Jiro Shiokawa

Physicochemical properties of rare earth perovskite oxides used as gas sensor material

The change of the conductivity in the rare earth perovskite oxide took place after the chemisorption of flammable gases. The sensitivity for methanol was highest. From the conductivity change of these perovskite oxides after the injection of methanol, the energy needed to promote an electron from a conducting to a nonconducting state, ΔE=Ec-Et, could be derived from the equationσ=Aσ0 exp (−ΔE/kT). LnCoO3 had the smallest ΔE and ΔH (metal-O), which is the binding energy of oxygen coordinating to the metal ions, but exhibited the highest activity for gas sensing. The gas sensing mechanism was also considered.

Luminescence properties of Eu(II)-borates and Eu2+- activated Sr-Borates

Abstract The luminescence properties of Eu(II)-borates and Eu 2+ -activated Sr-borates were studied, e.g. Sr 1− x Eu x B 2 O 4 Sr 1− x Eu x B 4 O 7 and Sr 1− x Eu x B 6 O 10 . It was found that Sr 1− x Eu x B 4 O 7 consisting of a three-dimensional network of BO 4 tetrahedra gave a remarkably stronger emission than the other compound consisting of BO 3- 3 ions, B 2 O 4- 5 ions, (BO 2 )∞ chains or a (B 3 O 5 )∞ network. This was understood by considering their crystallographic properties and with help of the Dexter theory.

The effect of hydrogen absorption on the electrical resistivities of LaNi5 and MmNi4.5Mn0.5 films (Mm misch metal)

Abstract Electrical resistivity measurements of LaNi 5 and MmNi 4.5 Mn 0.5 (Mm  misch metal) films which were obtained by vacuum vapour deposition were carried out in an atmosphere of hydrogen. Resistivities of the films initially increased immediately and then decreased during the absorption process of hydrogen. The reverse is true for the desorption process. The intermetallic compound films did not pulverize even after 150 cycles of absorption-desorption of hydrogen if the thickness of the film was less than 10 × 10 4 nm.

Magnetic properties of the rare-earth diborodicarbides (RB2C2)

Abstract The magnetic susceptibility of RB2C2 has been measured in the temperature range of 3–300 K. Curie-Weiss fits to the susceptibilities led to effective moments in agreement with those expected for R3+ ions. The RB2C2 (R = Ce, Nd, Sm, Gd, Tb, Er, and Tm) compounds are antiferromagnetic. Metamagnetic transitions at low fields were observed for CeB2C2 and TbB2C2. The compounds, DyB2C2 and HoB2C2, are ferromagnets with complex magnetic structures. Praseodymium borocarbide becomes a Van Vleck paramagnet at low temperature. The magnetic ordering temperatures of these compounds are discussed in terms of their crystal structure and the RKKY theory.

Synthesis and characterization of divalent-europium (Eu2+) compounds, EuB4O7, EuB2O4 and Eu2B2O5

Abstract Several new compounds, EuB4O7, EuB2O4 and Eu2B2O5 were synthesized. EuB4O7 and EuB2O4 crystallized in the orthorhombic, space group C2v7-Pnm21 with cell dimensions: a = 4.435(1), b = 10.731(1), c = 4.240(1) A and Z = 2 , and space group D2h14-Pnca with cell dimensions: a = 6.593(1), b = 12.063(2), c = 4.343(1) A and Z = 4 , respectively. Eu2B2O5 is monoclinic, space group C 2h 5 - P2 1 a with cell dimensions: a = 11.91(1), b = 5.36(1), c = 7.74(1) A , β = 92.7(1)° and Z = 4 . The IR patterns of EuB4O7, EuB2O4 and Eu2B2O5 were able to be ascribed to a three-dimensional network of BO4-tetrahedra, an endless chain of BO3 groups, (BO2)∞, and a B2O54− ion, respectively. The magnetic susceptibilities of these compounds were measured. EuB4O7 and Eu2B2O5 were paramagnetic in the ranges of measured temperature, and EuB2O4 was an antiferromagnet with the Neel temperature TN = 3 K. Their magnetic properties were understood by considering the relationship between their structures and the magnetic interactions.

Electrical properties of rare earth diborodicarbides (RB2C2-type layer compounds)

Abstract The electrical resistivities ρ, the pycnometric densities and the Seebeck coefficients of RB2C2-type layer compounds are reported. Superconducting transitions were observed for YB2C2 and LuB2C2 at 3.6 K and 2.4 K respectively, but the compound LaB2C2 remained in the normal state down to 1.8 K. The ρ-T curves for the borocarbides CeB2C2 and PrB2C2 have a knee at 110 K and 20 K respectively which can be ascribed to crystal field effects on the ground states of Ce3+ and Pr3+. The ρ-T curves of RB2C2 (R ≡ Ce, Nd, Gd, Tb, Dy, Ho, Er and Tm) show a sharp change in slope at low temperatures, verifying the magnetic ordering found from earlier magnetic susceptibility measurements. TmB2C2 exhibited unusual behaviour in that the resistivities increased rapidly below the Neel temperature.

The hydrogen electrode reaction characteristics of thin film electrodes of Ni-based hydrogen storage alloys

Abstract Film-type electrodes of hydrogen absorbing intermetallic compound and alloys, LaNi 5 , Ni 0.11 Ti 0.89 , Ni 0.50 Ti 0.50 and Ni 0.76 Ti 0.24 were prepared by a flash evaporation method. The hydrogen electrode reaction characteristics of the LaNi 5 and NiTi alloy films in 1 M NaOH are very similar to each other. The reaction proceeds via the Volmer-Tafel reaction route with mixed rate-determining characteristics. The exchange current densities of the constituent steps, as well as the overall reaction, are in the range of 10 −6 A cm −2 (true). Surface analysis by an XPS technique has shown that La or Ti on the electrode surface exists as an electrocatalytically innert oxide of La 2 O 3 or TiO 2 . Close similarities of these electrodes with pure Ni electrodes indicate that Ni is responsible for the electrocatalytic activity. No synergistic effect is thus noticeable.

The catalytic activity of rare earth manganites

Abstract The sequence of catalytic activities for the rare-earth manganites (LnMnO 3 , Ln=La/1 b Eu), La > Pr ⋍ Nd > Sm ⋍ Eu , is correlated with the paramagnetic Weiss constant; it was positive in LaMnO 3 , negative in EuMnO 3 .

Preparation and structure of sodium rare-earth sulfides, NaLnS2 (Ln; rare earth elements)

Abstract Ternary sodium rare-earth sulfides, NaLnS2 (Ln; rare earth elements), have been prepared and characterized. With an x-ray diffraction analysis of their crystal structure, it has been found that the materials containing La, Ce, and Pr crystallize in a cubic structure, including a Th3P4-type compound as a second phase, while the other NaLnS2 sulfides - except for NaNdS2 and NaSmS2 - crystallize in a rhombohedral NaHF2-type structure, which can be described with a hexagonal unit cell. In NaNdS2 and NaSmS2 these two phases, the cubic and the rhombohedral, coexist. The following relationships can be obtained from a comparision of the lattice constants of the cubic and hexagonal cells in NaNdS2; a h = a c √2 2 , ch=ac2 √3. Based on these relationships, the structural type of the cubic phase can be regarded as analogous to that of NaCl.

Catalytic activity of rare-earth orthoferrites and orthochromites

Abstract The catalytic activity of LnFeO 3 and LnCrO 3 (Ln=LaGd) for methanol oxidation has been studied by the measurement of conductivity change on methanol adsorption in the mixture gas ( O 2 (0.5%) + N 2 (99.5%)); The activity of these compounds appeared above the Neel points. The sequence of the activity for LnFeO 3 was Gd > Eu > Sm > Nd > Pr > La, which was correlated with the Neel temperature; GdFeO 3 , which has the lowest Neel temperature, showed the highest activity. However, a relationship between activity and Neel temperature was not clearly observed in LnCrO 3 .