In situ XRD study of La2Ni7H(x) during hydrogen absorption-desorption

Structural changes of La2Ni7H(x) during the first and second absorption-desorption processes along the P-C isotherm were investigated by in situ X-ray diffraction (XRD). Orthorhombic (Pbcn) and monoclinic (C2/c) hydrides coexisted in the first absorption plateau, but only a monoclinic (C2/c) hydride was observed in the first desorption plateau. Phase transformation of La2Ni7H(x) was irreversible between the first as well as the second absorption-desorption process. The lattice parameters and expansion of the La2Ni4 and LaNi5 cells during the absorption-desorption process were refined using the Rietveld method. The lattice parameters a and b of the orthorhombic hydride (Pbcn) decreased, while the lattice parameter c increased with increasing hydrogen content in the first absorption. During the first absorption, the volume of the orthorhombic La2Ni4 cell expanded by more than 50%, while the expansion of the LaNi5 cell was below 10%. The monoclinic La2Ni4 cell expanded to approximately four times the size of the LaNi5 cell in the first absorption. The lattice parameters a, b, and c of the monoclinic hydride (C2/c) decreased with decreasing hydrogen content in the first desorption. These La2Ni4 and LaNi5 cells contracted isotropically in the first desorption.