Phase transitions in tricyclopentadienyl-chloro-uranium(IV), U(C5H5)3Cl

The molecule crystallises in a monoclinic unit cell. On examining a single crystal of U(C5H5)3Cl, two phase transitions were found. This transition is characterised by the progressive appearance of peaks (O, k, O) with k odd from 230 K to 150 K. The appearance of those peaks transgresses one of the room temperature space-group extinction rules. The other transition (78 K) is of first order and produces twins. The peaks (O, k, O) with k odd do not exist below 78K. The structure between 230 K and 78 K is still monoclinic. X-ray powder experiments have also been performed in order to follow the evolution of the cell parameters as a function of the temperature from 300 K to 30K. The C5H5 dynamics is studied by neutron quasielastic experiments