Nuclear and magnetic structure of U3Ga2Ge3: A neutron powder diffraction study

The crystal structure of U3Ga2Ge3 has been investigated by means of neutron powder diffractometry in the range from 1.5 to 100 K, Full matrix 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 nonmetal occupation in the metal octahedra U1Ge1Ge2Ga2 or U2Ge2Ga2 or in metal tetrahedra Ge2Ga2 is ruled out, At temperatures lower than T-c=90 K uranium atoms order ferromagnetically as monitored by the intensity of the magnetic reflection I-M((011)). Due to the observation of the (M)(011) reflection, the magnetic spin structure is consistent with the symmetry of the noncentrosymmetric space group I422, The lower symmetry 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 uranium atoms in 8j (mu(U) = 2.57 mu(B)), only the uranium atoms in 2a were found to carry a magnetic moment mu(U) = 2.45 mu(B). All moments are parallel to the crystallographic a axis, To comply with the symmetry of the Ge/Ga atoms, magnetic and nuclear structure have to be described in space group I4. The reliability values for the 1.5 K diffractogram were: R-I=0.024, R-F=0.019, and R-mag=0.033. (C) 1997 Academic Press.