Crystal Structure of Nonstoichiometric Copper-Substituted La(Ni1-zCuz)x Compounds Studied by Neutron and Synchrotron Anomalous Powder Diffraction

The structural properties of the potential battery electrode materials LaNi4Cu, LaNi4.4Cu, LaNi5Cu, and LaNi4.5Cu1.5 have been investigated by neutron and anomalous synchrotron powder diffraction. The nonstoichiometry of such ABx (5< or=x< or=6) compounds has been described in a structure derived from the hexagonal CaCu5-type structure by replacement of the A atoms by dumbbells of B atoms. In these copper-substituted compounds, Cu can occupy any of the four available nickel sites. From a joint refinement procedure using both neutron and synchrotron data, the copper-to-nickel ratio at each B site has been determined. This shows that substitution is not random and that copper occupies some crystallographic sites preferentially. The results are discussed and compared to other substituted AB5 systems.