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 meas urements. They crystallize in the body-centered tetragonal ThCr2Si2-ty pe structure (space group I4/mmm) and order magnetically, with the exc eption of UCoNiGe2, which crystallizes in the primitive tetragonal CaB e2Ge2-type structure (space group P4/nmm) and does not order magnetica lly. UCoNiSi2, UNiCuSi2, and UNiCuGe2 order antiferromagnetically belo w T-N=(115 +/- 5), (150 +/- 2) and (140 +/- 5) K, respectively, with t he AF-I structure of alternate (+-+-) ferromagnetic uranium basal plan es and a wavevector k=(0, 0, 1). UCoCuSi2 and UCoCuGe2 order ferromagn etically 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, respectiv ely. The uranium ordered moments are along the tetragonal axis. The ma gnetic properties of the UMM'X(2) solid solutions are discussed and co mpared with those of the UM(2)X(2) compounds.