The structures of C-Ce2O3+delta, Ce7O12, and Ce11O20

The crystal structures of CeO1.68 (C-Ce2O3+delta), 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 (3) over bar with a = 11.111(2) Angstro m; one of the two oxygen positions is partially occupied with a probability of 37(3)%. For Ce7O12 the rhombohedral space group R (3) over bar with a 6 .785(1) Angstrom, alpha = 99.42(1)degrees was confirmed. Ce11O20 is isomorp hous to Tb11O20 with the triclinic space group P (1) over bar. The lattice parameters are a 6.757(3) Angstrom, b = 10.260(5) Angstrom, c = 6.732(3) An gstrom, alpha = 90.04(4)degrees, beta = 99.80(4)degrees, and gamma = 96.22( 4)degrees. For Ce11O20 there are no 1/2[1,1,1](F) oxygen vacancy pairs, i.e ., pairs corresponding to half of the space diagonal of the fluorite lattic e along [1, 1, 1](F). The shortest vacancy pair type in that structure is 1 /2[1, 1, 1](F), which is also present in the two other phases. High tempera ture neutron diffraction measurements showed that Ce7O12 is stable up to 10 72 K. Then it transforms into a fluorite type structure with disordered oxy gen vacancies. (C) 1999 Academic Press.