Preparation and structural study from neutron diffraction data of Pr 5Mo 3O 16

Abstract

The title compound has been prepared as polycrystalline powder by thermal treatments of mixtures of Pr 6O 11 and MoO 2 in air. In the literature, an oxide with a composition Pr 2MoO 6 has been formerly described to present interesting catalytic properties, but its true stoichiometry and crystal structure are reported here for the first time. It is cubic, isostructural with CdTm 4Mo 3O 16 (space group Pn-3 n, Z=8), with a=11.0897(1) Å. The structure contains MoO 4 tetrahedral units, with Mo–O distances of 1.788(2) Å, fully long-range ordered with PrO 8 polyhedra; in fact it can be considered as a superstructure of fluorite (M 8O 16), containing 32 MO 2 fluorite formulae per unit cell, with a lattice parameter related to that of cubic fluorite ( a f=5.5 Å) as a≈2 a f. A bond valence study indicates that Mo exhibits a mixed oxidation state between 5+ and 6+ (perhaps accounting for the excellent catalytic properties). One kind of Pr atoms is trivalent whereas the second presents a mixed Pr 3+–Pr 4+ oxidation state. The similarity of the XRD pattern with that published for Ce 2MoO 6 suggests that this compound also belongs to the same structural type, with an actual stoichiometry Ce 5Mo 3O 16.