Cationic Order-Disorder in Double Scheelite Type Oxides: the Case Study of Fergusonite La₂SiMoO₈

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. J. Inorg. Nucl. Chem. 1968, 30, 2091]. Using two different synthesis routes, both ordered and disordered forms of fergusonite La₂SiMoO₈ 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 [SiO₄] and [MoO₄] tetrahedra. The crystal chemistry of ordered double sheelite-type La^III₂(Si^IVO₄)(Mo^VIO₄) can be seen as an intermediate between those of simple regular scheelite or fergusonite Ln^III(Nb^VO₄) and of ordered triple scheelite Bi^III₃(Fe^IIIO₄)(Mo^VIO₄)₂. The structure of the disordered La₂SiMoO₈ 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 SiO₄/MoO₄ ordering in seemingly disordered compounds might explain the previous discrepancy between optical and X-ray diffraction measurements.