NUCLEAR AND MAGNETIC-STRUCTURE OF U3GA2GE3 - A NEUTRON POWDER DIFFRACTION STUDY

The crystal structure of U3Ga2Ge3 has been investigated by means of ne utron powder diffractometry in the range from 1.5 to 100 K, Full matri x full profile refinements of the nuclear structure at 100 K revealed a new structure type with almost fully ordered distribution of Ga and Ge atoms in the 8e sites of space group I4. The new U3Ga2Ge3 type is a low symmetry derivative of the ordered antitype-Cr5B3 (I4/mcm), R-I = 0.032, R-F = 0.030, From model refinements, eventual interstitial non metal occupation in the metal octahedra U1Ge1Ge2Ga2 or U2Ge2Ga2 or in metal tetrahedra Ge2Ga2 is ruled out, At temperatures lower than T-c=9 0 K uranium atoms order ferromagnetically as monitored by the intensit y of the magnetic reflection I-M((011)). Due to the observation of the (M)(011) reflection, the magnetic spin structure is consistent with t he symmetry of the noncentrosymmetric space group I422, The lower symm etry invokes a splitting of the 4a sites of the parent Cr5B3 type (I4/ mcm) into two twofold uranium sites 2a and 26 of I422. Besides the ura nium atoms in 8j (mu(U) = 2.57 mu(B)), only the uranium atoms in 2a we re found to carry a magnetic moment mu(U) = 2.45 mu(B). All moments ar e parallel to the crystallographic a axis, To comply with the symmetry of the Ge/Ga atoms, magnetic and nuclear structure have to be describ ed in space group I4. The reliability values for the 1.5 K diffractogr am were: R-I=0.024, R-F=0.019, and R-mag=0.033. (C) 1997 Academic Pres s.