The ternary system: Silicon–tantalum–uranium

Phase equilibria in the ternary system Si-Ta-U have been established in an isothermal section at 1000 °C by optical microscopy, electron probe microanalysis and X-ray diffraction. Two novel ternary compounds were observed and were characterised by X-ray powder Rietveld refinement: stoichiometric ?1-U2Ta3Si4 (U 2Mo3Si4-type, P21/c; a = 0.70011(1), b = 0.70046(1), c = 0.68584(1) nm, ß = 109.38(1); RF = 0.073, X-ray powder Rietveld refinement) and ?2-U2-xTa3+xSi4 at x ? 0.30 (Sc 2Re3Si4-type = partially ordered Zr 5Si4-type, P41212; a = b = 0.69717(3)(1), c = 1.28709(4) nm; RF = 0.056; X-ray single crystal data). Mutual solubility of U-silicides and Ta-silicides are found to be very small i.e. below about 1 at.%. Due to the equilibrium tie-line Ta 2Si-U(Ta), no compatibility exists between the U-rich silicides U3Si or U3Si2 and tantalum metal. Single crystals obtained from alloys slowly cooled from liquid (2000 °C), yielded a fully ordered compound U2Ta2Si3C (unique structure type; Pmna, a = 0.68860(1); b = 2.17837(4); c = 0.69707(1) nm; R F2 = 0.048).