A new hydride phase in the LaNi5-H2 system has been found by precise pressure-composition isotherm measurements. Two plateaux on the isotherms have been clearly observed at temperatures higher than 353 K. The gap between the two plateaux that indicates the existence of the new phase appeared at the composition LaNi5H3. Results of in situ X-ray diffraction measurements show that the new phase LaNi5H3 has hexagonal structure which is the same as the solid solution phase (α phase) and the full hydride phase. The new phase is named β phase and the full hydride phase (LaNi5H6) which has been called β phase is named here γ phase. It was found that the β phase formed after reactions were carried out at temperatures higher than 343 K in desorption and 367 K in absorption. However, once the β phase appeared it formed even at room temperature. Change in lattice parameters has also been measured along the phase transformations. It was found that the lattice expansion accompanying hydride formation was anisotropic. The structure of the β-phase hydride is discussed.
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