The niobium-silicon-uranium system

Phase relations in the ternary system Nb-Si-U were established for the isot hermal sections at 1000 degrees C and 850 degrees C. Experimental technique s used were optical microscopy, X-ray microanalyser (XMA) and X-ray powder analysis of are melted samples which were annealed at 1000 degrees C for 14 4 h and/or at 850 degrees C for 300 h. Phase equilibria are characterised b y the formation of two ternary compounds. Rietveld X-ray and neutron powder data refinements confirmed the stoichiometric behaviour for U2Nb3Si4, and established isotypism with the ordered Zr5Si4-type (Sc2Re3Si4-type). The se cond phase, U2-xNb3+xSi4,x similar or equal to 0.25, crystallises in the pa rtially ordered Sm5Ge4-type (Ce2Sc3Si4-type). At 1000 degrees C both compou nds form a narrow two-phase equilibrium. Due to the established two-phase e quilibria: gamma-(U, Nb) + U2Nb3Si4 and Nb5Si3 + gamma-(U,Nb) there is no c ompatibility between U3Si2 and niobium metal at 1000 degrees C and at 850 d egrees C, respectively. U2Nb3Si4 is weakly ferromagnetic below T-c similar or equal to 35 K. The remanence of only 0.0088 mu(B)/U atom probably result s from a non-linear antiferromagnetic structure, too weak to be discovered by neutron powder diffraction. (C) 2000 Elsevier Science B.V. All rights re served.