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 UAl3-UGe3 solid-solution and for the unique ternary intermediate phase U3Al2-xGe3+x. The experimental isopleth section of the pseudo-binary UAl3-UGe3 reveals an isomorphous solid solution based on the Cu3Au-type below the solidus. The U3Al2-xGe3+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 U3Al2-xGe3+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 I4cm in the paramagnetic domain. The magnetic measurements confirm the ferromagnetic ordering of the solid solution U3Al2-xGe3+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 5f electrons. (C) 2016 Elsevier Inc. All rights reserved.