Magnetic structures of U 3M 2M 3′, M=Al, Ga; M′=Si, Ge: a neutron powder diffraction study

Abstract

The crystal and magnetic structures of U 3Al 2Si 3, U 3Ga 2Si 3 and U 3Al 2Ge 3 were investigated by means of neutron powder diffractometry in the range 1.5–80 K. Rietveld full profile refinements of the nuclear structure above the magnetic ordering temperatures in all cases confirmed the almost fully ordered distribution of M (Al or Ga) and M′ (Si or Ge) atoms in the 8c sites of space group I4. Thus isotypism is established with the U 3Ga 2Ge 3-type as a low-symmetry derivative of the ordered antitype-Cr 5B 3 (space group I4/mcm). From model refinements, interstitial nonmetal occupation in the metal octahedra U 1M 2M 1′M 2′ or U 2M 2M 2′ or in metal tetrahedra M 2M 2′ is ruled out. Magnetic ordering temperatures, earlier derived from susceptibility measurements, were confirmed from the neutron data: U 3Al 2Si 3 (T m =33 K), U 3Ga 2Si 3 (T m =64 K) and U 3Al 2Ge 3 (T m =63 K). All uranium moments are parallel to the crystallographic a- axis. Whereas uranium atoms (in 8c, 2a 1, 2a 2; space group I4) in both silicides were found to order ferrimagnetically with the moments in the sites 2a 1,2 being antiparallel but not equal, magnetic order in U 3Al 2Ge 3 is ferromagnetic in the range 1.5–40 K. For the latter compound (at 1.5 K), besides the uranium atoms in 8c (μ U =2.37 μ B ), only the uranium atoms in 2a 1 were found to carry a magnetic moment μ U =2.12 μ B . In the range 40 K<T<63 K an additional sinusoidal modulation of the moments in the direction of the c- axis was observed with a vector k=[0, 0, 0.076]. The residual values for the magnetic structures at about 1.5 K were: U 3Al 2Si 3 ( R m=0.061), U 3Ga 2Si 3 ( R m=0.084) and U 3Al 2Ge 3 ( R m1=0.050; for the sinusoidal modulation at 45 K, R m2=0.264). A symmetry analysis for the magnetic structure employing space group irreducible representations is consistent with the magnetic structure derived experimentally.