Figure 2: Simulated diffraction patterns for X-rays (a, d, g) and neutrons (b, e, h) of the three structures for the cubic high-temperature UC2 phase proposed by Bowman et al. (a, b, c) , Bredig (d, e, f) , and Wilson (g, h, i) . While the three proposed structures are substantially different, the X-ray diffraction patterns look very similar due to the bias of the diffraction signal to the uranium sublattice, virtually ignoring the carbon atoms. The neutron diffraction patterns, however, show distinct differences, allowing to unambiguously determine which of the proposed structures is present. The structure proposed by Bowman et al.  from neutron diffraction data suggests C-C dumbbells freely rotating, which is symbolized here by a cluster of carbon atoms. The structure by Bredig, determined from X-ray diffraction data, also suggests C-C dumbbells, but in a static position, resulting in a pyrite structure type. The structure proposed by Wilson, also based on X-ray diffraction data, is similar to the fluorite structure observed for UO2. The diffraction data for X-rays is simulated for radiation ( = 1.5405 Å), while the neutron diffraction data are simulated for the 90° bank of the HIPPO neutron time-of-flight diffractometer (see Figure 4) at LANSCE [77, 78]. Diffraction pattern simulations were performed using GSAS  and gsaslanguage . Structure plots were produced with VESTA .