A study of oxygen self-diffusion in tile c-direction of rutile using a a nuclear technique : D.J. Derry, J.M. Calvert and D.G. Lees, Mettalurgy Department, the University, Manchester, M13 9PL, U.K. and Physics Department, The University, Manchester, M13 9PL, U.K.

Abstract

Investigation of the Cs–Ba–Mo–O system by fused salt electrolysis at 960°C has led to the new related-hollandite compound Ba∼8/7Mo8O16. Single-crystals of Ba∼8/7Mo8O16 were examined by electron diffraction and high-resolution electron microscopy. Ba∼8/7Mo8O16 was found to present a tetragonal I basic unit-cell (a=10.21 Å, c=2.89 Å) with a one-dimensional modulation of wavevector q close to 4/7 along c* at room temperature. Refinement of the crystal structure was made on X-ray single crystal data in the seven-fold supercell in the three-dimensional space group I4 (a=10.214(2) Å, c=20.255(5) Å, Rw(F2)=0.0844 for all 7767 independent reflections and 149 parameters). In the latter supercell approach, the Mo atoms in the double edge-sharing rutile-type chains form Mo3 triangles and planar rhomboidal Mo4 clusters that coexist in equal proportions. The Mo–Mo distances vary from 2.5341(8) to 2.696(2) Å and from 2.5341(8) to 2.894(4) Å in the Mo3 and Mo4 clusters, respectively. The shortest intercluster distance is 3.047(4) Å. The Mo–O distances range between 1.81(1) to 2.418(8) Å as usually observed in reduced molybdenum oxides. The Ba2+ cations occupy the large square channels in square prismatic environment of oxygen atoms with the distribution sequence …Ba–□–Ba–Ba–□Ba–□–….