In situ X-ray and neutron powder diffraction study of LaNi 5-x Sn x -H systems
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In situ X-ray and neutron powder diffraction study of LaNi5-xSnx–H systems Yumiko Nakamura1, Robert C. Bowman, Jr. 2, Etsuo Akiba1, 1) National Institute of Advanced Industrial Science and Technology, AIST Central 5, 1-1-1, Higashi, Tsukuba, Ibaraki, 305-8565, Japan 2) Jet Propulsion Laboratory, California Institute of Technology, Mail Stop 79-24, 4800 Oak Grove Dr., Pasadena, CA 91109-8099, USA ABSTRACT We have studied structural change and lattice strain formation during absorption and desorption of hydrogen (deuterium) by high purity LaNi5-xSnx alloys (x = 0.22, 0.25) using in situ X-ray and neutron diffraction along with simultaneously measuring the P-C isotherms. From profile analysis of the X-ray diffraction data, lattice parameter and lattice strain in each hydriding state are evaluated. In situ neutron diffraction data provided hydrogen occupation changing with hydrogen content. Significant lattice contract and strain formation were observed in the hydride phase in desorption. This behavior was related with decrease in hydrogen occupation in the hydride phase revealed from the neutron diffraction data. INTRODUCTION LaNi5 is a representative intermetallic compound which has a reversible hydrogen absorbing property under ambient conditions. The alloy phase transforms into hydride phase through two-phase coexistence region. The lattice parameters and the cell volume increase by 7 % and 24 %, respectively [1,2], in the transformation from the alloy phase to the hydride phase, respectively. The hydriding reaction with such large lattice expansion in absorption and contraction in desorption could induce various kinds of lattice defects and strain. Nakamura and Akiba have made an in situ X-ray diffraction (XRD) study of LaNi5 and LaNi4.75Al0.25 in the initial absorption-desorption [3]. LaNi5 showed heavy anisotropic lattice strain in the direction of . It formed in absorption and did not disappear in the following desorption reaction. LaNi4.75Al0.25 did not show such type of strain but showed another type of anisotropic strain in the direction of , which disappeared in the following phase transformation. These results suggest that strain formation strongly depends on the substitution elements. Hydriding properties of LaNi5-xSnx have been reported as a good candidate of heat pump material for space use by one of the present authors [4,5]: these alloys show a wide and flat plateau region and good stability for repeated absorption-desorption cycles. They show little peak broadening in XRD profiles even after hydriding [6,7]. The good stability in repeated hydriding and little peak broadening in XRD profiles are similar to properties of LaNi5-xAlx [8,9]. From the viewpoint of the relation between strain and stability in repeated cycles, introduction and properties of lattice strain in LaNi5-xSnx in absorption-desorption are of much interest.
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This paper will present results of in situ XRD measurements of LaNi4.75Sn0.25 during the initial absorption-desorption cycle. These measurements were performed un
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