Naoichi Yamamoto

Layered structures of vanadium pentoxide gels

Abstract Layered structures of V 2 O 5 gels, V 2 O 5 · n H 2 O, were studied by means of the X-ray Rietveld method from a series of 00l reflections. The phases with 1.15-nm and 0.87-nm layer spacings which appeared at room temperature and 260°C, respectively, were investigated. The 1-D Patterson maps suggested that a V 2 O 5 layer consists of two V 2 O 5 sheets facing each other at a distance of 0.28nm. The calculated patterns from this structure of a double-sheet V 2 O 5 layer and bilayer arrangements of interstitial water fit to the observed ones for both phases and this structural type agreed well with the 1-D Fourier maps. The observed sample density confirmed the structure of double-sheet V 2 O 5 layer. The 3-D structure of the V 2 O 5 layer is proposed to be closely related to the structure of the V 2 O 5 layer of δ-Ag x V 2 O 5 .

Protonic conduction and defect structures in Sr-doped LaPO4

Abstract Protonic conduction and defect structures in Sr-doped LaPO 4 (Sr=0–7 mol%) were investigated by using conductivity, MAS-NMR spectroscopy and FT-Raman spectroscopy measurements. The results of MAS-NMR and FT-Raman spectroscopy measurements revealed that hydrogen phosphate groups, possibly hydrogen phosphate ions HPO 4 2− , and pyrophosphate ions P 2 O 7 4− existed in Sr-doped LaPO 4 under wet and dry conditions, respectively, in addition to orthophosphate ions PO 4 3− . Based on these results, we proposed a defect structure model for Sr-doped LaPO 4 . According to this model, La substitution with Sr leads to formation of P 2 O 7 4− as intrinsic positive defects, and in wet atmosphere, protons are introduced into the phosphate forming hydrogen phosphate groups through the equilibrium between P 2 O 7 4− and water vapor. The conductivity behavior of Sr-doped LaPO 4 versus water vapor and oxygen partial pressures supported this defect structure model.

Powder X-ray crystal structure of VO2(A)

Abstract VO2(A), a metastable phase of vanadium dioxide, was synthesized by hydrothermal treatment of VO(OH)2. The powder X-ray structure analysis of VO2(A) has been performed by the use of EXAFS and the Rietveld method; possible structure models based on the EXAFS results were examined by the Rietveld analysis. VO2(A) has a tetragonal symmetry with a space group P4 2 nmc ; a = 8.4336(7) A, c = 7.6782(7) A, and Z = 16. The structure consists of a three-dimensional framework of VO6 octahedra. Four units of two edge-sharing octahedra are linked to each other by sharing corners forming a 2 × 2 square block in the c-plane. The blocks are piled up along the c-axis by sharing edges of the octahedra resulting in zigzag chains of V ions running along the c-axis. Taking the revealed structure into consideration, a comparison with the structure of VO2(B) is made and a tentative mechanism of the phase transition in VO2(A) is proposed.

Hydrothermal synthesis and electrochemical properties of Li-Mn-spinel

Abstract The Li–Mn-spinel was synthesized using hydrothermal reaction through γ-MnOOH in 0.050∼0.150 mol/l LiOH aqueous solution at 130∼170°C. The composition of the as-precipitated Li–Mn-spinel depended on both the reaction temperature and the concentration of LiOH aqueous solution. The particle was rod-shaped and the average size was around 0.2×0.2×1 μm. The charge–discharge characteristics of the as-precipitated Li–Mn-spinel were investigated in the system of Li–Mn-spinel/EC–DEC/Li. The as-precipitated Li–Mn-spinels showed an electrochemical behavior comparable to those prepared using the conventional calcination method, and the first discharge capacity varied from 62 to 107 mAh/g depending on Li content in the spinel.

Single-electrode Peltier heats of Li-Si alloy electrodes in LiCl-KCl eutectic melt

This paper reports single-electrode Peltier heats for cathodic (or anodic) reactions of Li-Si alloy electrodes in coexisting phase states evaluated from their thermoelectric power values. The thermoelectric power and thermodynamic properties for various coexisting phase states in Li-Si alloy were determined from electromotive force measurements of the following nonisothermal and isothermal cells, respectively, (T) Li I LiCl-KCl I Li-Si (T + ΔT) and (T) Li | LiCl-KCl | Li-Si (T). General methods of estimating the thermoelectric power for a lithium alloy electrode from its thermodynamic properties have been developed thermodynamically. The estimation methods were validated by detailed comparison with experimental results of Li-Si alloy electrode systems.

Structure determination of H2V3O8 by powder X-ray diffraction

Abstract A vanadium oxyhydroxide H2V3O8 with a green fibrous shape was hydrothermally synthesized; it was originally formulated as V3O7 · H2O. It has an orthorhombic symmetry: a = 16.9298(2), b = 9.3589(1), c = 3.64432(4), A, and Z = 4. The structure analysis was performed by the Rietveld method using powder X-ray diffraction data. A structure model for the space group Pnam was found to be compatible with the X-ray data leading to Rp = 0.059 and Rwp = 0.063. The structure is described as a layer structure composed of V3O8 layers which are made up with VO6 octahedra (V(1)O6 and V(2)O6) and VO5 trigonal bipyramids (V(3)O5), where V(3) was assigned to V5+ and V(2) was likely V4+. The arrangement of VO polyhedra resembles to that in β-Na0.33V2O5.

Electrical Conduction Properties of Sr-Doped LaPO4 and CePO4 under Oxidizing and Reducing Conditions

Electrical conduction properties of Sr-doped LaPO 4 and Sr-doped CePO 4 under H 2 O/O 2 and H 2 O/H 2 conditions were investigated with conductivity measurements. Conductivities of I mol % Sr-doped LaPO 4 were 10 - 5 . 2 -10 - 3 . 5 S cm - 1 at 500-925°C under wet reducing conditions and were close to those under wet oxidizing conditions. It was found from the H/D isotope effect on conductivity that the material showed dominant protonic conduction under wet reducing conditions. As for 1 mol % Sr-doped CePO 4 , conductivities were 10 - 2 . 8 -10 - 2 . 0 S cm - 1 at 500-925°C under wet oxidizing conditions and much higher than those of 1 mol % Sr-doped LaPO 4 under the same conditions. Such high conductivities of Sr-doped CePO 4 seemed attributable to electronic conduction due to partial oxidation of Ce 3 + to Ce 4 + caused by substituting Sr2 + for Ce 3 + . Under wet reducing conditions, however, conductivities of the material decreased to 10 - 5 . 2 -10 - 3 . 4 S cm - 1 at 500-925°C, which was almost comparable with those of 1 mol % Sr-doped LaPO 4 . It was concluded that Sr-doped CePO 4 showed mixed protonic and p-type electronic conductions under wet reducing conditions. Based on the results of conductivity measurements, defect structures in Sr-doped LaPO 4 and CePO 4 under wet oxidizing and reducing conditions are discussed.

The Shift of the Spin Flip Temperature of α-Fe2O3 Fine Particles

The particle size dependences of the spin flip transition temperature (Morin temperature) and of the lattice constant of α-Fe 2 O 3 were studied experimentally. With decreasing particle size, it was found that the Morin temperature shifts downwards and crystal lattice expands both in a - and c -axis directions. As the origin of the Morin temperature shift, the change of the dipolar magnetic field due to the lattice dilatation was considered. Experimental values were compared to the theoretical calculation of Artmann and a qualitative agreement was obtained.

Crystallization Behavior and Partially Melted States in Bi-Sr-Ca-Cu-O

High-temperature X-ray measurements were carried out on Bi-Sr-Ca-Cu-O. It was found that the 30 A phase showed incongruent melting forming new crystalline phases which differed depending on the compositions. Crystallization from the melt by cooling first produced the 30 A phase on solidification and then the 24 A phase crystallized around 730°C. Crystallization from the glassy phase by heating started at 430°C and led to the 24 A phase, which changed to the 30 A phase above 800°C. As a rule, low-temperature crystallization from the amorphous phase takes place below 730°C to form the 24 A phase in Bi-Sr-Ca-Cu-O.

Protonic conduction in rare earth orthophosphates with the monazite structure

Abstract Electrical conduction in rare earth orthophosphates, LnPO4 (Ln=La, Pr, Nd and Sm), with the monazite structure (P21/n) was investigated by using conductivity measurements at 500–925 °C. From the conductivities of undoped and 1 mol% Sr-doped LnPO4 under wet (H2O and D2O) and dry atmospheres, it was found that LnPO4 began to conduct protons under wet atmosphere by substituting Sr for Ln. The conductivity behavior of 1 mol% Sr-doped LnPO4 versus p(H2O) and p(O2) was discussed in terms of the defect equilibria. It was concluded that protonic conduction was dominant in the materials though electron holes contributed slightly to the total conductivity as temperature increased. All the 1 mol% Sr-doped LnPO4 investigated in this study exhibited similar electrical conduction regardless of rare earth element used.