Structural environments and oxidation states of metal cations in bismuth cerium molybdate solid solutions by x-ray absorption spectroscopy

Abstract

Bismuth and cerium L/sub 3/-edge and molybdenum K-edge X-ray absorption investigations of silica-supported bismuth cerium molybdates were undertaken to probe the local structural environments and oxidation states of the metal cations. A total of 10 single-phase compositions in the bimodal Bi/sub 2-x/Ce/sub x/(MoO/sub 4/)/sub 3/ solid solution (for 0 less than or equal to x less than or equal to 0.25 and 1 less than or equal to x less than or equal to 1.5) were examined; namely, x = 0, 0.025, 0.075, 0.125, 0.2, 0.25, 1, 1.125, 1.25, and 1.5. For low cerium concentrations (x less than or equal to 0.25), the Bi/sub 2-x/Ce/sub x/(MeO/sub 4/)/sub 3/ materials possess severely distorted defect-scheelite structure with cerium cations dissolved in the Bi/sub 2/(MoO/sub 4/)/sub 3/ (x = 0) lattice, whereas for high cerium concentrations (x greater than or equal to 1), they possess undistorted defect-scheelite structures with bismuth cations dissolved in the Ce/sub 2/(MoO/sub 4/)/sub 3/ (x = 2) lattice. The EXAFS results provide the first quantitative structural information about the individual Bi/sup 3 +/ and Ce/sup 3 +/ sites in the Bi/sub 2-x/Ce/sub x/(MoO/sub 4/)/sub 3/ compositions for 0.025 less than or equal to x less than or equal tomore » 1.5; only site-averaged metrical parameters for an admixture of bismuth and cerium cation positions are available from X-ray and neutron diffraction measurements.« less