Identification of the atomic scale structure of the La0.65Nd0.15Mg0.20Ni3.5 alloy synthesized by spark plasma sintering

Magnesium and neodymium substituted La2Ni7 hydride forming alloys are studied as possible active materials for negative electrodes in Ni–MH battery. La0.65Nd0.15Mg0.20Ni3.5 has been prepared by Spark Plasma Sintering (SPS) and characterized by X-ray diffraction, microprobe, Transmission Electron Microscopy (TEM) and High-Angle annular Dark-Field (HAADF) Scanning Transmission Electron Microscopy (STEM). HAADF-STEM is the optimal technique for the atomic resolution imaging of the crystal structure and defects of this complex material by providing directly interpretable images of the crystal structure with chemical contrast. The rhombohedral [3R] structure has been directly identified from the HAADF images, in agreement with the results obtained from Rietveld analysis of the X-ray diffraction patterns. HAADF image simulations fit the experimental data well. Local hexagonal-type defects and occasional changes in the sequence of building units for these stacking structures have been observed within the grains and are much more readily identifiable by HAADF-STEM imaging. ► The structure of La0.65Nd0.15Mg0.20Ni3.5 was identified from HAADF imaging. ► A localization of the atom species was proposed from HAADF imaging and simulations. ► Stacking faults corresponding to additional RM5 units were observed. ► Local defects corresponding to the hexagonal structure were highlighted.