TRANSMISSION ELECTRON-MICROSCOPY STUDY OF ORDER-DISORDER PHENOMENA IN NONSTOICHIOMETRIC LANI5+X AND LANI6-XCUX ELECTRODE MATERIALS

High-energy electron diffraction, X-ray powder diffraction and high-resolution electron microscopy have been used to investigate the crystallographic properties of non-stoichiometric LaNi5+x (0 less-than-or-equal-to x less-than-or-equal-to 0.4) and LaNi6-xCux (0 less-than-or-equal-to x less-than-or-equal-to 3-0) electrode materials, which are based on the CaCu5 structure type. Structural aspects with respect to Ni/Cu rich deviations from the exact AB5 stoichiometry are studied. X-ray powder diagrams show only the basic CaCu5 diffraction lines without any superstructure lines, and are therefore interpreted in terms of random substitutions of A atoms (La) by dumbbell pairs of B atoms (Ni/Cu) oriented along the c axis, thereby leading to a contraction and expansion of the lattice in the a and c direction, respectively. The observation of diffuse scattering in electron diffraction, which is located on a specific geometry in diffraction space, reveals that a large amount of the substitutions of A atoms by B2 dumbbells occur in a partially ordered manner. High-resolution electron microscopy shows that small Ni/Cu rich domains exist, in which the ordering is related to the stacking of A atoms and B2 dumbbells in the ordered rhombohedral and hexagonal A2B17 superstructures, and in the tetragonal AB12 superstructure.