Defect control in oxidation catalysts

Abstract

The crystal structure of a complex molybdenum oxide Ag1/8Pr5/8MoO4 is reported. The Ag1/8Pr5/8MoO4 sub-structure can be described on the base of the scheelite (CaWO4) structure. Transmission electron microscopy reveals that the real structure is better described in a (3+1)D formalism. According to electron diffraction study the new scheelite-type complex oxide Ag1/8Pr5/8MoO4 crystallizes in the B2/b(αβ0)00 (3+1)D superspace group with unit cell parameters a≈7.50Å, b≈5.30Å, c≈11.74Å and γ≈135° (Z=4) and modulation vector q=0.56a*+0.59b*. The structure of Ag1/8Pr5/8MoO4 is refined from X-ray powder data in the scheelite setting I2/b(αβ0) with a=b=5.3013(4)Å, c=11.7407(11)Å, q=1.14690(14)a*+0.58921(12)b* with fixed γ=90° angle (Rp=0.033, R=0.033, Rm=0.029, R1=0.047, S=1.36). Displacement modulations apply for all atoms. The occupancy modulation shows that one-fourth of the Ag/Pr atoms are absent. The structure can be considered as a crystallographic shear structure with incommensurate ordering of vacancies and displacement modulations for all atoms. The arrangement of Ag/Pr atoms and vacancies is at the origin of the incommensurate modulation in the cation-deficient Ag1/8Pr5/8MoO4 phase.