The niobium–silicon–uranium system

PPhase relations in the ternary system Nb–Si–U were established for the isothermal sections at 1000°C and 850°C. Experimental techniques used were optical microscopy, X-ray microanalyser (XMA) and X-ray powder analysis of arc melted samples which were annealed at 1000°C for 144 h and/or at 850°C for 300 h. Phase equilibria are characterised by 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 second phase, U2−xNb3+xSi4,x ≃ 0.25, crystallises in the partially ordered Sm5Ge4-type (Ce2Sc3Si4-type). At 1000°C both compounds form a narrow two-phase equilibrium. Due to the established two-phase equilibria: γ-(U,Nb) + U2Nb3Si4 and Nb5Si3 + γ-(U,Nb) there is no compatibility between U3Si2 and niobium metal at 1000°C and at 850°C, respectively. U2Nb3Si4 is weakly ferromagnetic below Tc≃35 K. The remanence of only 0.0088 μB/U atom probably results from a non-linear antiferromagnetic structure, too weak to be discovered by neutron powder diffraction.