Investigation of the phase relations in the U-Al-Ge ternary system: Influence of the Al/Ge substitution on the properties of the intermediate phases

Abstract The phase relations within the U-Al-Ge ternary system were studied for two isothermal sections, at 673 K for the whole Gibbs triangle and at 1173 K for the concentration range 25–100 at% U. The identification of the phases, their composition ranges and stability were determined by x-ray powder diffraction, scanning electron microscopy coupled to energy dispersive spectroscopy and differential thermal analysis. The tie-lines and the solubility domains were determined for the U-Ge and U-Al binaries, the UAl 3 -UGe 3 solid-solution and for the unique ternary intermediate phase U 3 Al 2−x Ge 3+x . The experimental isopleth section of the pseudo-binary UAl 3 -UGe 3 reveals an isomorphous solid solution based on the Cu 3 Au-type below the solidus. The U 3 Al 2−x Ge 3+x solid solution extends for −0.1≤x≤1.35 and −0.2≤x≤1.5 at 673 K and 1173 K respectively. It crystallizes in the I -centered tetragonal symmetry. The reciprocal lattice of several compositions of the U 3 Al 2−x Ge 3+x solid solution was examined by electron diffraction at room temperature, revealing the presence of a c -glide plane. Their crystal structure was refined by single crystal x-ray diffraction suggesting an isomorphous solid solution best described with the non-centrosymmetric space group I 4 cm in the paramagnetic domain. The magnetic measurements confirm the ferromagnetic ordering of the solid solution U 3 Al 2−x Ge 3+x with an increase of Tc with the Al content. The thermal variation of the specific heat bear out the magnetic transitions with some delocalized character of the uranium 5 f electrons.