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

Hartenbach, I.; Hertweck, R.F.

doi:10.1107/S2056989024004365

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Volume 80| Part 6| June 2024| Pages 607-609

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

Ingo Hartenbach ^a ^* and Robin F. Hertweck ^b

^aUniversity of Stuttgart, Institute of Inorganic Chemistry, Pfaffenwaldring 55, 70569 Stuttgart, Germany, and ^bGymnasium in der Glemsaue, Gröninger Str. 29, 71254 Ditzingen, Germany
^*Correspondence e-mail: ingo.hartenbach@iac.uni-stuttgart.de

Edited by M. Weil, Vienna University of Technology, Austria (Received 2 May 2024; accepted 10 May 2024; online 17 May 2024)

Coarse colorless single crystals of lithium lutetium bis[orthomolybdate(VI)], LiLu[MoO₄]₂, were obtained as a by-product from a reaction aimed at lithium derivatives of lutetium molybdate. The title compound crystallizes in the scheelite structure type (tetragonal, space group I4₁/a) with two formula units per unit cell. The Wyckoff position 4b (site symmetry $[\overline{4}]$ ) comprises a mixed occupancy of Li⁺ and Lu³⁺ cations in a 1:1 ratio. In comparison with a previous powder X-ray study [Cheng et al. (2015 ). Dalton Trans. 44, 18078–18089.] all atoms were refined with anisotropic displacement parameters.

Keywords: crystal structure; scheelite; molybdates; lithium; lutetium; mixed occupancy.

CCDC reference: 2312843

1. Chemical context

The mineral powellite (CaMoO₄) 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 (CaWO₄) in space group type I4₁/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.

2. Structural commentary

In the crystal structure of LiLu[MoO₄]₂ (Fig. 1) the Li⁺ and Lu³⁺ cations reside at Wyckoff position 4b (site symmetry $[\overline{4}]$ ) 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) Å] in the shape of a trigonal dodecahedron (Fig. 2). The Mo⁶⁺ cations are situated in the centers of oxygen tetrahedra at Wyckoff position 4a (site symmetry $[\overline{4}]$ ) with distances of 4 × 1.774 (3) Å. The existence of LiLu[MoO₄]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) Å] 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:0.517 Lu when refined freely). For electroneutrality, the site occupancies were fixed to ideal values (0.5:0.5) in the final refinement step.

Table 1
Experimental details

Crystal data
Chemical formula	LiLu[MoO₄]₂
M_r	501.79
Crystal system, space group	Tetragonal, I4₁/a
Temperature (K)	293
a, c (Å)	5.1052 (3), 11.0800 (7)
V (Å³)	288.78 (4)
Z	2
Radiation type	Ag Kα, λ = 0.56083 Å
μ (mm⁻¹)	25.07
Crystal size (mm)	0.14 × 0.09 × 0.08

Data collection
Diffractometer	Stoe Stadivari
Absorption correction	Multi-scan [X-RED32 (Stoe & Cie, 2019 ) using Gaussian integration, analogous to Coppens (1970 ). Afterwards scaling of reflection intensities was performed within LANA (Koziskova et al., 2016 )]
T_min, T_max	0.031, 0.155
No. of measured, independent and observed [I > 2σ(I)] reflections	4315, 352, 162
R_int	0.037
(sin θ/λ)_max (Å⁻¹)	0.833

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.019, 0.048, 0.96
No. of reflections	352
No. of parameters	15
Δρ_max, Δρ_min (e Å⁻³)	1.14, −1.22
Computer programs: X-AREA (Stoe & Cie, 2019), LANA (Koziskova et al., 2016), SHELXT (Sheldrick, 2015a ), SHELXL (Sheldrick, 2015b ), DIAMOND (Brandenburg & Putz, 2023 ) and publCIF (Westrip, 2010 ).

Figure 1
The augmented unit cell of LiLu[MoO₄]₂ in a view approximately along [010], with the [MoO₄]^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.

Figure 2
Oxidic coordination environment around the mixed cationic Li⁺/Lu³⁺ position in the shape of a trigonal dodecahedron; displacement ellipsoids are drawn at the 95% probability level [Symmetry codes: (i) y − $[{1\over 4}]$ , −x + $[{3\over 4}]$ , z + $[{3\over 4}]$ ; (ii) x − $[{1\over 2}]$ , y, −z + $[{1\over 2}]$ ; (iii) −x + $[{1\over 2}]$ , −y + $[{1\over 2}]$ , −z + $[{1\over 2}]$ ; (iv) −y + $[{1\over 4}]$ , x − $[{1\over 4}]$ , z + $[{3\over 4}]$ ; (v) x − $[{1\over 2}]$ , y − $[{1\over 2}]$ , z + $[{1\over 2}]$ ; (vi) −x + $[{1\over 2}]$ , −y + 1, z + $[{1\over 2}]$ ; (vii) −y + $[{3\over 4}]$ , x − $[{1\over 4}]$ , −z + $[{3\over 4}]$ ; (viii) y − $[{3\over 4}]$ , −x + $[{3\over 4}]$ , −z + $[{3\over 4}]$ ].

Since Na⁺ and K⁺ cations are larger than Li⁺ cations and thus closer to the size of Ln³⁺ cations, it is not astonishing that the crystal volumes of NaLn[MoO₄]₂ and KLn[MoO₄]₂ compounds are considerably larger than those of the respective LiLn[MoO₄]₂ series. In case of the larger lanthanoids, lithium-containing scheelite-type structures according to the formula LiLn[MoO₄]₂ with Ln = Ce³⁺ (Egorova et al., 1982 ) and Nd³⁺ (Kolitsch, 2001 ) are known so far, while for Yb³⁺ as a representative of the smaller lanthanides, the crystal structure shows deviations from the Laue group 4/m, crystallizing in space group I $[\overline{4}]$ (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. LiLn₅[W₈O₃₂] 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.

3. Synthesis and crystallization

Colorless single crystals of LiLu[MoO₄]₂, which remain stable towards atmospheric influences, were obtained as a by-product of synthesis attempts for LiLu₅[Mo₈O₃₂]. 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.

4. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 1. The 1:1 ratio of Li⁺ and Lu³⁺ was reached by fixed occupation factors (0.5:0.5) of the respective atoms at Wyckoff position 4b.

Supporting information

CCDC reference: 2312843

Crystal structure: contains datablock I. DOI: https://doi.org/10.1107/S2056989024004365/wm5719sup1.cif

Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S2056989024004365/wm5719Isup2.hkl

checkCIF report

Computing details top

Lithium lutetium bis[orthomolybdate(VI)] top

Crystal data top

LiLu[MoO₄]₂	D_x = 5.771 Mg m⁻³
M_r = 501.79	Ag Kα radiation, λ = 0.56083 Å
Tetragonal, I4₁/a	Cell parameters from 2522 reflections
a = 5.1052 (3) Å	θ = 3.5–31.7°
c = 11.0800 (7) Å	µ = 25.07 mm⁻¹
V = 288.78 (4) Å³	T = 293 K
Z = 2	Coarse, colorless
F(000) = 444	0.14 × 0.09 × 0.08 mm

Data collection top

Stoe Stadivari diffractometer	352 independent reflections
Radiation source: Axo Ag	162 reflections with I > 2σ(I)
Graded multilayer mirror monochromator	R_int = 0.037
Detector resolution: 5.81 pixels mm^-1	θ_max = 27.9°, θ_min = 3.5°
rotation method, ω scans	h = −8→8
Absorption correction: multi-scan [X-Red32 (Stoe & Cie, 2019) using Gaussian integration, analogous to Coppens (1970). Afterwards scaling of reflection intensities was performed within LANA (Koziskova et al., 2016)]	k = −8→8
T_min = 0.031, T_max = 0.155	l = −15→18
4315 measured reflections

Refinement top

Refinement on F²	0 restraints
Least-squares matrix: full	w = 1/[σ²(F_o²) + (0.0206P)² + 1.1482P] where P = (F_o² + 2F_c²)/3
R[F² > 2σ(F²)] = 0.019	(Δ/σ)_max < 0.001
wR(F²) = 0.048	Δρ_max = 1.14 e Å⁻³
S = 0.96	Δρ_min = −1.22 e Å⁻³
352 reflections	Extinction correction: SHELXL (Sheldrick 2015b), Fc^*=kFc[1+0.001xFc²λ³/sin(2θ)]^-1/4
15 parameters	Extinction coefficient: 0.044 (2)

Special details top

Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å²) top

	x	y	z	U_iso*/U_eq	Occ. (<1)
Li	0.000000	0.250000	0.625000	0.00754 (18)	0.5
Lu	0.000000	0.250000	0.625000	0.00754 (18)	0.5
Mo	0.000000	0.250000	0.125000	0.01028 (19)
O	0.2480 (4)	0.4067 (5)	0.0394 (2)	0.0175 (6)

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Li	0.0075 (2)	0.0075 (2)	0.0075 (3)	0.000	0.000	0.000
Lu	0.0075 (2)	0.0075 (2)	0.0075 (3)	0.000	0.000	0.000
Mo	0.0097 (2)	0.0097 (2)	0.0114 (3)	0.000	0.000	0.000
O	0.0203 (15)	0.0157 (14)	0.0177 (13)	−0.0012 (12)	0.0049 (11)	0.0027 (14)

Geometric parameters (Å, º) top

Li—Oⁱ	2.369 (3)	Lu—O^v	2.371 (3)
Li—Oⁱⁱ	2.369 (3)	Lu—O^vi	2.371 (3)
Li—Oⁱⁱⁱ	2.369 (3)	Lu—O^vii	2.371 (3)
Li—O^iv	2.369 (3)	Lu—O^viii	2.371 (3)
Li—O^v	2.371 (3)	Lu—Lu^ix	3.7668 (2)
Li—O^vi	2.371 (3)	Lu—Lu^x	3.7668 (2)
Li—O^vii	2.371 (3)	Lu—Lu^xi	3.7668 (2)
Li—O^viii	2.371 (3)	Lu—Lu^xii	3.7668 (2)
Lu—Oⁱ	2.369 (3)	Mo—O^xiii	1.774 (3)
Lu—Oⁱⁱ	2.369 (3)	Mo—O^xiv	1.774 (3)
Lu—Oⁱⁱⁱ	2.369 (3)	Mo—O^xv	1.774 (3)
Lu—O^iv	2.369 (3)	Mo—O	1.774 (3)

Oⁱ—Li—Oⁱⁱ	126.20 (9)	O^iv—Lu—O^viii	74.75 (12)
Oⁱ—Li—Oⁱⁱⁱ	126.20 (9)	O^v—Lu—O^viii	99.23 (6)
Oⁱⁱ—Li—Oⁱⁱⁱ	79.57 (15)	O^vi—Lu—O^viii	99.23 (6)
Oⁱ—Li—O^iv	79.57 (15)	O^vii—Lu—O^viii	132.78 (15)
Oⁱⁱ—Li—O^iv	126.20 (9)	Oⁱ—Lu—Lu^ix	158.93 (7)
Oⁱⁱⁱ—Li—O^iv	126.20 (9)	Oⁱⁱ—Lu—Lu^ix	70.38 (7)
Oⁱ—Li—O^v	153.18 (13)	Oⁱⁱⁱ—Lu—Lu^ix	37.40 (7)
Oⁱⁱ—Li—O^v	69.36 (7)	O^iv—Lu—Lu^ix	101.37 (7)
Oⁱⁱⁱ—Li—O^v	74.75 (12)	O^v—Lu—Lu^ix	37.35 (7)
O^iv—Li—O^v	73.93 (6)	O^vi—Lu—Lu^ix	101.88 (8)
Oⁱ—Li—O^vi	73.93 (6)	O^vii—Lu—Lu^ix	85.81 (7)
Oⁱⁱ—Li—O^vi	74.75 (12)	O^viii—Lu—Lu^ix	131.46 (8)
Oⁱⁱⁱ—Li—O^vi	69.36 (7)	Oⁱ—Lu—Lu^x	37.40 (7)
O^iv—Li—O^vi	153.18 (13)	Oⁱⁱ—Lu—Lu^x	158.93 (7)
O^v—Li—O^vi	132.78 (15)	Oⁱⁱⁱ—Lu—Lu^x	101.37 (7)
Oⁱ—Li—O^vii	74.75 (12)	O^iv—Lu—Lu^x	70.38 (8)
Oⁱⁱ—Li—O^vii	153.18 (13)	O^v—Lu—Lu^x	131.46 (7)
Oⁱⁱⁱ—Li—O^vii	73.93 (6)	O^vi—Lu—Lu^x	85.81 (7)
O^iv—Li—O^vii	69.36 (7)	O^vii—Lu—Lu^x	37.35 (7)
O^v—Li—O^vii	99.23 (6)	O^viii—Lu—Lu^x	101.88 (8)
O^vi—Li—O^vii	99.23 (6)	Lu^ix—Lu—Lu^x	122.737 (3)
Oⁱ—Li—O^viii	69.36 (7)	Oⁱ—Lu—Lu^xi	70.38 (8)
Oⁱⁱ—Li—O^viii	73.93 (6)	Oⁱⁱ—Lu—Lu^xi	101.37 (7)
Oⁱⁱⁱ—Li—O^viii	153.18 (13)	Oⁱⁱⁱ—Lu—Lu^xi	158.93 (7)
O^iv—Li—O^viii	74.75 (12)	O^iv—Lu—Lu^xi	37.40 (7)
O^v—Li—O^viii	99.23 (6)	O^v—Lu—Lu^xi	85.81 (7)
O^vi—Li—O^viii	99.23 (6)	O^vi—Lu—Lu^xi	131.46 (7)
O^vii—Li—O^viii	132.78 (15)	O^vii—Lu—Lu^xi	101.88 (8)
Oⁱ—Lu—Oⁱⁱ	126.20 (9)	O^viii—Lu—Lu^xi	37.35 (7)
Oⁱ—Lu—Oⁱⁱⁱ	126.20 (9)	Lu^ix—Lu—Lu^xi	122.737 (3)
Oⁱⁱ—Lu—Oⁱⁱⁱ	79.57 (15)	Lu^x—Lu—Lu^xi	85.322 (6)
Oⁱ—Lu—O^iv	79.57 (15)	Oⁱ—Lu—Lu^xii	101.37 (7)
Oⁱⁱ—Lu—O^iv	126.20 (9)	Oⁱⁱ—Lu—Lu^xii	37.40 (7)
Oⁱⁱⁱ—Lu—O^iv	126.20 (9)	Oⁱⁱⁱ—Lu—Lu^xii	70.38 (7)
Oⁱ—Lu—O^v	153.18 (13)	O^iv—Lu—Lu^xii	158.93 (7)
Oⁱⁱ—Lu—O^v	69.36 (7)	O^v—Lu—Lu^xii	101.88 (8)
Oⁱⁱⁱ—Lu—O^v	74.75 (12)	O^vi—Lu—Lu^xii	37.35 (7)
O^iv—Lu—O^v	73.93 (6)	O^vii—Lu—Lu^xii	131.46 (7)
Oⁱ—Lu—O^vi	73.93 (6)	O^viii—Lu—Lu^xii	85.81 (7)
Oⁱⁱ—Lu—O^vi	74.75 (12)	Lu^ix—Lu—Lu^xii	85.322 (5)
Oⁱⁱⁱ—Lu—O^vi	69.36 (7)	Lu^x—Lu—Lu^xii	122.737 (3)
O^iv—Lu—O^vi	153.18 (13)	Lu^xi—Lu—Lu^xii	122.737 (3)
O^v—Lu—O^vi	132.78 (15)	O^xiii—Mo—O^xiv	106.65 (10)
Oⁱ—Lu—O^vii	74.75 (12)	O^xiii—Mo—O^xv	106.65 (10)
Oⁱⁱ—Lu—O^vii	153.18 (13)	O^xiv—Mo—O^xv	115.3 (2)
Oⁱⁱⁱ—Lu—O^vii	73.93 (6)	O^xiii—Mo—O	115.3 (2)
O^iv—Lu—O^vii	69.36 (7)	O^xiv—Mo—O	106.65 (10)
O^v—Lu—O^vii	99.23 (6)	O^xv—Mo—O	106.65 (10)
O^vi—Lu—O^vii	99.23 (6)	Mo—O—Liⁱⁱⁱ	130.25 (16)
Oⁱ—Lu—O^viii	69.36 (7)	Mo—O—Li^xvi	120.42 (15)
Oⁱⁱ—Lu—O^viii	73.93 (6)	Liⁱⁱⁱ—O—Li^xvi	105.25 (12)
Oⁱⁱⁱ—Lu—O^viii	153.18 (13)

Symmetry codes: (i) y−1/4, −x+3/4, z+3/4; (ii) x−1/2, y, −z+1/2; (iii) −x+1/2, −y+1/2, −z+1/2; (iv) −y+1/4, x−1/4, z+3/4; (v) x−1/2, y−1/2, z+1/2; (vi) −x+1/2, −y+1, z+1/2; (vii) −y+3/4, x−1/4, −z+3/4; (viii) y−3/4, −x+3/4, −z+3/4; (ix) −x, −y, −z+1; (x) −x+1/2, −y+1/2, −z+3/2; (xi) −x−1/2, −y+1/2, −z+3/2; (xii) −x, −y+1, −z+1; (xiii) −x, −y+1/2, z; (xiv) −y+1/4, x+1/4, −z+1/4; (xv) y−1/4, −x+1/4, −z+1/4; (xvi) x+1/2, y+1/2, z−1/2.

Acknowledgements

The authors thank Dr Falk Lissner for measuring the single-crystal of the title compound.

Funding information

Funding for this research was provided by: German Research Foundation (DFG) grant ‘Open Access Publication Funding/2023–2024/University of Stuttgart’ (512689491).

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