The rare-earth metal(III) ortho-oxomolybdates(VI) of the empirical formula Ln_0.667[MoO₄] (Ln = Ce, Pr, Nd, and Sm) in the defect scheelite-type structure crystallize tetragonally with the space group I4₁/a (a = 533−525, c = 1183−1158 pm) and four formula units per unit cell. In their crystal structure one crystallographically unique lanthanoid trication is present at Wyckoff position 4b (see Table 1 site symmetry: 4̄..), showing a coordination sphere of eight oxygen atoms in the shape of a trigonal dodecahedron (Figure 1, left top). The distances between the Ln³⁺ cations and their surrounding O²⁻ anions range between 255 pm for the cerium compound, the largest of the four lanthanoid representatives, and 249 pm in the samarium derivative, the smallest one in this case (see Table 2). These distances are in good agreement with those of other rare-earth metal compounds with complex oxoanions such as the monazite-type phosphates with the formula Ln[PO₄] (Ce[PO₄]: d(Ce³⁺−O²⁻) = 245–265 pm; Sm[PO₄]: d(Sm³⁺−O²⁻) = 239–259 pm) [4]. To maintain electroneutrality, the atom site of the Ln³⁺ cations cannot be fully occupied, but by about two thirds, which is the case for all four title compounds (see Table 1). The molybdenum(VI) cations are also crystallographically unique and situated at the Wyckoff position 4b (see Table 1, site symmetry: 4̄..). They are surrounded by four oxygen atoms forming bisphenoidally distorted tetrahedra whose molybdenum–oxygen bond lengths, as well as their O–Mo–O angles, do not vary much throughout the presented series (d(Mo⁶⁺−O²⁻) = 175–177 pm, (∢)(O²⁻−Mo⁶⁺−O²⁻) = 107–108°, 4×, and 114–116°, 2×, see Table 2). For comparison, the Mo–O bond lengths and angles found in powellite (scheelite-type Ca[MoO₄]) lie at 176 pm, 107° (4×), and 115° (2×) [18] and thus agree very well with the herein presented data for the title compounds. The isolated [MoO₄]²⁻ tetrahedra (see Figure 1, left bottom) are exclusively vertex-connected to the polyhedra around the Ln³⁺ cations, therefore the crystallographically unique O²⁻ anions are surrounded by one molydenum and two lanthanoid cations. The crystal structure of the Ln_0.667[MoO₄] series (Ln = Ce, Pr, Nd, and Sm) is shown in Figure 1, on the right.

A simple structure type for compounds containing complex building blocks can usually be determined when the complex unit is shrunk to its center of gravity. In the case of the scheelite-type structure, the result can be considered as an AB structure with a coordination number ratio of 8:8. The first structure type that comes to mind with these “real” coordination numbers would be cesium chloride (CsCl) [20], but no further similarity can be found between these two structures. Interconnecting the Ln³⁺ and the Mo⁶⁺ cations with themselves, they show a tetrahedral coordination environment towards each other and; thus, the structure ends up in two interpenetrating diamond-like lattices [21] (Figure 2, middle), which is the description of the NaTl-type structure [17] (Figure 2, left). The same is also true considering zircon-type structures [2] if stripped off the ligand O²⁻ anions (Figure 2, right).

In the crystal structure of sodium thallide each Tl⁻ anion is surrounded by ten Na⁺ cations and four Tl⁻ anions resulting in an overall coordination number of 14. If only the sodium cations are considered, the thallium atoms are enclosed by adamantane-like cage with four shorter (323 pm, Na in Figure 3, left) and six longer distances (373 pm, Na′ in Figure 3, left). While the longer-bonded (Na′)⁺ cations form an octahedron around the central Tl⁻ anion, the shorter connected (Na)⁺ cations arrange themselves tetrahedrally, building up a heterocuban cage together with the four next thallium neighbors, which show the same distance to the center as the four closest Na⁺ cations. Vice versa, the same is of course true, if the environment of Na⁺ is described. This symmetrically ideal setup (NaTl: cubic, Fd3̄, a = 748.8(3) pm [17], Na at Wyckoff position 8a, Tl in 8b, both with site symmetry 4̄3m) is tetragonally distorted in the scheelite-type structure (site symmetry: 4̄.. for both Ln³⁺ and Mo⁶⁺). Therefore eight short (≈ 375 pm, 4× and ≈ 400 pm, 4×) and two long distances (≈ 590 pm, 2×) between Mo⁶⁺ and Ln³⁺ are determined in the alike adamantane cage of Ln³⁺ around the Mo⁶⁺ (and thus around the [MoO₄]²⁻ anions) with the four closest neighbors (Ln in Figure 3, middle) forming a square plane around the central Mo⁶⁺ cation and the four slightly more distant ones (Ln′ in Figure 3, middle) arrange tetrahedrally like the (Na)⁺ cations in NaTl. Here also the description of the Ln³⁺ environment around the oxomolybdate unit can be interchanged to the alternative situation. In the case of the the zircon-type structure (Zr[SiO₄]: tetragonal, I4₁/amd, a = 660.7(1), c = 598.2(1) pm [22], Zr at the Wyckoff position 4a, Si at 4b, both with site symmetry 4̄m2) a further distortion of the aforementioned arrangement is detectable. Again, four out of the ten Zr⁴⁺ cations of the adamantane cage build a tetrahedron around the central Si⁴⁺ cation (and thus around the [SiO₄]⁴⁻ anion). However, in this structure type once again these four are not the nearest surrounding atoms (363 pm, Zr′ in Figure 3, right), but two of the remaining six show a very short Zr⁴⁺⋯Si⁴⁺ distance of 299 pm (Zr in Figure 3, right). The other four are about 170 pm further away (467 pm, Zr″ in Figure 3, right). This distortion is easily explained by the interconnection of the tetrahedral complex oxoanion with the anionic polyhedra around the Zr⁴⁺ cations. While in the scheelite structure these are exclusively vertex-connected (at all eight O²⁻ anions), in the zircon structure two edges and four vertices are the joining links, and the aforementioned short zirconium–silicon distances of 299 pm are those running through the connecting edges. Here again the partial structures around Zr⁴⁺ and [SiO₄]⁴⁻ can be interchanged.

In general, the adamantane cage can be dismembered resulting in an octahedron with an interpenetrating tetrahedron. While the tetrahedron contributes to the linking in all three cases (Figure 3: Na in NaTl, Ln′ in scheelite-type Ln_0.667[MoO₄], and Zr′ in zircon-type Zr[SiO₄]), in the case of the octahedron, only for NaTl do all six members show the same distance to the central Tl⁻ anion. For scheelite-type compounds the octahedron is stretched, leaving two very far (Ln″) and four short (Ln) contacts behind, and in zircon-type compounds it is compressed, comprising two very short (Zr) and four long (Zr″) distances to the central unit. In all cases the structures can also be described vice versa.

All four representatives of the short Ln_0.667[MoO₄] series (Ln = Ce, Pr, Nd, and Sm) were only obtained as by-products so far. The direct synthesis using Ln₂O₃ and MoO₃ in 1:3 molar ratios experiences a direct competition with the rare-earth metal “sesquimolybdates” Ln₂[MoO₄]₃ (better: Ln₂Mo₃O₁₂ since not all of the these structures contain isolated [MoO₄]²⁻ units), which are known in literature for some of the rare-earth elements, comprising Ln = Ce [23], Nd [16], and Sm [24], although in different structure types, depending on the size of the lanthanoid. In the case of Ce_0.667[MoO₄], the single crystals emerged from an unsuccessful attempt to synthesize Ce[MoO₄]₂. Pr_0.667[MoO₄] and Sm_0.667[MoO₄] were obtained in experiments planned to prepare the respective fluoride oxodimolybdates PrFMo₂O₇ and SmFMo₂O₇ [25], and the neodymium representative Nd_0.667[MoO₄] occurred as by-product in the synthesis of NdBr[MoO₄] [26]. The single crystals of all four title compounds are coarse, transparent and remain stable when exposed to air and water. They show the color of the respective Ln³⁺ cation, i. e. green in the case of Pr, violet for Nd, and pale yellow for Sm. The crystals of the cerium derivative display an orange color, which is not quite uncommon, since very often in cerium(III) compounds the transition energy is lowered and the compound exhibits a color in the range between yellow and red, depending on the actual chemical surrounding of the Ce³⁺ cations. This can be assigned to the effect that the orbital, which contains the single f-electron lies within the band gap between the valence and the conduction band [27,28].

Intensity data sets for single crystals of all four Ln_0.667[MoO₄] representatives (Ln = Ce, Pr, Nd, and Sm) were collected on a Nonius Kappa-CCD diffractometer using graphite-monochromatized Mo-Kα radiation (wavelength: λ = 71.07 pm). A numerical absorption correction was performed with the help of the program HABITUS [29]. The structure solutions and refinements were carried out by using the program package SHELX-97 [30]. Details of the data collections and the structure refinements [31] are summarized in Table 3, atomic positions and coefficients of the equivalent isotropic displacement parameters [19] can be found in Table 1, while interatomic distances and selected bond angles are listed in Table 2. Further details of the crystal structure investigations can be obtained from the Fachinformationszentrum (FIZ) Karlsruhe, D-76344 Eggenstein-Leopoldshafen, Germany (Fax: +497247-808-666; E-Mail: crysdata@fiz-karlsruhe.de), on quoting the depository numbers CSD-423509 for Ce_0.667[MoO₄], CSD-423510 for Pr_0.667[MoO₄], CSD-423511 for Nd_0.667[MoO₄], and CSD-423512 for Sm_0.667[MoO₄].