Mixed occupancy: the crystal structure of scheelite-type LiLu[MoO4]2

The crystal structure of LiLu[MoO4]2, which emerged as a by-product of the synthesis to obtain lithium derivatives of lutetium molybdate, adopts the scheelite type.


Chemical context
The mineral powellite (CaMoO 4 ) is one of the main sources for molybdenum on this planet.Its tetragonal crystal structure can be described as isotypical with that of the mineral scheelite (CaWO 4 ) in space group type I4 1 /a with the c axis roughly twice as long as the respective a axis (Dickinson, 1920).The predomination of divalent cations, such as alkaline earth metals, can be changed by introducing a mixed occupancy of monovalent (i.e.alkali metals) and trivalent cations (i.e.rare-earth metals) at the respective Wyckoff position.Since the coordination number of eight around the alkaline earth metal cations in the scheelite structure usually requires larger cations, it is remarkable that the title compound also adopts the scheelite structure type although it comprises the smallest cations of both the alkali metals and the lanthanides.

Structural commentary
In the crystal structure of LiLu[MoO 4 ] 2 (Fig. 1) the Li + and Lu 3+ cations reside at Wyckoff position 4b (site symmetry 4) exhibiting a 1:1 mixed occupancy.The coordination environment around this position is built up by eight oxide anions [d Li/Lu-O = 4 � 2.369 (3) and 4 � 2.371 (3) A ˚] in the shape of a trigonal dodecahedron (Fig. 2).The Mo 6+ cations are situated in the centers of oxygen tetrahedra at Wyckoff position 4a (site symmetry 4) with distances of 4 � 1.774 (3) A ˚.The existence of LiLu[MoO 4 ] 2 was first mentioned by Cheng et al. (2015), with the crystal structure being refined by the Rietveld method on basis of X-ray data from microcrystalline powder.While their refinement of the lattice parameters [a = 5.10332 (11), c = 11.0829(3) A ˚] resulted in similar values as for the current single-crystal study (see Table 1), no anisotropic displacement parameters of the refined atoms were given in the previous powder study.Furthermore, the structure refinement on basis of single-crystal data not only allows for a more accurate determination of the oxygen site, but also for a rather precise determination of the Li:Lu ratio found at Wyckoff position 4b (occupancy ratio 0.483 Li:0.517Lu when refined freely).For electroneutrality, the site occupancies were fixed to ideal values (0.5:0.5) in the final refinement step.
Since Na + and K + cations are larger than Li + cations and thus closer to the size of Ln 3+ cations, it is not astonishing that the crystal volumes of NaLn[MoO 4 ] 2 and KLn[MoO 4 ] 2 compounds are considerably larger than those of the respective LiLn[MoO 4 ] 2 series.In case of the larger lanthanoids, lithium-containing scheelite-type structures according to the formula LiLn[MoO 4 ] 2 with Ln = Ce 3+ (Egorova et al., 1982) and Nd 3+ (Kolitsch, 2001) are known so far, while for Yb 3+ as a representative of the smaller lanthanides, the crystal structure shows deviations from the Laue group 4/m, crystallizing in space group I4 (Volkov et al., 2005;Armand et al., 2021).In all the aforementioned compounds, the rather small Li + cations assume a mixed occupancy with the respective lanthanoid, which is also found in the crystal structures of e.g.LiLn 5 [W 8 O 32 ] for Ln = Y (Dorn et al., 2017) and Dy-Lu (Dorn et al., 2021).However, in these structures the Li + cations show a sixfold coordination in contrast to the scheelite-type title compound with a coordination number of eight.

Synthesis and crystallization
Colorless single crystals of LiLu[MoO 4 ] 2 , which remain stable towards atmospheric influences, were obtained as a by- Oxidic coordination environment around the mixed cationic Li + /Lu 3+ position in the shape of a trigonal dodecahedron; displacement ellipsoids are drawn at the 95% probability level [Symmetry codes: (

Figure 1
Figure 1The augmented unit cell of LiLu[MoO 4 ] 2 in a view approximately along [010], with the [MoO 4 ] 2-anions in polyhedral representation and displacement ellipsoids drawn at the 95% probability level.Atomic positions marked with the subscript "a" build up the asymmetric unit.

Table 1
Experimental details. of synthesis attempts for LiLu 5 [Mo 8 O 32 ].Lithium chloride, lutetium sesquioxide and molybdenum trioxide in molar ratios of 3:8:24 were fused together in evacuated silica ampoules and treated with a stepwise temperature program with a peak value of 1123 K for four days.After a slow cooling ramp of another four days, the desired compound was obtained as a microcrystalline powder with single crystals of the title compound found in the bulk.