(MBi)46V8Oy-family type (M = Pb, Sr, Ca, Cd, Na0.5Bi0.5): Syntheses, crystal structures and conductivity properties

Phases, with nominal formulas (MBi)46V8Oy (M = Pb, Sr, Ca, Cd, Na0.5Bi0.5) were prepared as powder samples as well as single crystals of closely related compositions. Single-crystal structural determinations were realized using X-ray diffraction on a Bruker 4KCCD Apex diffractometer. In this series, the cell is related to the fluorite δ-Bi2O3: it is orthorhombic, with a→0=3/2a→F−3/2b→F, b→0=3/2a→F+3/2b→F, c→0=3c→F when nominal ratio is either M10/Bi36 (M = Sr–Na0.5Bi0.5) or Pb3/Bi43. Two types of monoclinic structures can be obtained from Pb6/Bi40 nominal ratio, either with a→m=3/2a→F+3/2b→F+3c→F, b→m=−3/2a→F+3/2b→F, c→m=−3a→F−3b→F (1) or a→m=3/2a→F+3/2b→F−3c→F, b→m=−3/2a→F+3/2b→F, c→m=−5/2a→F+5/2a→F+4c→F (2). For nominal Sr–Na0.5Bi0.5 compositions, the structural refinements led to M10−δBi36+δV8Oy non-stoichiometric materials. In all structures, a cationic “slab” model was identified by the stacking of [(Bi,M, V)/Bi,M/(Bi,M, V)] layers. In these slabs, vanadium atoms are located in the external layers. The systematic stacking of 2 slabs (i.e. 6 cationic layers) allows the description of all the structures, except for Pb6/Bi40 nominal composition where a stacking of either 2 or 3 successive slabs is needed. In this description, the interslab distances are systematically larger than interlayer distances within the slabs. Along with the oxygen atoms, the cations determine a long range network of OBi4 and O(Bi,M)4 anti-tetrahedrons distributed over the whole structure and VO4 located at the slab–interslab interfaces. The conductivity properties of materials with M10Bi36V8O84 nominal compositions are presented. They likely result from the oxide ion mobility (of the OBi4 and O(Bi,M)4 entities) within the interslab spaces.

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