Crystal structure and non-stoichiometry of cerium brannerite: Ce0.975Ti2O5.95

The crystal structure of cerium brannerite, Ce0.975Ti2O5.95, was determined from Rietveld analysis of high resolution neutron powder diffraction data (space group C2/m, a=9.8320(1) Å, b=3.75287(6) Å, c=6.8852(1) Å, β=119.230(1)°). The presence of both cerium and oxygen vacancies was confirmed from Rietveld analysis and defect energy calculations, combined with careful investigation of the phase diagram. Formation of oxygen vacancies at the O1 site, charge compensated by Ce vacancies, relieves considerable Coulombic repulsion and hence structural strain associated with short O1–O1 contacts forming the shared edge of neighbouring TiO6 polyhedra. Consideration of the bond valence sum of Ce4+ in Ce0.975Ti2O5.95 afforded a revised value of the r0=2.098±0.039 Å for the reference bond length, based on analysis of 21 crystal structures mined from the ICSD.