Preparation and structural study from neutron diffraction data of Pr5Mo3O16

The title compound has been prepared as polycrystalline powder by thermal treatments of mixtures of Pr6O11 and MoO2 in air. In the literature, an oxide with a composition Pr2MoO6 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 CdTm4Mo3O16 (space group Pn-3n, Z=8), with a=11.0897(1) Å. The structure contains MoO4 tetrahedral units, with Mo–O distances of 1.788(2) Å, fully long-range ordered with PrO8 polyhedra; in fact it can be considered as a superstructure of fluorite (M8O16), containing 32 MO2 fluorite formulae per unit cell, with a lattice parameter related to that of cubic fluorite (af=5.5 Å) as a≈2af. 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 Pr3+–Pr4+ oxidation state. The similarity of the XRD pattern with that published for Ce2MoO6 suggests that this compound also belongs to the same structural type, with an actual stoichiometry Ce5Mo3O16.

Graphical AbstractFormerly formulated as Pr2MoO6, the title compound is a cubic superstructure of fluorite (a=11.0897(1) Å, space group Pn-3n) due to the long-range ordering of PrO8 scalenohedra and MoO4 tetrahedral units, showing noticeable shifts of the oxygen positions in order to provide a tetrahedral coordination for Mo ions. A mixed valence Mo5+–Mo6+ is identified, which could account for the excellent catalytic properties of this material.Figure optionsDownload as PowerPoint slide