Hiroki Okudera

The crystal structure of γ-P4, a low temperature modification of white phosphorus

Abstract The crystal structure of γ-P4, one of three modifications hitherto reported on white phosphorus, was determined from X-ray powder diffraction data collected at T = 123 K on a Guinier-Simon camera equipped with a cold gas blower and an image plate detector. Crystallographic data at T = 123 K are: space group C2/m, a = 9.1709(5) Å, b = 8.3385(5) Å, c = 5.4336(2) Å, and β = 90.311(3)°, V = 415.51(6) Å3, Z = 4. The crystal structure of γ-P4 was solved by the method of simulated annealing. The subsequent Rietveld refinement in the range 12° < 2θ < 92° employing rigid-body constraints on the P4 molecule converged at Rp = 3.8, wRp = 5.0, and RF2 = 14.0%. The asymmetric unit of γ-P4 contains three P atoms; two P atoms in a molecule are related by a mirror plane which bisects the molecule. The centers of gravity of these P4 molecules show a distorted body-centered cubic arrangement. The four apices of the P4 tetrahedron point to the largest possible voids formed by neighbor molecules. The difference to the crystal structures of SiF4 and GeF4 with an exact bcc arrangement of tetrahedral molecules is discussed as well as, in terms of layer stackings, the similarity of the structures of γ- and β-P4.

Determinations of crystallographic space group and atomic arrangements in oxide-ion-conducting Nd9.33(SiO4)6O2

Abstract In this paper we report single crystal X-ray diffraction study of an oxide-ion-conductor Nd9.33(SiO4)6O2 at 150 K. By detailed examinations of the atomic positions and an electron density distribution, it was clearly shown that Nd9.33(SiO4)6O2 has an apatite structure (space group P63/m) with no symmetry lowering or site-splitting. All oxygens in the hexagonal channel were located at their ideal positions (2a site at 0, 0, ¼) of the apatite structure, while their thermal displacements were highly anisotropic and that in the c-axis direction was more than four-times larger than those in other directions. Cation vacancies were located at the nine-coordinated 4f Nd site. The other Nd site (seven-coordinated 6h site) were basically fully occupied. This full occupation of the 6h Nd site makes potential waves inside the hexagonal channel smooth and transports of oxide-ions in the channel much easier. The distortion of SiO4 tetrahedron was not apparent. Crystallographic data at 150 K: Cell dimensions a = 9.5556(6) Å and c = 7.019(2) Å; F(000) = 852; Z = 1; Dc = 5.779(2) g/cm3.

Wurtzite-type ZnS nanoparticles by pulsed electric discharge

The synthesis of wurtzite-type ZnS nanoparticles by an electric discharge submerged in molten sulfur is reported. Using a pulsed plasma between two zinc electrodes of diameter 5 mm in molten sulfur, we have synthesized high-temperature phase (wurtzite-type) ZnS nanocrystals with an average size of about 20 nm. The refined lattice parameters of the synthesized wurtzite-type ZnS nanoparticles were found to be larger than those of the reported ZnS (JCPDS card no 36-1450). Synthesis of ZnMgS (solid solution of ZnS and MgS) was achieved by using ZnMg alloys as both cathode and anode electrodes. UV-visible absorption spectroscopy analysis showed that the absorption peak of the as-prepared ZnS sample (319 nm) displays a blue-shift compared to the bulk ZnS (335 nm). Photoluminescence spectra of the samples revealed peaks at 340, 397, 423, 455 and 471 nm, which were related to excitonic emission and stoichiometric defects.

High temperature stable WC1−x@C and TiC@C core–shell nanoparticles by pulsed plasma in liquid

High temperature phase tungsten carbide and titanium carbide coated with graphitic carbon (WC1−x@C and TiC@C) nanoparticles were synthesized by pulsed plasma in liquid. HRTEM results show that the average size of WC1−x@C and TiC@C nanoparticles are 30 and 35 nm, respectively. Raman spectroscopy revealed well-organized graphitic coatings, which make the nanoparticles stable at higher temperatures and resistant to oxidation. Refinement of the XRD profiles showed larger cell parameters compared to JCPDS cards. Furthermore, thermogravimetric analyses showed the higher thermal stability up to 601 and 543.4 °C, for the WC1−x@C and TiC@C nanoparticles, these are 43% and 36% more compared to the thermal stability of previously reported nanoparticles, which degraded at around 350 °C.

Temperature Dependence of XANES Spectra for ATiO3, A2TiO4 and TiO2 Compounds with Structural Phase Transitions

Ti K‐edge X‐ray absorption near edge structure (XANES) spectra of various titanates was measured at various temperatures. The composition, structure and temperature dependence of XANES spectra was investigated especially on the phase transition. Ti atoms are located in TiO6 octahedral sites for the all samples. Ti K‐edge XANES spectra change largely with different compositions, while the temperature dependence of XANES spectra is small in each compound even if undergoing structural phase transition. For the temperature dependence, the pre‐edge intensity has two types of behavior. One is decreasing as temperatures increase, the other is increasing. The former is for PbTiO3 and BaTiO3 perovskite and the later is for CaTiO3 and SrTiO3 perovskite, Mg2TiO4 spinel and TiO2 rutile.

The role of water in coesite crystallization from silica gel

Structural changes in silica gel after compression to 5 GPa and 100 °C for one hour were investigated using Raman and IR spectroscopies and X-ray diffraction measurements. The non-powdered silica gel sample was entirely crystallized to coesite under these experimental conditions, showing that the crystallization of silica gel to coesite can be readily achieved in spite of the low temperature conditions and short duration. This behavior may result because the average structure of silica gel is dominated by four-membered rings of SiO 4 tetrahedra, there is a less-polymerized network structure due to the presence of silanol (Si-OH), and larger ring structures, such as 6- and 8-membered rings, of SiO 4 tetrahedra are disrupted by water molecules under pressure. These properties may lead to successive polymerization and reconstruction of the SiO 4 tetrahedra in the bulk silica-gel sample, enabling the easy formation of the coesite-like network structure. This shows the importance of the contribution of silanol and water molecules for coesite formation.

Relationships among channel topology and atomic displacements in the structures of Pb5(BO4)3Cl with B = P (pyromorphite), V (vanadinite), and As (mimetite)

Abstract Rare-earth silicate oxy-apatites have been found to exhibit high ion-conductivity along channels within their structure, which makes them candidate materials for solid oxide fuel cell electrolytes. It is not understood so far why this high ion-conductivity is restricted to oxide-ion transport and does not occur for halide ions common in apatite type minerals. This study reports on the relationship between the topology of these structural channels and the spatial displacement of the chloride ion in three different structures of natural apatite group minerals [Pb5(BO4)3Cl (Z = 2) with B = P (pyromorphite), V (vanadinite), and As (mimetite)] using single-crystal X‑ray diffraction. All of these minerals crystallize in the hexagonal chlorapatite structure with space group P63/m with no symmetry lowering or site splitting. The anion channel is built from a face-sharing array of nearly regular Pb26 octahedra running parallel to the c-axis, and the chloride ions were found at the center of each octahedron with bond-valence sums of 1.10 for mimetite and vanadinite and 1.25 for pyromorphite. The mean square displacement (msd) of the chloride ion in [001] was found to be a function of the size of Pb26 octahedron in that direction. This position is also the center of a flat O36 trigonal antiprism. The msds of the chloride ion in the x-y plane were found to be correlated to the size of the antiprism in this plane, namely the distance between the chloride ion and its nearest oxide ions, and the amount of roto-oscillation motion of the BO4 tetrahedra around the B-O1 axis. While the chloride ion in the channel was bonded to six Pb cations, the repulsion from six neighboring oxygen ions is also apparent and this repulsion restricts the motion of the chloride ion within the x-y plane.

Synthesis of WO3·H2O nanoparticles by pulsed plasma in liquid

Pure orthorhombic-phase WO3·H2O nanoparticles with sizes of about 5 nm were synthesized by pulsed plasma in deionized water, in which tungsten electrodes provide the source of tungsten and the water is the source of oxygen and hydrogen. The quenching effect and liquid environment inherent in this pulsed plasma in liquid method resulted in ultra-small particles with lattice lengths (a = 5.2516 A, b = 10.4345 A, c = 5.1380 A) larger than those of reference lattices. The emission lines of W I atoms, W II ions and H I atoms were observed by an optical emission spectrum in order to gather information on the synthetic mechanism. These nanoparticles showed higher absorption in the visible region than did ST-01 TiO2 and Wako WO3 nanoparticles. The WO3·H2O nanoparticles displayed more activity in the photocatalytic test than did the commercial TiO2 sample (ST-01). Also, the absorption edge of WO3·H2O shifted to longer wavelengths in the UV-Vis absorption pattern relative to that of the anhydrous tungsten oxide.

Shock compression of synthetic opal

Structural change of synthetic opal by shock-wave compression up to 38.1 GPa has been investigated by using SEM, X-ray diffraction method (XRD), Infrared (IR) and Raman spectroscopies. Obtained information may indicate that the dehydration and polymerization of surface silanole due to high shock and residual temperature are very important factors in the structural evolution of synthetic opal by shock compression. Synthetic opal loses opalescence by 10.9 and 18.4 GPa of shock pressures. At 18.4 GPa, dehydration and polymerization of surface silanole and transformation of network structure may occur simultaneously. The 4-membered ring of TO4 tetrahedrons in as synthetic opal may be relaxed to larger ring such as 6-membered ring by high residual temperature. Therefore, the residual temperature may be significantly high at even 18.4 GPa of shock compression. At 23.9 GPa, opal sample recovered the opalescence. Origin of this opalescence may be its layer structure by shock compression. Finally, sample fuse by very high residual temperature at 38.1 GPa and the structure closes to that of fused SiO2 glass. However, internal silanole groups still remain even at 38.1 GPa.

La3Cl3BC – Structure, Bonding and Electrical Conductivity

A new rare earth carbide boride halide, La3Cl3BC, has been prepared by heating a mixture of stoichiometric quantities of LaCl3, La, B and C at 1050 °C for 10 days. La3Cl3BC (La3Br3BC type) crystallizes in the monoclinic system with space group P21/m (No. 11), a = 8.2040(16), b = 3.8824(8), c=11.328(2)Å , β =100.82(3)°. In the structure, monocapped trigonal prisms containing B-C units are condensed into chains along the b direction, and the chains are further linked by Cl atoms in the a and c directions. The condensation results in a polymeric anion 1∞[BC] with a spine of B atoms in a trigonal prismatic coordination by La, and the C atoms attached in a square pyramidal coordination. The B-B and B-C distances are 2.16 and 1.63 Å , respectively. La3Cl3BC is metallic. The EH calculation shows that the distribution of valence electrons can be formulated as (La3+)3(Cl−)3(BC)5− · e−.