CESIUM SUBSTITUTION IN THE TITANATE HOLLANDITES BAXCSY(TI-Y+2X(3+)TI-8-2X-Y(4+)O16 FROM 5 TO 400 K

Neutron powder diffraction data were collected for the titanate hollandites Cs1.36Ti8O16, Cs0.82Ba0.41Ti8O16 and Cs0.40Ba0.79Ti8O16 over the temperature range 5 to 400 K. Rietveld refinement was used to determine the tetragonal lattice parameters and the structural parameters of these compounds. The lattice parameters a and c increase with Cs concentration from a = 10.1688 (2) and c = 2.9595 (1) angstrom in Cs0.40Ba0.79Ti8O16 at 5 K to a = 10.2682 (2) and c = 2.9643 (1) angstrom in Cs1.36Ti8O16 at the same temperature. The expansion in these compounds is isotropic only in Cs1.36Ti8O16 with a linear-expansion coefficient of approximately 8 x 10(-6) K-1 at 300 K. The presence of Cs is evident by the compression of the centres of the oxygen octahedral walls separating adjacent tunnels and by the enlargement of the tunnel cross-section. It is evident that the mean <Ti-O> bond length in the oxygen octahedra is influenced not only by the relative Ti3+/Ti4+ concentration in the octahedra but also by the Cs in the tunnels. The oxygen box forming the cavity around each tunnel ion, Ba or Cs, is approximately 10% larger in Cs1.36Ti8O16 than in the pure Ba hollandite Ba1.07Ti8O16. All the tunnel ions are off-centred along the tunnel directions owing to the large size of Cs in relation to the intrinsically small tunnel cavities and the pairing of these ions in the tunnels. Positional disorder of all the ions is evident in the temperature factors, which possess a large temperature-independent component. The X-ray Debye temperature theta-M of each hollandite is in the range 420 to 460 K and the r.m.s. displacement of the tunnel ions along the tunnel axis arising from positional disorder is between 0.16 and 0.20 angstrom. In the context of hollandites being used as hosts for radioactive Cs in nuclear waste, an analysis is presented of the possibility of Cs or Ba migration along the tunnels.