LANTHANOID AND URANIUM NICKEL ARSENIDES WITH CABE2GE2-TYPE AND THCR2SI2-TYPE STRUCTURES

New ternary arsenides of the lanthanoids and uranium with nickel were obtained by reaction of the elemental components in powder form. The compounds MNi//2As//2 (M EQUVLNT La, Ce, Pr, Nd, Sm and U) crystallize with the primitive tetragonal CaBe//2Ge//2-type structure, which was refined for NdNi//2As//2 from single-crystal X-ray data to a residual of R equals 0. 035 for 15 variable parameters and 207 structure factors. The refinement of the occupancy parameters shows that all atomic positions are fully occupied. The closely related body-centered tetragonal ThCr//2Si//2-type structure was found for the new compounds MNi//2Si//2-type structure (M EQUVLNT La, Ce, Pr, Nd and Sm). A structure refinement of previously reported EuNi//2As//2 with ThCr//2Si//2-type structure gives some indication of a deviation from full occupancy of the nickel (and arsenic) positions.