Akinori Hoshikawa

Observation of hydrogen in deuterated methane hydrate by maximum entropy method with neutron powder diffraction

The crystal structure of deuterated methane hydrate (structure I, space group: Pm(-)3n) was investigated by neutron powder diffraction at temperatures of 7.7-185 K. The scattering amplitude density distribution was examined by a combination of Rietveld method and maximum entropy method (MEM). The distribution of the D atoms in both D(2)O and CD(4) molecules was clarified from three-dimensional graphic images of the scattering amplitude density. The MEM results showed that there were low-density sites for the D atom of D(2)O in a particular location within the D(2)O cage at low temperatures. The MEM provided more reasonable results because of the decrease in the R factor that is attainable by this method. Accordingly, the low-density sites for the D atom of D(2)O probably exist within the D(2)O cage. This suggests that a spatial disorder of the D atom of D(2)O occurs at these sites and that hydrogen bonds between D(2)O molecules become partially weakened. With regard to the CD(4) molecules, there were high-density sites for the D atom of CD(4), and the density distribution of the C and D atoms was observed separately in the scattering amplitude density image. Consequently, the C-D bonds of CD(4) were not observed clearly because the CD(4) molecules had an orientational disorder. The D atoms of CD(4) were displaced from the line between the C and O atoms, and were located near the face center of the polygon in the cage. Accordingly, the D atoms of CD(4) were not bonded to specific O atoms. This result is consistent with the hydrophobicity of the CD(4) molecule. We also report the difference between the small and the large cages in the density distribution map and the temperature dependence of the scattering amplitude density.

Doping Effect on Crystal Structure of Fast Oxide Ion Conductor LaGaO3-Based Perovskite Compounds

Doping effects of Sr and/or Mg on the crystal structures of LaGaO3 perovskites were investigated. The doping with Mg into Ga-site in LaGaO3 led to structural change from the orthorhombic primitive lattice (Pbnm) to the orthorhombic body-centered lattice (Ibmm) at room temperature. The orthorhombic cell (Ibmm) of LaGa0.9Mg0.1O2.95 changed to the monoclinic (I2/a) by doping with Sr into La site. Neutron powder diffraction analyses revealed that these compounds had GaO6 octahedral tilt. The GaO6 octahedral tilt angles were reduced with doping with Sr and/or Mg.

Investigation of the Oxide Ion Conduction Mechanism in LaGaO3-Based Electrolytes through High-Temperature Neutron Powder Diffraction

The crystal structure and oxide ion conduction properties of La0.9Sr0.1Ga0.9Mg0.1O2.9 were compared with those of LaGa0.8Mg0.2O2.9 with the same oxygen vacancy concentration. Although La0.9Sr0.1Ga0.9Mg0.1O2.9 had a smaller free space through which oxide ions pass compared to LaGa0.8Mg0.2O2.9, the activation energy of oxide ion conduction in the former was lower. This was attributed to the large displacement of La and Ga in La0.9Sr0.1Ga0.9Mg0.1O2.9. The GaO6 octahedral tilts in the LaGaO3 perovskite structure were reduced by double doping with Sr and Mg. Reduction of the GaO6 octahedral tilt proved effective for oxide ion conduction.