The Structures of C–Ce2O3+delta, Ce7O12, and Ce11O20

The crystal structures of CeO1.68 (C–Ce2O3+), Ce7O12, and Ce11O20 with ordered oxygen vacancy distributions have been determined by refinement of single crystal neutron diffraction data collected at ambient temperature. CeO1.68 is cubic, of space group Ia with a=11.111(2) ?; one of the two oxygen positions is partially occupied with a probability of 37(3)%. For Ce7O12 the rhombohedral space group R with a=6.785(1) ?, =99.42(1)° was confirmed. Ce11O20 is isomorphous to Tb11O20 with the triclinic space group P . The lattice parameters are a=6.757(3) ?, b=10.260(5) ?, c=6.732(3) ?, =90.04(4)°, =99.80(4)°, and gamma=96.22(4)°. For Ce11O20 there are no [1, 1, 1]F oxygen vacancy pairs, i.e., pairs corresponding to half of the space diagonal of the fluorite lattice along [1, 1, 1]F. The shortest vacancy pair type in that structure is [1, 1, 1]F, which is also present in the two other phases. High temperature neutron diffraction measurements showed that Ce7O12 is stable up to 1072 K. Then it transforms into a fluorite type structure with disordered oxygen vacancies.