Synthesis, crystal structure and TEM study of the new hollandite-type Ba∼8/7Mo8O16

Investigation of the Cs-Ba-Mo-O system by fused salt electrolysis at 960 degreesC has led to the new related-hollandite compound Ba?????. Single-crystals of Bax8/7Mo8O16 were examined by election diffraction and high-resolution electron microscopy. Bax8/7Mo8O16 was found to present a tetragonal I basic unit-cell (a=10.21 Angstrom, c=2.89 Angstrom) 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) Angstrom, c=20.255(5) Angstrom, R-w(F-2)=0.0844 for all 7767 independent triangles and planar rhomboidal MO4 clusters that coexist in equal proportions. The Mo-Mo distances vary from 2.5341(8) to 2.696(2) Angstrom (3) and from 2.5341(8) to 2.894(4) Angstrom in the Mo-3 and Mo-4 clusters, respectively. The shortest intercluster distance is 3.047(4) Angstrom. The Mo-O distances range between 1.81(1) to 2.418(8) Angstrom 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-square -Ba-Ba-square Ba-square .... (C) 2001 Elsevier Science B.V. All rights reserved.