Phase transition and hydrogenation properties of Ce2Ni7-type Pr2Co7 during the hydrogen absorption process

The crystal structure and hydrogenation properties of Pr2Co7 with a Ce2Ni7-type structure were investigated by X-ray diffraction (XRD) and observation of the pressure-composition (PC) isotherms. The reversible hydrogen capacity reached 0.8 H/M, and two plateaus were observed in the absorption-desorption process. The two observed hydride phases, Pr2Co7H2.7 and Pr2Co7H7.2, were determined to have hexagonal (space group: P63/mmc) and orthorhombic (space group: Pbcn) crystal structures, respectively. The crystal structure transformed in the order of hexagonal with a Ce2Ni7-type structure (original alloy) → same Ce2Ni7-type structure (Pr2Co7H2.7) → orthorhombic (Pr2Co7H7.2). The crystal lattice of the Pr2Co7H2.7 underwent anisotropic expansion along the c-axis of the original alloy, whereas that of Pr2Co7H7.2 exhibited isotropic expansion. The full width at half maximum (FWHM) values for the original alloy and hydride phases during the hydrogen absorption-desorption process were evaluated based on the XRD data. The FWHM values for the main peaks decreased as the hydrogen content increased during the absorption process, indicating that the number of lattice defects did not increase upon hydrogenation. The plateau pressures during the absorption process of the second cycle were the same as those of the first cycle, which also suggests that there were no lattice defects.