Lanthanoid and uranium nickel arsenides with CaBe 2Ge 2- and ThCr 2Si 2-type structures

Abstract

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 ≡ La, Ce, Pr, Nd, Sm and U) crystallize with the primitive tetragonal (space group P4/ nmm) CaBe 2Ge 2-type structure, which was refined for NdNi 2As 2 from single-crystal X-ray data to a residual of R = 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-centred tetragonal ThCr 2Si 2-type structure (space group I4/ mmm) was found for the new compounds MNi 2As 2 (M ≡ La, Ce, Pr, Nd and Sm). A structure refinement of previously reported EuNi 2As 2 with ThCr 2Si 2-type structure ( R = 0.054 for 7 variables and 99 structure factors) gives some indication of a deviation from full occupancy of the nickel (and arsenic) positions. This agrees with the fact that the cell volumes of the ThCr 2Si 2-type compounds are slightly smaller than those of the corresponding CaBe 2Ge 2-type compounds. The study of the ternary system Nd-Ni-As for compositions close to 1:2:2 also shows that the ThCr 2Si 2-type compound is formed in samples with a lower nickel content. Further support for this view was obtained from energy dispersive X-ray fluorescence analyses and from band structure considerations.