Abstract
The rare-earth metal(III) ortho-oxomolybdates with the formula Ln0.667[MoO4] (Ln = Ce, Pr, Nd, and Sm) and defect scheelite-type structure crystallize in the tetragonal space group I41/a (a = 533–525, c = 1183–1158 pm) with four formula units per unit cell. The Ln3+ cations at Wyckoff position 4b exhibit a coordination sphere of eight oxygen atoms in the shape of a trigonal dodecahedron. The same site symmetry (..) is observed for the tetrahedral oxomolybdate(VI) entities [MoO4]2–, since their central Mo6+ cation is situated at the 4a position. Due to this equal site multiplicity, the lanthanoid(III) cations have to be statistically under-occupied to maintain electroneutrality, thus a defect scheelite structure emerges. The partial structure of both the Ln3+ cations and the [MoO4]2– anions (if shrunk to their centers of gravity) can be best described as distorted diamond-like arrangements. Therefore, these two interpenetrating partial structures exhibit a similar setup as found in the zircon-type as well as in the NaTl-type structure.
Highlights
The mineral scheelite (Ca[WO4]) is named after the German-Swedish pharmacist and chemist CarlWilhelm Scheele, who, besides other elements, discovered oxygen and tungsten, and was able to synthesize tungstic acid from this mineral in the first place
Besides compounds containing the larger lanthanide cations, which crystallize in the monazite-type (C.N.(Ln3+) = 9) [3,4,5,6,7,8], rare-earth metal phosphates, arsenates and vanadates prefer the xenotime- (Ln[PO4]) [4,7,8,9,10,11] and the wakefieldite-type Ln[VO4] [12,13], which are both equal to the zircon-type (C.N.(Ln3+) = 8); only a high-pressure modification of Sm[AsO4] is known to crystallize in the scheelite-type [14]
We focus on the close relationship between the scheelite-type (Ca[WO4], here: the title compounds Ln0.667[MoO4], Ln = Ce, Pr, Nd, and Sm) and the zircon-type structure (Zr[SiO4]), which can both be derived from the crystal structure of sodium thallide (NaTl) [17]
Summary
The mineral scheelite (Ca[WO4]) is named after the German-Swedish pharmacist and chemist Carl. For trivalent rare-earth metal compounds, tetrahedral entities with pentavalent central atoms as counteranions, such as phosphates, arsenates and vanadates, are widely known. Besides compounds containing the larger lanthanide cations, which crystallize in the monazite-type (C.N.(Ln3+) = 9) [3,4,5,6,7,8], rare-earth metal phosphates, arsenates and vanadates prefer the xenotime- (Ln[PO4]) [4,7,8,9,10,11] and the wakefieldite-type Ln[VO4] [12,13], which are both equal to the zircon-type (C.N.(Ln3+) = 8); only a high-pressure modification of Sm[AsO4] is known to crystallize in the scheelite-type [14].
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