(MBi) 46V 8O y-family type (M = Pb, Sr, Ca, Cd, Na 0.5Bi 0.5): Syntheses, crystal structures and conductivity properties

Abstract

Phases, with nominal formulas ( MBi) 46V 8O y ( M = Pb, Sr, Ca, Cd, Na 0.5Bi 0.5) 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-Bi 2O 3: it is orthorhombic, with a → 0 = 3 / 2 a → F − 3 / 2 b → F , b → 0 = 3 / 2 a → F + 3 / 2 b → F , c → 0 = 3 c → F when nominal ratio is either M 10/Bi 36 ( M = Sr–Na 0.5Bi 0.5) or Pb 3/Bi 43. Two types of monoclinic structures can be obtained from Pb 6/Bi 40 nominal ratio, either with a → m = 3 / 2 a → F + 3 / 2 b → F + 3 c → F , b → m = − 3 / 2 a → F + 3 / 2 b → F , c → m = − 3 a → F − 3 b → F (1) or a → m = 3 / 2 a → F + 3 / 2 b → F − 3 c → F , b → m = − 3 / 2 a → F + 3 / 2 b → F , c → m = − 5 / 2 a → F + 5 / 2 a → F + 4 c → F (2). For nominal Sr–Na 0.5Bi 0.5 compositions, the structural refinements led to M 10− δ Bi 36+ δ V 8O y 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 Pb 6/Bi 40 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 OBi 4 and O(Bi, M) 4 anti-tetrahedrons distributed over the whole structure and VO 4 located at the slab–interslab interfaces. The conductivity properties of materials with M 10Bi 36V 8O 84 nominal compositions are presented. They likely result from the oxide ion mobility (of the OBi 4 and O(Bi, M) 4 entities) within the interslab spaces.