Bia(FeO4) (MOO4)2, prepared from aqueous solutions of the components, is microcrystalline and has I41/a scheelite-type structure (a = 5.32, c = 11"67 A) with a random distribution of the Fe and Mo atoms on the tetrahedral cation site. This phase is metastable and transforms on heating to an ordered scheelite superstructure: C2/c, a= 16"904, b= 11.653, c=5.254 ,~, fl= 107.15°; Z=4. The structure was solved and refined from single-crystal counter data: R = 0.061 for 1083 structure factors. Both crystallographically different Bi atoms show lone-pair distortions. The Fe and Mo atoms are tetrahedrally coordinated to O atoms. Strong Bi-O bonds result in unusually long Fe-O distances which correlate well with a large isomer shift of 0"282 mm s -1 relative to ~-Fe for tetrahedral Fe 3+ in the 57Fe M6ssbauer spectrum. The M6ssbauer data also indicate a trend towards higher coordination of the Fe atom in the disordered phase. Infrared spectra suggest considerable short-range disorder in that phase. Bi3(GaO4)(MoO4)z forms disordered and ordered scheelite phases isotypic with Bi3(FeO4) (M004)2. Scheelite-related phases are classified according to their group-subgroup relationships. Scheelite-related YNbO4 which was previously reported with a noncentrosymmetric space group appears to be centrosymmetric.

No abstract

No abstract

No abstract

No abstract