Hydrogenation characteristics of Ce2Ni7-type La2Co7 and its phase transformation during hydrogen absorption–desorption processes

Abstract

We have investigated the crystal structure and hydrogenation properties of La2Co7 with a Ce2Ni7-type structure by X-ray diffraction (XRD) and P–C isotherm (pressure composition isotherm) analyses. The maximum hydrogen capacity reached 0.79 ​H/M (1.04 mass%). After the first full desorption, 0.50 ​H/M (0.66 mass%) of hydrogen remained in the sample, and the reversible hydrogen capacity decreased to 0.24 ​H/M (0.32 mass%) in the second absorption–desorption cycle. Three plateaus were observed in the first absorption cycle, and the crystal structures of the two hydride phases were identified. A Ce2Ni7-type structure (space group: P63/mmc) was identified for La2Co7H2.8 (I), which is similar to that of La2Co7. La2Co7H5.0 (II) was identified to have a hexagonal structure with a lower symmetry (space group: P-62c). The lattice parameters are a ​= ​0.50428(9) nm and c ​= ​2.9861(4) nm, and the expansions of a and c relative to the lattice parameters of La2Co7 are −1.9 and 21.3%, respectively, indicating anisotropic expansion. For La2Co7H7.2 (III, full hydride phase), severe peak broadening was observed in the XRD pattern, and it was difficult to construct a suitable structural model. The crystal structure transformation occurred in the order of a hexagonal Ce2Ni7-type structure (La2Co7) → the same Ce2Ni7-type structure (La2Co7H2.8) → a hexagonal structure with a lower symmetry (La2Co7H5.0) → an unknown structure (La2Co7H7.2).