Crystal structures and conductivity properties of MBi6V2O15 family type compounds (M = Pb, Sr, Ca, Cd, Na0.5Bi0.5)

A family of compounds was prepared as powder samples with nominal formulas MBi6V2O15 (M = Pb, Sr, Ca, Cd, Na0.5Bi0.5), as well as single crystals with closely related compositions. Single crystal structure determinations were realized using X-ray diffraction. Each sample crystallizes in a monoclinic cell related to the δ-Bi2O3 fluorite-type cell, through the relationship: am→=3/2aF→+3/2bF→+3cF→,bm→=3/2aF→−3/2bF→,cm→=2aF→+2bF→−4cF→. With M = Na0.5Bi0.5, the non-centrosymetric C2 space group was selected, while for M = Pb–Cd, the structures were determined in the centrosymetric space group C2/c. Whatever the M element, a non-stoichiometric phase can be obtained. For M = Sr, Ca and Cd, mixed sites were observed; from their refined M/Bi occupancy factors, a determination method of Pb/Bi ratios has been proposed. These phases adopt a layered structure related to the δ-Bi2O3 fluorite structure, built from stacking slab units of cationic triple layers [(Bi, V)/(Bi, Bi/M)/(Bi, V)] where the vanadium atoms are located in the two external layers. Oxygen atoms are trapped either in cationic tetrahedrons (OBi4, O(Bi/M)4) for those within the slabs, or in VO4 vanadates for those in the inter-slab spaces, thus limiting the materials oxide conductivity.

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