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

Structural changes of La2Ni7Hx 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 La2Ni7Hx 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...