A novel ternary uranium-based intermetallic U₃₄Fe_4−xGe₃₃: Structure and physical properties

Highlights: • A new uranium intermetallic compound U₃₄Fe_4−xge₃₃ was prepared and characterized. • It crystallizes in the tetragonal system with an original structure-type. • U lattice stacking is made of cupolae, orthobicupola and irregular cubes. • U₃₄Fe_4−xGe₃₃ orders magnetically at T_C = 28 K, with pure U magnetism. • T_C and electrical resistivity increase with applied magnetic field. - Abstract: The new ternary phase U₃₄Fe_4−xGe₃₃ has been synthesized and characterized by means of single crystal X-ray diffraction, magnetization, Mössbauer spectroscopy, specific heat, electrical resistivity, magnetoresistivity and thermopower measurements. It crystallizes in its own tetragonal structure type which can be described as derived from the one of the binary USi (U₃₄Si_34.5 structure-type, space group I4/mmm), with lattice parameters at room temperature, a = 10.873(5) Å and c = 25.274(3) Å. Structure refinement confirmed six inequivalent U atoms, occupying sites with dissimilar coordination, the Ge atoms staying on seven positions and Fe on two positions, one of the Fe sites with a partial occupancy. The U sub-lattice is composed by the stacking of a square cupola, two distorted cubes and a square orthobicupola. U₃₄Fe_4−xGe₃₃ with x = 0.68 undergoes a ferromagnetic-type transition below 28 K. Mössbauer spectroscopy shows that the magnetism is ruled by the U sub-lattice, as Fe atoms have no ordered moments. The Sommerfeld coefficient of the electronic specific heat is γ = 131 mJ/(mol_U K²), whereas the estimated magnetic entropy at T_C is 0.22Rln2. A residual resistivity of 314 μΩ cm and a resistivity ratio of 1.1 were found in the electrical resistivity curve, which also exhibits an upturn below T_C that shifts towards higher temperatures with the applied magnetic field. This behavior may be related to some disorder in the non-magnetic lattice and/or partial ordering of the magnetic lattice