Abstract
The hydrogen absorption properties of the Sm2Fe17 alloy were investigated by measurements of pressure–composition isotherms over the temperature range 423–623 K. The hydrogen atoms occupy not only the octahedral 9e sites but also the tetrahedral 18g sites in the rhombohedral Sm2Fe17 lattice. It is believed that the full occupation of the 9e sites, corresponding to the hydrogen composition H/Sm2Fe17=3.0, is followed by a partial occupation of the 18g sites. In the present work, however, the partial molar entropy of hydrogen suggests that the 18g sites start to be occupied even at low hydrogen concentrations. This behavior is followed by drastic changes in the partial thermodynamic properties of hydrogen starting around H/Sm2Fe17=2.0, implying that a large amount of hydrogen can start to enter the 18g sites before the 9e sites are completely occupied. This thermodynamic behavior was confirmed by discontinuous changes in the lattice constants and the saturation magnetization of Sm2Fe17 alloy with hydrogen composition.
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