Neutron diffraction and ac susceptibility studies of the solid solutions UMM′X₂ (M, M′ = Co, Ni, Cu; X = Si, Ge)

Abstract

Polycrystalline samples of the intermediate solid solutions of the UM₂X₂ compounds (M, M′ = Co, Ni, Cu; X = Si, Ge), namely UMM′X₂, have been prepared and studied by ac susceptibility and neutron diffraction measurements. They crystallize in the body-centered tetragonal ThCr₂Si₂-type structure (space group I4/mmm) and order magnetically, with the exception of UCoNiGe₂, which crystallizes in the primitive tetragonal CaBe₂Ge₂-type structure (space group P4/nmm) and does not order magnetically. UCoNiSi₂, UNiCuSi₂, and UNiCuGe₂ order antiferromagnetically below T_N = (115±5), (150±2) and (140±5) K, respectively, with the AF-I structure of alternate (+-+-) ferromagnetic uranium basal planes and a wavevector k = (0, 0, 1). UCoCuSi₂ and UCoCuGe₂ order ferromagnetically below T_C=(107±5) and (114±5) K and undergo transition to the AF-I structure below T_o=(95±5) and (94±5) K, respectively. The uranium ordered moments are along the tetragonal axis. The magnetic properties of the UMM′X₂ solid solutions are discussed and compared with those of the UM₂X₂ compounds.