Carbon species in the crystal structures of uranium-transition-element carbides,UMC₂

The carbon species present in the ternary uranium-transition-metal carbide phasesUMC₂ (M =Fe, Cr, W, Mo), prepared by arc melting, have been investigated by mild hydrolysis, thermal analysis, and X-ray and (forM =W, Mo) neutron powder diffraction. ForM =W and Mo, the Rietveld neutron-diffraction profile analyses confirm thatUMC₂ (M =W, Mo) are isostructural at room temperature with orthorhombic (Pnma) unit cell dimensionsa = 5.612(2), b = 3.241(1), c = 10.956(3)A °for UMoC₂ anda = 5.614(2), b = 3.245(1), c = 10.964(4)A °for UWC₂. Accurate determinations of carbon positions establish that carbon-carbon interatomic distances correspond to the presence of C₁ rather than C₂ groups in both crystal structures. ForM =Fe and Cr, the hydrolysis, thermal, and X-ray results also show the presence of isolated C₁ carbon units.