Disorder in solid CBr₂Cl₂ and CBrCl₃

The low-temperature structures of CBr₂Cl₂ and CBrCl₃ have been determined using powder neutron diffraction. The two phases are isostructural with one another, CBr₄ and CCl₄, belonging to the C 2/c space group with 32 molecules per unit cell. A preliminary discussion of the high-T phases of these molecules is presented. The high-T phases show patterns consistent with an orientationally disordered (plastic) crystalline phase, consisting of rotating pseudospheres on an FCC lattice. A second disordered phase, between the other two phases, has also been observed in CBrCl₃. It is still under investigation.