Crystal structure and electronic properties of the new compounds, U6Fe16Si7 and its interstitial carbide U6Fe16Si7C
D. Berthebauda, O. Tougaita, , , M. Potela, E.B. Lopesb, A.P. Gonçalvesb and H. Noëla
aLaboratoire de Chimie du Solide et de Matériaux. UMR CNRS 6226, Université de Rennes 1, 263 Avenue de Général Leclerc, 35042 Rennes, France bDepartamento de Química, Instituto Tecnológico e Nuclear/CFMC-UL, P-2686-953 Sacavém, Portugal Received 1 June 2007; revised 17 July 2007; accepted 29 July 2007. Available online 22 August 2007.
The new compounds U6Fe16Si7 and U6Fe16Si7C were prepared by arc-melting and subsequent annealing at 1500 °C. Single-crystal X-ray diffraction showed that they crystallize in the cubic space group (No. 225), with unit-cell parameters at room temperature a=11.7206(5) Å for U6Fe16Si7 and a=11.7814(2) Å for U6Fe16Si7C. Their crystal structures correspond to ordered variants of the Th6Mn23 type. U6Fe16Si7 adopts the Mg6Cu16Si7 structure type, whereas U6Fe16Si7C crystallizes with a novel “filled” quaternary variant. The inserted carbon is located in octahedral cages formed by six U atoms, with U–U interatomic distances of 3.509(1) Å. Insertion of carbon in the structure of U6Fe16Si7 has a direct influence on the U–Fe and Fe–Fe interatomic distances. The electronic properties of both compounds were investigated by means of DC susceptibility, electrical resistivity and thermopower. U6Fe16Si7 is a Pauli paramagnet. Its electrical resistivity and thermopower point out that it cannot be classified as a simple metal. The magnetic susceptibility of U6Fe16Si7C is best described over the temperature range 100–300 K by using a modified Curie–Weiss law with an effective magnetic moment of 2.3(2) μB/U, a paramagnetic Weiss temperature, θp=57(2) K and a temperature-independent term χ0=0.057(1) emu/mol. Both the electrical resistivity and thermopower reveal metallic behavior.
The new compounds U6Fe16Si7 and U6Fe16Si7C crystallize with the ternary ordered variant of the Th6Mn23 type, commonly referred as Mg6Cu16Si7 and with a novel “filled” variant of this type of structure, respectively.
Keywords: Actinide; Uranium; Iron; Heavy Fermion; Magnetic susceptibility; Electrical resistivity; Thermopower