A novel ternary uranium-based intermetallic U34Fe4−xGe33: Structure and physical properties

Abstract

The new ternary phase U34Fe4−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 (U34Si34.5 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. U34Fe4−xGe33 with x=0.68 undergoes a ferromagnetic-type transition below 28K. 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=131mJ/(molUK2), whereas the estimated magnetic entropy at TC is 0.22Rln2. A residual resistivity of 314μΩcm and a resistivity ratio of 1.1 were found in the electrical resistivity curve, which also exhibits an upturn below TC 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.