MAGNETIC-ORDERING IN U(CO)2GE2, U(NI)2GE2, U(CU)2GE2 COMPOUNDS

Several U(M,M')2Ge2 compounds (M,M'=Co,Ni,Cu) have been studied by neutron diffraction. U(Co1-xCux) 2Ge2 and UNiCuGe2 exhibit ThCr 2Si2-type tetragonal structure, with a=4.02-4.10 Å and c=9.86-10.25 Å, and a magnetically ordered uranium sublattice with moments along c. The ordering is antiferromagnetic with a wave vector k=(0,0,1) for x=0,0.25, and 0.50 and UNiCuGe2 (TN=175, 130, 100, and 140 K, respectively), and ferromagnetic for x=0.75 (TC=102 K) and 1 (TC=107 K). In UCoNiGe2 (a=4.07 Å, c=9.52 Å) no magnetic ordering has been found down to 10 K. Two other samples of UCo 2Ge2 (a=4.04 Å, c=9.44 and 9.38 Å) were previously reported not to order magnetically down to 4.2 K. The absence of magnetic ordering is related to smaller c parameters and lower crystal symmetry.