NEUTRON-DIFFRACTION AND AC SUSCEPTIBILITY STUDIES OF THE SOLID-SOLUTIONS UMM'X(2) (M, M'=CO, NI, CU, X=SI, GE)

Polycrystalline samples of the intermediate solid solutions of the UM(2)X(2) compounds (M=Co, Ni, Cu; X=Si, Ge), namely UMM'X(2), 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'X(2) solid solutions are discussed and compared with those of the UM(2)X(2) compounds.