Synthesis and characterization of the structural and magnetic properties of new uranium and copper-based silicides and germanides: study of the physical and hydridation properties of some compounds belonging to the Gd-Ni-X systems, where X = Ga, Al, Sn

Three novel phases, U3Cu4Si4, U3Cu4Ge4 and UCuGe1,77, were prepared in the U - Cu - X (X = Si or Ge) ternary system. Their structural and magnetic properties were investigated. The magnetic structures of the first two compounds were determined by neutron diffraction. Structural and magnetic behaviour transitions occur as copper substitutes silicon atoms in the UCuxSi2-x (0,28 ≤ x ≤ 0,96) solid solution. Thus, the structure of the compositions changes in the α-ThSi2(tetragonal) → AlB2(hexagonal) → Ni2In(hexagonal) sequence while a transition from a nonmagnetic to ferromagnetic then antiferromagnetic behaviour is observed. The magnetic properties of the different compositions are governed by a Kondo - RKKY -type interactions competition. Crystallographic disorder and magnetic frustrations are at the origin of a spin glass state between the ferro- and antiferromagnetic areas. The investigations of the GdNi3X2 (X =Ga, Al, Sn) compounds revealed that their structural and magnetic properties are strongly dependent on the nature of the X element as well as the on thermal treatment. A CaCu5 → HoNi2,6Ga2,4 - type structure transition and a ferro - to antiferromagnetic behaviour evolution are favoured by the increase of the X - atom size. A commensurate modulated crystal structure (described also as a aHoNi2,6Ga2,4 x aHoNi2,6Ga2,4 x 2cHoNi2,6Ga2,4 -type superstructure) has been observed for GdNi3Al2. Hydrogen absorption in Gd3Ni6Al2 and GdNi3Al2 weakens the strength of the magnetic interactions. (author)