302408.fig.003
Figure 3: Overview of the diffraction data obtained in a neutron diffraction experiment on the HIPPO diffractometer to experimentally verify which of the three proposed structures for the high temperature cubic phase of UC2 is experimentally observed. The three proposed structures are discussed in Figure 2. In this experiment, uranium carbide was synthesized in situ in a furnace from a mix of UO2 and graphite powders. The experiment in the overview plot proceeds from bottom to top with the sample temperature shown on the left. UO2 peaks are observed in the beginning, with the first transition to more intense diffraction peaks without changing the location of the peaks observed at 750°C. At this temperature the two low-temperature urania phases UO2 and become a single hyperstoichiometric phase. At 1500°C, the urania and graphite powders react to form . At 1890°C the transition to the cubic UC2 phase is observed. The substantial weakening of the diffraction signal is recovered upon cooling and results from the rotational disorder of the dumbbells discussed in Figure 2 (so-called sublattice melting). Detailed analysis of the data allowed to unambiguously confirm the structure determined by Bowman et al. [40] and predicted by Wen et al. [44]. See [207] for more details on these experiments.