Structural and Hydriding/Dehydriding Properties of HoMn 2 H x with Cubic and Hexagonal Laves Phase Structure
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0927-EE03-09
Structural and Hydriding/Dehydriding Properties of HoMn2Hx with Cubic and Hexagonal Laves Phase Structure Yoshikazu Makihara1, Youichi Iwata1, Kazumi Umeda1, Yoshio Miyairi1, and Hironobu Fujii2 1 Department of Mechanical and Electronic System Engineering, Kyushu Kyoritsu University, 18 Jiyugaoka, Yahatanishi-ku, Kitakyushu, 807-8585, Japan 2 Natural Science Center for Basic Research and Development, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, 739-8526, Japan
ABSTRACT Both the cubic C15-type and the hexagonal C14-type Laves phase compounds HoMn2 were successfully prepared. We found that they absorbed hydrogen up to 3.4 atoms per formula unit at room temperature and discharged it at around 490 K, reversibly. Structural and hydriding/dehydriding properties of the host HoMn2 and their hydrides HoMn2Hx have been investigated by powder X-ray diffraction (XRD) and thermogravimetry/differential thermal analysis (TG/DTA). With increasing the hydrogen content, the host C15-type HoMn2 is distorted into a rhombohedral phase at above x = 2.9, while the host C14-type one keeps the hexagonal structure up to x = 3.4. The experimental results obtained from TG/DTA measurement suggest that two possible interstitial sites with different activation energies for hydrogen desorption exist in the C14 HoMn2, while one site in the C15 HoMn2. INTRODUCTION It is known that hydrogen storage properties of the compounds are closely related to their constituent atoms and the crystal structure. The constituent atoms strongly influences the properties such as chemical affinity with hydrogen and catalytic effect that promotes the hydriding/dehydriding kinetics by resolving hydrogen molecules into atoms or combining hydrogen atoms. On the other hand, the crystal structure of the compound is also an important factor for the kinetics and the hydrogen storage capacity because the number and the hole size of the interstitial sites occupied by hydrogen atoms are directly dependent on the crystal structure. If we could synthesize two samples which have the same constituent atoms and composition, and the different crystal structures, we might prove the structural effect on the hydrogen storage properties more clearly. It has been reported for Zr-based Laves phase compounds that ZrCr2 can be stabilized in either the cubic C15- or the hexagonal C14-type structure and absorb 4.0 hydrogen atoms per formula for the C-15 type [1] and 3.4 atoms per formula for the C-14 type [2]. The hydrogen positions absorbed in the ZrCr2 hydrides have been determined to be two tetrahedral sites of 2Zr+2Cr and 1Zr+3Cr for the C15-type and four different tetrahedral sites of 2Zr+2Cr for the C14-type [3,4]. The Laves phase RMn2 (R= rare earth and Y) compounds, which crystallize in the cubic C15-type structure for R=Y, Gd, Tb, Dy, Ho and in the hexagonal C14-type structures for R=Pr, Nd, Sm, Er, Tm and Lu, can easily absorb large quantities of hydrogen. Recently, the structural
and magnetic properties of the RMn2Hx hydrides have been investigated as a
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