Crystal and local structures of La2Co7H obtained by synchrotron X-ray total scattering

Abstract

We investigated the crystal and local structures of La2Co7Hx using synchrotron X-ray diffraction (SXRD) in the first absorption process. La2Co7 with a Ce2Ni7-type structure was transformed into La2Co7H7.4 with an orthorhombic structure (space group: Pbcn), determined by Rietveld refinement, through the formation of La2Co7H4.1. The expansions of the lattice parameters of La2Co7H7.4 from the original alloy were Δa = −3.2 %, Δb = −3.4 %, and Δc = 29.2 %. The unit cell expanded anisotropically. The local structure of La2Co7Hx (x = 0, 4.1, and 7.4) was determined by performing an atomic pair distribution function (PDF) analysis. The results showed no significant difference between the crystal and local structures of La2Co7 and La2Co7H4.1. The refined structural parameters of La2Co7H7.4 obtained by the PDF analysis were inconsistent with the Rietveld analysis results. The local distortion was not negligible, and the local atomic positions of the metal atoms deviated from the crystal structure. The local distortion of the CaCu5-type cell of La2Co7H7.4 was greater than that of the MgZn2-type cell.