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Crystal Structure Evolution of La2Ni7 during Hydrogenation Yuki Iwatake, Kyosuke Kishida and Haruyuki Inui Department of Materials Science and Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan ABSTRACT Atomic scale characterization of the La2Ni7 hydrides by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) revealed that not only the anisotropic expansion of the La2Ni4 unit layer previously reported but also the shearing on the basal plane of the La2Ni4 unit layers occur during one-cycle of hydrogen absorption/desorption process. Two different types of orthorhombic La2Ni7 hydrides with the same atomic arrangement of La and different atomic arrangement of Ni were observed depending on the maximum hydrogen concentration achieved during one hydrogen absorption/desorption cycle. INTRODUCTION (RE,Mg)2Ni7-based hydrogen-absorbing alloys (RE: Rare Earth) have attracted a great deal of interest as negative electrode materials in nickel-metal-hydride (Ni-MH) batteries with very low self-discharge property and high capacity. Such attractive properties are closely related to the crystal structure of (RE,Mg)2Ni7 phase and its stability upon hydrogen absorption/ desorption cycles. For further development of a new class of hydrogen-absorbing materials for negative electrodes of Ni-MH batteries, it is essential to understand the details of crystal structure evolution during hydrogen absorption/desorption cycles. Most of La-Ni binary intermetallic compounds such as LaNi3, La2Ni7 and La5Ni19 possess common block-stacking structures, in which each block layer is composed of one La2Ni4 (Laves-type) unit layer and one or more LaNi5 (Haucke-type) unit layers stacking along the c-axis (figure 1) [1]. La2Ni7 with the Ce2Ni7-type hexagonal structure (space group: P63/mmc, #194) is made with two block layers, each consisting of one La2Ni4-unit layer and two LaNi5-unit layers [2]. It has long been believed that reversibility of the hydrogen absorption/desorption behavior of La2Ni7 is poor because of its amorphization upon hydrogenation, therefore structural variation during hydrogen absorption/desorption cycles has not been studied in detail for a long time [3]. Recently, good reversibility of hydrogen storage for La2Ni7 has been confirmed and crystal structural variation during hydrogenation has been studied by powder x-ray and neutron diffraction, respectively [46]. Common characteristics suggested by the previous studies is that hydrogen is preferentially absorbed in the La2Ni4 unit layer at low pressure and the hydride formed at this stage is stable after desorption. The crystal structure of the hydride has been proposed to be characterized by the preferential and anisotropic expansion of the La2Ni4 unit layers along the stacking direction [5,6]. However, the details of the crystal structure of the La2Ni7 hydride are still controversial. In this study, the structural change of La2Ni7 phase during hydrogenation was investigated by atomic-resolution high-angle annular dark-field scanning trans