Yoshinobu Ishii

Modulated Structure of the Thermoelectric Compound [Ca2CoO3]0.62CoO2

We have determined the crystal structure of the composite crystal [Ca 2 CoO 3 ] 0.62 CoO 2 , known as Ca 3 Co 4 O 9 , by a superspace group approach. Structural parameters were refined with a super...

Crystal and magnetic structures and their temperature dependence of Co2Z-type hexaferrite (Ba,Sr)3Co2Fe24O41 by high-temperature neutron diffraction

We have prepared nonoriented and magnetically oriented specimens of Co2Z-type Ba ferrite Ba3Co2Fe24O41 (Ba3Z) and those with Sr2+ substitution for Ba2+, i.e., Ba1.5Sr1.5Co2Fe24O41 (Ba1.5Sr1.5Z) and Sr3Co2Fe24O41 (Sr3Z) with the conventional solid-state reaction method. Permeability measurements of nonoriented specimens have shown that this substitution improves the frequency characteristic of permeability, though the permeability in Sr3Z significantly decreases. X-ray diffraction (XRD) and magnetization measurements of magnetically oriented specimens have shown that the magnetic moments of iron and cobalt ions in Ba3Z and Ba1.5Sr1.5Z lie in the c plane, but that those in Sr3Z deviate from the c plane. We have studied the substitution effect of Sr2+ for Ba2+ on the crystal structures and the effective sizes and directions of magnetic moments and their temperature dependences with high-temperature neutron diffraction technique. This substitution induces the change in the distribution of cobalt ions and mome...

Existence of Ferroelectric Ice in the Universe

The question as to whether or not ferroelectric ice, named ice XI, exists in a stable low-temperature phase attracts much interest. This question arose as a condensed-matter issue and became of interest in astronomy (e.g., does ice XI exist on Pluto?) because astronomical observations identified the existence of crystalline ice in our solar system. From neutron diffraction experiments, we found the temperature conditions for the transformation of the largest fraction of ice into ice XI using the lowest level of impurity dopant. The finding of bulk crystal of ordered structure firmly supports that ice XI is stable. This suggests the existence of naturally occurring ice XI at a narrow temperature range (57-66 K) in our solar system.

Studies on the magnetic and structural phase transitions of Nd3RuO7

Crystal structures and magnetic and thermal properties of neodymium ruthenate Nd3RuO7 have been investigated. The specific heat and neutron diffraction measurements showed a monoclinic–orthorhombic structural phase transition at 130 K. The monoclinic crystal structure at 100 K is well described with the space group P21/m (a = 10.8893(4) A, b = 7.3864(2) A, c = 7.4731(2) A and β = 90.008(6)°). The results of the magnetic susceptibility measurements show the existence of the antiferromagnetic transition with a weak ferromagnetic component at 19 K. Specific heat and neutron diffraction measurements also indicated long-range magnetic ordering of both Nd3+ and Ru5+ ions below 19 K. The magnetic sublattice of Nd3+ ions was twice as large as the size of the crystal lattice along the b-axis. On the other hand, the size of the magnetic sublattice of Ru5+ was the same as that of the crystal lattice. Ru5+ ions were coupled anti-ferromagnetically along the c-axis.

The time dependence of in-situ tritium release from lithium oxide and lithium aluminate (VOM-22H experiment)

An in-situ tritium recovery experiment was conducted in the JRR-2 reactor on Li 2 O and LiAlO 2 spheres. A technique for obtaining tritium diffusion coefficients from the time dependency of tritium release after incremental temperature changes was developed. The tritium retention in LiA1O 2 was higher than in Li 2 O for the same temperature. The apparent diffusion coefficients for LiAlO 2 at temperatures up to 900°C was one order magnitude higher than the previous, lower temperature TRIO data.

Neutron scattering study of dipolar spin ice Ho2Sn2O7: Frustrated pyrochlore magnet

By means of neutron scattering techniques we have investigated the frustrated pyrochlore magnet Ho 2 Sn 2 O 7 , which was found to show ferromagnetic spin-ice behavior below T≃ 1.4 K by susceptibility measurements. High-resolution powder neutron diffraction shows no detectable disorder of the lattice, which implies the appearance of a random magnetic state solely by frustrated geometry, i.e., the corner sharing tetrahedra. Magnetic inelastic scattering spectra show that Ho magnetic moments behave as an Ising spin system at low temperatures and that the spin fluctuation has static character. The system remains in a short-range-ordered state down to at least T = 0.4 K. By analyzing the wave-vector dependence of the magnetic scattering using a mean-field theory, it is shown that the Ising spins interact via the dipolar interaction. Therefore we conclude that Ho 2 Sn 2 O 7 belongs to the dipolar-spin-ice family. Slow spin dynamics is exhibited as thermal hysteresis and time dependence of the magnetic scattering.

Irradiation damage in lithium oxide

Abstract Irradiation damage studies of lithium oxide (Li20) using optical absorption and electron spin resonance (ESR) methods are reviewed. F+-centers and the defects associated with F aggregate centers were introduced in Li2O irradiated with thermal neutrons and oxygen ions. In addition, colloidal Li metal centers were produced at the high fluences. The production process of the F+-centers was discussed from results of the oxygen-ion irradiation. The recovery behavior of F+-centers was studied by isochronal and isothermal annealing experiments.

Magnetic and neutron diffraction studies on double perovskites A2LnRuO6(A = Sr, Ba; Ln = Tm, Yb)

Magnetic properties of double perovskites A2LnRuO6 (A = Sr, Ba; Ln = Tm, Yb) have been reported. Powder neutron diffraction measurements have been performed at 10 K and higher temperatures (≥100 K) to determine their crystal and magnetic structures. As a result of the Rietveld analysis of the diffraction profiles, it is found that they are monoclinic with space group P21/n (A = Sr) or cubic with space group Fmm (A = Ba). From the magnetic susceptibility and specific heat measurements, a magnetic transition at 36–48 K is observed in each compound. The neutron diffraction data collected at 10 K show that this magnetic transition is due to a long range antiferromagnetic ordering involving both Ru5+ and Ln3+ ions. Each of the magnetic moments of Ru5+ and Ln3+ orders in a type I arrangement.

Neutron Scattering Study of the Crystal and Magnetic Structures in Itinerant-5f Antiferromagnets UNiGa5 and UPtGa5

The crystal and magnetic structures of 5 f itinerant antiferromagnets UNiGa 5 and UPtGa 5 were studied by means of neutron scattering. The powder diffraction data were well explained by the HoCoGa 5 tetragonal structure with space group P4/mmm. In UNiGa 5 , uranium spins in the simple tetragonal lattice have a Neel-type structure with a relatively large moment of 0.9 µ B /U along the [0,0,1] direction. On the other hand, magnetic moments of uranium atoms in UPtGa 5 are aligned ferromagnetically in the (0,0,1) plane and are directed along the [0,0,1] direction in an antiferromagnetic sequence (↑↓). An ordered moment is 0.24 µ B /U. The nearest-neighbor interaction might be different in these isostructural compounds.

Modulated Structure of Misfit Layered Cobalt Oxide [(Ca0.90Bi0.10)2(Co0.95Bi0.05)O3]pCoO2

We have determined the crystal structure of Bi-substituted and Bi-free misfit layered cobalt oxides [Ca2CoO3]0.62CoO2, by a (3+1)-dimensional superspace group approach. Structural parameters have been refined with a superspace group of C2/m(1p0)s0 using powder neutron diffraction data. Bismuth atoms are found to substitute for both Ca and Co atoms in the rock salt-type [Ca2CoO3] subsystem. The resulting structural formula is expressed as [(Ca0.90Bi0.10)2(Co0.95Bi0.05)O3]pCoO2 with a refined b-axis ratio of p=0.6183. By Bi substitution, the modulation of the Co–O distances in the [Ca2CoO3] subsystem is markedly decreased relative to the Bi-free counterpart, whereas such a modulation in the [CoO2] subsystem is slightly increased. The observed increase in the Seebeck coefficient and electrical resistivity of the Bi-substituted phase can be explained in terms of the decrease in hole concentration in the CoO2 sheets.