, resistivity and specific heat.
Abstract: Temperature variation of the magnetic susceptibility \ensuremath{\chi}, resistivity \ensuremath{\rho}, specific heat C, and lattice constants has been investigated in electron-doped ${\mathrm{CaMnO}}_{3}.$ The parent ${\mathrm{CaMnO}}_{3}$ is an antiferromagnetic band insulator, and shows an insulator-metal crossover with electron doping, together with an enhanced ferromagnetic component. We have found an enhancement of the specific heat jump $\ensuremath{\Delta}C$ at the spin-ordering temperature ${T}_{\mathrm{spin}}$ and interpreted the enhancement in terms of the intrinsic charge separation.
TL;DR: In this paper, a new metallofullerene obtained by arc discharge of Sc2 O3 /graphite rods was deduced from 13 C NMR spectroscopic and synchrotron X-ray powder diffraction studies.
Abstract: Not Sc2 @C86 but Sc2 C2 @C84 : that is the conclusion of 13 C NMR spectroscopic and synchrotron X-ray powder diffraction studies on a new metallofullerene obtained by arc discharge of Sc2 O3 /graphite rods. The picture shows a schematic representation of the deduced structure, in which the novel scandium carbide cluster Sc2 C2 is encaged in D2d -C84 .
TL;DR: In this article, high-resolution powder neutron diffraction has been used to study the crystal structure of the Mg11B2 superconductor (Tc = 39.2 K) prepared at high pressure in the temperature range 2 to 293 K.
Abstract: High-resolution powder neutron diffraction has been used to study the crystal structure of the Mg11B2 superconductor (Tc = 39.2 K) prepared at high pressure in the temperature range 2 to 293 K. The experimental data provide clear evidence of temperature-independent phase inhomogeneity, which was modelled by coexisting Mg1+δB2 phases. Precise structural information for temperatures near Tc also reveals extremely small volume and c-axis discontinuities, consistent with the existence of a specific heat anomaly at Tc.
TL;DR: In this paper, the temperature dependence of the generalized phonon density-of-states showed clear anomalous behaviour near 24 meV in the acoustic phonon region, which may be interpreted as evidence of a substantial contribution to the total electron-phonon coupling strength deriving from these phonons.
Abstract: Neutron inelastic scattering measurements have been performed on superconducting MgB 2 above and below T c . The temperature dependence of the generalized phonon density-of-states showed clear anomalous behaviour near 24 meV in the acoustic phonon region, which may be interpreted as evidence of a substantial contribution to the total electron-phonon coupling strength deriving from these phonons. Weaker evidence for a corresponding response in the high-energy B bond stretching phonons was also encountered.
15 Aug 2001-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this article, a charge density study of alkali metal doped fullerenes, Rb2CsC60, K2RbC60 and Li2C60 was carried out at room temperature by MEM/Rietveld analysis.
Abstract: The charge density study of alkali metal doped fullerenes, Rb2CsC60, K2RbC60, Na2RbC60 and Li2CsC60, was carried out at room temperature by MEM/Rietveld analysis, which is the combination of the Maximum Entropy Method (MEM) and Rietveld analysis using synchrotron radiation powder data. The distinctly different features of charge densities of the C60 molecules relating to the superconducting properties were revealed in the obtained MEM charge densities. For Na2RbC60, K2RbC60 and Rb2CsC60, the rotations of the C60 molecules were virtually quenched while the C60 molecule in Li2CsC60 was in an orientational disorder. In the interatomic region, the overlapped charge density indicating the existence of a covalent bond was not found significantly between the C60 molecules and doped metal atoms for the four materials. However, in the MEM charge densities, the relative differences among them were found significantly for the charge deficiencies of the doped metal atoms which should be associated with charge transfer from the metal atoms to the C60 molecule.
TL;DR: A double shell of Platonic solids: A trajectory that connects the six-membered rings of Ih -C80 is followed by the two La atoms of La2 @C80 according to the maximum entropy/Rietveld analysis of the synchrotron powder-diffraction data.
Abstract: A double shell of Platonic solids: A trajectory that connects the six-membered rings of Ih -C80 is followed by the two La atoms of La2 @C80 according to maximum entropy/Rietveld analysis of the synchrotron powder-diffraction data. The result is a pentagonal-dodecahedral charge-density distribution for La2 (brown in the picture) inside the icosahedral C80 cage (green).