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The U-Nb-Al ternary system: experimental and simulated investigations of the phase equilibria and study of the crystal structure and electronic properties of the intermediate phases

Abstract : The phase relations in the ternary U-Nb-Al system were established for the whole concentration range for 900 K and 1200 K. They were derived from quenched samples annealed at 900 K for three months and at 1200 K for two months by using x-ray powder diffraction, scanning electron microscopy and energy dispersive spectroscopic analyses. The formation of the two ternary phases, UNb2Al20 and U6Nb4Al43 was confirmed. Both phases form by peritectic reaction, at 1200(5) K and at 1262(5) K for UNb2Al20 and U6Nb4Al43 respectively. Single-crystal structure refinements confirmed that UNb2Al20 adopts the CeCr2Al20 type (cubic, Fd (3) over barm) and that U6Nb4Al43 crystallizes with the Ho6Mo4Al43 type (hexagonal, P6(3)/mcm). The isothermal sections are characterized by (i) the extended homogeneity ranges due to mutual exchange between U and Nb and between Nb and Al in the bcc-phases (gamma U and Nb), at 1200 K, and (ii) a substantial ternary extension with limits of about 6(1) to 10(1) at.% U at 900 K and 1200 K respectively, of the sigma-phase (Nb2Al, tetragonal, P4(2)/mnm). The modeling of these isothermal sections was carried out by the Calphad method. The liquidus projection and the invariant reactions along with their corresponding temperatures were calculated using our optimized database. The electronic properties of both intermediate compounds were measured by means of dc-susceptibility, specific heat and resistivity measurements in the temperature range 2-300 K, revealing for UNb2Al20 enhanced Pauli paramagnetism and complex magnetic features for U6Nb4Al43 at low temperature. For the latter compound a composite behavior involving long-and short-range orderings, resulting from intra and inter U dimeric chains along the c-axis is suspected below 12 K.
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Submitted on : Wednesday, March 28, 2018 - 11:37:47 AM
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Chantal Moussa, Alexandre Berche, Mathieu Pasturel, José Barbosa, Bertrand Stepnik, et al.. The U-Nb-Al ternary system: experimental and simulated investigations of the phase equilibria and study of the crystal structure and electronic properties of the intermediate phases. Journal of Alloys and Compounds, Elsevier, 2017, 691, pp.893-905. ⟨10.1016/j.jallcom.2016.08.257⟩. ⟨hal-01438111⟩

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