Pressure-composition isotherms have been measured for the LaNi5-H2 system. The isotherms at higher than 80 °C clearly showed two plateaux on desorption, while the gap between two plateaux at the composition LaNi5H≈3 was much smaller in absorption than it was in desorption. In situ X-ray diffraction measurements have been performed at 84 °C in the pressure range up to 4.5 MPa and a new hydride phase corresponding to LaNi5H≈3 was found. The lower plateau on the isotherms is ascribed to the coexisting region of a phase (solid-solution phase) and β phase (LaNi5H≈3) and the higher plateau to that of β phase and 7 phase (LaNi5H6). Changes in lattice parameters have also been measured along these phase transformations. No distortion of the original hexagonal lattice has been observed in the present measurements. The representative lattice parameters for a, β and γ were obtained as follows. α: a = 5.01 - 5.06 A ̊, c = 4.00 A ̊; β: a = 5.27 - 5.31 A ̊, c = 4.05 - 4.10 A ̊; γ: a = 5.36 - 5.4 A ̊, c = 4.18 - 4.25 A ̊.
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