MAGNETIC ORDERING OF THE UCu_2-xGa_2+y SYSTEM DETERMINED BY NEUTRON DIFFRACTION. N. Stüßer, Z. Zolnierek^*, Hahn-Meitner-Institut Berlin, FRG, ^*W.Trzebiatowski Institute, Wroclaw, PL

In the last few years there has been a great interest in studying the (RE,An)T₂(Si,Ge)₂ ternaries which exhibit a variety of magnetic structures. These compounds, where RE stands for the rare earth, An for actinide and T for transition metal atom crystallize in the space group I4/mmm. We have examined by neutron powder diffraction the ternaries UCu_2-xGa_2+y which are structural analogues to the ThCr₂Si₂ compound often referred to as the 1:2:2 compounds.

Two types of magnetic ordering were established. For UCu_1.16Ga_2.66 and UCu_1.33Ga_2.66 a simple AF-I-type structure was found. In the AF-I phase the uranium atoms at (0[[opthyphen]]0[[opthyphen]]0) have a spin parallel to the c-axis whereas those at (1/2[[opthyphen]]1/2[[opthyphen]]1/2) are antiparallel. This type of magnetic ordering is very common in the UT₂X₂-series.

For the copper ternaries UCu_2-xGa_2.5 with x=0.75, 0.66, and 0.5 a new type of magnetic ordering was detected which has not been observed so far for any of the known 1:2:2 type intermetallics. The new magnetic unit cell is orthorhombic with lattice parameters A=2^1/2a, B=2^3/2a, and C=c, where a and c specify the tetragonal unit cell of the chemical structure. The magnetic structure can be described by a propagation vector k=(0[[opthyphen]]1[[opthyphen]]0) corresponding to (1/4[[opthyphen]]1/4[[opthyphen]]0) for the chemical unit cell. In this new type of magnetic structure the moments are canted from the c-axis. The c-components still exhibit the AF-I[[opthyphen]]structure and the perpendicular components point along the B-direction forming an antiferromagnetic lattice as well.