Investigation of selected lanthanide and actinide compounds by neutron diffraction

Magnetic ordering phenomena of different classes of lanthanide and actinide compounds were systematically studied by means of elastic and inelastic neutron scattering. The chemical structure of the uranium compounds K2UX5 (X=Cl,Br,I) is built up by U chains along the b-axis. The magnetic part of the specific heat data was analyzed in terms of an anisotropic two-dimensional Ising model. These investigations prove the existence of a short-range ordered magnetic correlation followed by a long-range ordered magnetic phase below TN. The long-range magnetic structures have been determined by means of elastic magnetic neutron scattering. K2UCl5 and K2UBr5 order magnetically corresponding to the Shubnikov space group Pn'm'a' at the Neel-temperatures TN = 3.8 and 2.8 K, respectively. The magnetic structure of K2UI5 below TN = 1.45 K corresponds to the magnetic space group Pnm'a. In the series X = Cl, Br and I, the ordering temperatures and the ordered magnetic moments at saturation decrease with increasing uranium distances. By means of inelastic neutron scattering, crystal-field transitions of K2UX5 (X=Cl,Br,I) were measured. Including additional luminescence measurements the crystalline-electric-field level scheme was determined and explained by the theoretical model. The chemical and magnetic structures of the Al substituted compounds RCU5-xAlx (R=La,Ce,Yb) were systematically studied by means of elastic neutron scattering. The magnetic properties are strongly dependent on the concentration of Al. In YbCu5 the Yb is divalent and non-magnetic, whereas the YbCu3Al2 is an Yb3+ compound with an ordered magnetic phase. The crossover from a non-magnetic state as a function of x was monitored by neutron scattering. In addition the magnetic structures of the cerium compounds Ce Cu5 and CePd2Ga3 were determined. (author) figs., tabs., 73 refs