Cationic Order-Disorder in Double Scheelite Type Oxides: the Case Study of Fergusonite LaSiMoO.

Up to now, the possible occurrence of a cationic ordering on the tetrahedral sublattices of stoichiometric double scheelite-type oxides was not settled, with somewhat contradictory X-ray diffraction and optical measurements [Blasse, G. . , , 2091]. Using two different synthesis routes, both ordered and disordered forms of fergusonite LaSiMoO were prepared. The crystal structure of the ordered form was determined using powder X-ray and neutron diffraction, which clearly evidence a tridimensional ordering between [SiO] and [MoO] tetrahedra. The crystal chemistry of ordered double sheelite-type La(SiO)(MoO) can be seen as an intermediate between those of simple regular scheelite or fergusonite Ln(NbO) and of ordered triple scheelite Bi(FeO)(MoO). The structure of the disordered LaSiMoO phase was analyzed using powder X-ray diffraction. A few small and larger diffraction peaks or bumps are observed in addition to the sharper peaks of a simple fergusonite cell. DIFFaX and FAULTS programs helped showing that these faint peaks originate from stacking faults between 2D ordered layers. The intermediate 2D-3D nature of SiO/MoO ordering in seemingly disordered compounds might explain the previous discrepancy between optical and X-ray diffraction measurements.