Kouji Sakaki

Positron annihilation study of lattice defects induced by hydrogen absorption in some hydrogen storage materials

Some AB5 and AB2 hydrogen storage compounds have been characterized by using positron-annihilation lifetime spectroscopy. It has been shown that they contain no constitutional vacancies and that deviations from the stoichiometric compositions are all compensated by antistructure atoms. Positron lifetimes in fully-annealed LaNi5−xAlx and MmNi5−xAlx alloys show good correlation with their hydrogen desorption pressures. On the other hand, surprising amounts of vacancies together with dislocations have been found to be generated during the first hydrogen absorption process of LaNi5 and ZrMn2. These lattice defects may play a key role in initial activation processes of hydrogen storage materials.

Development of an energy-domain 57Fe-Mössbauer spectrometer using synchrotron radiation and its application to ultrahigh-pressure studies with a diamond anvil cell.

An energy-domain (57)Fe-Mössbauer spectrometer using synchrotron radiation (SR) with a diamond anvil cell (DAC) has been developed for ultrahigh-pressure measurements. The main optical system consists of a single-line pure nuclear Bragg reflection from an oscillating (57)FeBO(3) single crystal near the Néel temperature and an X-ray focusing device. The developed spectrometer can filter the Doppler-shifted single-line (57)Fe-Mössbauer radiation with a narrow bandwidth of neV order from a broadband SR source. The focused incident X-rays make it easy to measure a small specimen in the DAC. The present paper introduces the design and performance of the SR (57)Fe-Mössbauer spectrometer and its demonstrative applications including the newly discovered result of a pressure-induced magnetic phase transition of polycrystalline (57)Fe(3)BO(6) and an unknown high-pressure phase of Gd(57)Fe(2) alloy placed in a DAC under high pressures up to 302 GPa. The achievement of Mössbauer spectroscopy in the multimegabar range is of particular interest to researchers studying the nature of the Earths core.

Control of the Orientation and Photoinduced Phase Transitions of Macrocyclic Azobenzene

Photoinduced phase transitions caused by photochromic reactions bring about a change in the state of matter at constant temperature. Herein, we report the photoinduced phase transitions of crystals of a photoresponsive macrocyclic compound bearing two azobenzene groups (1) at room temperature on irradiation with UV (365 nm) and visible (436 nm) light. The trans/trans isomer undergoes photoinduced phase transitions (crystal-isotropic phase-crystal) on UV light irradiation. The photochemically generated crystal exhibited reversible phase transitions between the crystal and the mesophase on UV and visible light irradiation. The molecular order of the randomly oriented crystals could be increased by irradiating with linearly polarized visible light, and the value of the order parameter was determined to be -0.84. Heating enhances the thermal cis-to-trans isomerization and subsequent cooling returned crystals of the trans/trans isomer.

Phase transformation and crystal structure of La(2)Ni(7)H(x) studied by in situ X-ray diffraction.

The phase transformation of La(2)Ni(7) during hydrogenation was investigated by in situ X-ray diffraction. We found two hydride phases, La(2)Ni(7)H(7.1) (phase I) and La(2)Ni(7)H(10.8) (phase II), during the first absorption cycle. The metal sublattice of phase I was orthorhombic (space group Pbcn) with lattice parameters a = 0.50128(6) nm, b = 0.8702(1) nm, and c = 3.0377(1) nm. The sublattice for phase II was monoclinic (space group C2/c) with lattice parameters a = 0.51641(9) nm, b = 0.8960(1) nm, c = 3.1289(1) nm, and β = 90.17(1)°. The lattice parameter c increased with the hydrogen content, while a and b decreased in the formation of phase I from the alloy. Phase transformation from phase I to phase II was accompanied by isotropic expansion. The La(2)Ni(4) and LaNi(5) subunit expanded by 48.9% and 6.0% in volume, respectively, during hydrogenation to phase I. They expanded an additional 14% and 5.8%, respectively, in the formation of phase II. The obtained volume expansion suggested different hydrogen distribution in the La(2)Ni(4) and LaNi(5) subunit during hydrogenation.

Synthesis and crystal structure of a Pr5Ni19 superlattice alloy and its hydrogen absorption-desorption property.

The intermetallic compound Pr(5)Ni(19), which is not shown in the Pr-Ni binary phase diagram, was synthesized, and the crystal structure was investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Two superlattice reflections with the Sm(5)Co(19)-type structure (002 and 004) and the Pr(5)Co(19)-type structure (003 and 006) were observed in the 2θ region between 2° and 15° in the XRD pattern using Cu Kα radiation. Rietveld refinement provided the goodness-of-fit parameter S = 6.7 for the Pr(5)Co(19)-type (3R) structure model and S = 1.7 for the Sm(5)Co(19)-type (2H) structure model, indicating that the synthesized compound has a Sm(5)Co(19) structure. The refined lattice parameters were a = 0.50010(9) nm and c = 3.2420(4) nm. The high-resolution TEM image also clearly revealed that the crystal structure of Pr(5)Ni(19) is of the Sm(5)Co(19) type, which agrees with the results from Rietveld refinement of the XRD data. The P-C isotherm of Pr(5)Ni(19) in the first absorption was clearly different from that in the first desorption. A single plateau in absorption and three plateaus in desorption were observed. The maximum hydrogen storage capacity of the first cycle reached 1.1 H/M, and that of the second cycle was 0.8 H/M. The 0.3 H/M of hydrogen remained in the metal lattice after the first desorption process.

Theoretical calculation of positron lifetimes for LaNi5–H system

Abstract Positron lifetime spectroscopy is a powerful tool for the study of vacancy-type defects in solids. Our positron lifetime measurements have revealed that huge numbers of excess vacancies are formed in addition to dislocations during the first hydrogen absorption process of LaNi 5 and excess vacancies becomes mobile and form microvoids by a thermal activation process. For further investigation, theoretical approaches using electronic structure calculations are indispensable. In this work, we have performed theoretical calculations of positron lifetime for LaNi 5 –H system using first principles electronic structure calculations. By comparison between the theoretical and experimental positron lifetimes, one of the defect components during hydrogen absorption can be ascribed to the annihilation at vacancy-clusters composed of two or three Ni vacancies. The positron lifetime of the vacancy-cluster component increases over 400 ps during isochronal annealing after hydrogen desorption. The vacancy-cluster may contain not only Ni vacancies but also La vacancies, since the vacancy-cluster composed of only Ni vacancies cannot yield such a long positron lifetime.

Crystal structure and local structure of Mg(2-x)Pr(x)Ni4 (x = 0.6 and 1.0) deuteride using in situ neutron total scattering.

We studied crystal structure and local structure of Mg(2-x)Pr(x)Ni4 (x = 0.6 and 1.0) and their deuterides using in situ neutron total scattering and first-principles calculations. The total scattering data were analyzed using Rietveld refinement and pair distribution function analysis (PDF). The crystal structure of Mg(2-x)Pr(x)Ni4 before deuterium absorption was C15b in space group F43m. No difference between the crystal and local (PDF) structures was observed. The crystal structure of Mg1.0Pr1.0Ni4D(∼4) was found to be orthorhombic in space group Pmn2(1), with three deuterium occupation sites: PrNi3 and two types of bipyramidal Pr2MgNi2 that have a plane of symmetry composed of MgNi2. There is no significant difference between the crystal structure and the local structure of Mg1.0Pr1.0Ni4D(∼4). On the other hand, the average crystal structure of the Mg-rich Mg1.4Pr0.6Ni4D(∼3.6) was C15b with two deuterium occupation sites: PrNi3 and MgPrNi2 suggesting that the deuterium occupation shifts away from the Pr2MgNi2 bipyramid. First-principles relaxed structures also showed the shift of the hydrogen occupation site toward the Pr atom of the bipyramid, when induced by Mg substitution for the opposing Pr, resulting in hydrogen occupation in the MgPrNi2 tetrahedral site. The PDF pattern of Mg1.4Pr0.6Ni4D(∼3.6) cannot be refined below 7.2 Å in atomic distances using the C15b structure which was obtained from Rietveld refinement but can be done using an orthorhombic structure. It suggests that Mg1.4Pr0.6Ni4D(∼3.6) was locally distorted to the orthorhombic.

In situ XRD study of La2Ni7H(x) during hydrogen absorption-desorption.

Structural changes of La2Ni7H(x) during the first and second absorption-desorption processes along the P-C isotherm were investigated by in situ X-ray diffraction (XRD). Orthorhombic (Pbcn) and monoclinic (C2/c) hydrides coexisted in the first absorption plateau, but only a monoclinic (C2/c) hydride was observed in the first desorption plateau. Phase transformation of La2Ni7H(x) was irreversible between the first as well as the second absorption-desorption process. The lattice parameters and expansion of the La2Ni4 and LaNi5 cells during the absorption-desorption process were refined using the Rietveld method. The lattice parameters a and b of the orthorhombic hydride (Pbcn) decreased, while the lattice parameter c increased with increasing hydrogen content in the first absorption. During the first absorption, the volume of the orthorhombic La2Ni4 cell expanded by more than 50%, while the expansion of the LaNi5 cell was below 10%. The monoclinic La2Ni4 cell expanded to approximately four times the size of the LaNi5 cell in the first absorption. The lattice parameters a, b, and c of the monoclinic hydride (C2/c) decreased with decreasing hydrogen content in the first desorption. These La2Ni4 and LaNi5 cells contracted isotropically in the first desorption.

Melting of Pb Charge Glass and Simultaneous Pb–Cr Charge Transfer in PbCrO3 as the Origin of Volume Collapse

A metal to insulator transition in integer or half integer charge systems can be regarded as crystallization of charges. The insulating state tends to have a glassy nature when randomness or geometrical frustration exists. We report that the charge glass state is realized in a perovskite compound PbCrO3, which has been known for almost 50 years, without any obvious inhomogeneity or triangular arrangement in the charge system. PbCrO3 has a valence state of Pb(2+)(0.5)Pb(4+)(0.5)Cr(3+)O3 with Pb(2+)-Pb(4+) correlation length of three lattice-spacings at ambient condition. A pressure induced melting of charge glass and simultaneous Pb-Cr charge transfer causes an insulator to metal transition and ∼10% volume collapse.

Application of metal hydride paper to simple pressure generator for use in soft actuator systems.

Metal hydride (MH) actuators have a simple structure and a number of features that make them attractive for use in rehabilitation engineering and assistive technology. The MH actuator provides a high power-to-weight ratio, high-strain actuation, human-compatible softness, and noiseless operation, while being environmentally benign. On the other hand, there remain technical challenges to be overcome to improve the MH actuator regarding its speed of operation and energy efficiency, given the low heat conductivity of the MH powder that is used as the pressure generator for soft actuation. To overcome the issues of low heat conductivity and the handling of MH powder, we developed an MH paper, which is a special paper incorporating MH powder and carbon fiber, for use as a new pressure-generating element for a soft MH actuator system. In addition, the basic properties and structure of the proposed MH paper were investigated through scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and several thermodynamic experiments. The results of these experiments showed that the hydrogen absorption and desorption rates of the MH paper were significantly higher than those of the MH powder around room temperature.