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 La 2 SiMoO 8 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 4 ] and [MoO 4 ] tetrahedra. The crystal chemistry of ordered double sheelite-type La III 2 (Si IV O 4 )(Mo VI O 4 ) can be seen as an intermediate between those of simple regular scheelite or fergusonite Ln III (Nb V O 4 ) and of ordered triple scheelite Bi III 3 (Fe III O 4 )(Mo VI O 4 ) 2 . The structure of the disordered La 2 SiMoO 8 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 4 /MoO 4 ordering in seemingly disordered compounds might explain the previous discrepancy between optical and X-ray diffraction measurements.