Defect structure and transport properties of Cr 2 (MoO 4 ) 3 and Al 2 (MoO 4 ) 3

Cr2(MoO4)3 and Al(MoO4)3 were prepared by complexation and characterized by different techniques. The solids are monophasic in nature, with no significant differences in the unit cell parameters reported in the literature for monoclinic Cr2(MoO4)3 and Al2(MoO4)3. IR, Raman and UV-Visible diffuse reflectance spectra indicated the presence of Mo atoms octahedrally coordinated to oxygen. DSC results showed that the kinetics of the monoclinic ⇔ orthorhombic transformation is complex in these materials. For both molybdates the electrical conductivity was always found to be independent on PO2, and probably reflects an intrinsic electronic mechanism. The defect structure and the transport properties of these materials are discussed and compared with those of isostructural Fe2(MoO4)3. It is proposed that Mo atoms octahedrally coordinated to oxygen are localized in the interstitial sites of the M2(MoO4)3 structure and that the transport properties of Al- and Cr- molybdate differ from those of Fe-molybdate, in sympathy with the catalytic behavior in the oxidation of propylene, because of the different reducibility of Cr3+, Al3+ and Fe3+.