Scheelite-type NaEr(MoO₄)₂

Abstract

Explorations of the A ¹⁺–RE ³⁺–Mo⁶⁺–O²⁻ (A ¹⁺ is an alkali metal cation, RE ³⁺ is a rare-earth metal cation) quaternary systems prepared by the high-temperature solution growth method led to the title structure, sodium erbium bis­(molyb­date), NaEr(MoO₄)₂. It is isostructural to the scheelite structure (CaWO₄) and is composed of [MoO₄]²⁻ tetra­hedra with symmetry and [(Na/Er)O₈]¹⁴⁻ polyhedra. The [(Na/Er)O₈]¹⁴⁻ polyhedron is a distorted tetra­gonal anti­prism, also with symmetry, with statistically mixed Na/Er atoms at its centre. There are two sets of Na/Er—O bond lengths [2.420 (4) and 2.435 (3) Å], but just one set of Mo—O bond lengths [1.774 (4) Å].

Related literature

For the structures, properties and applications of the alkali rare-earth tungstates and molybdates with the general formula A ¹⁺ RE ³⁺(M ⁶⁺O₄)₂ (A ¹⁺ is an alkali metal cation, RE ³⁺ is a rare-earth metal cation, M ⁶⁺ is Mo⁶⁺ or W⁶⁺), see: Huang et al. (2006 ▶); Klevtsova (1975 ▶); Klevtsova et al. (1972 ▶); Kolitsch (2001 ▶); Kuzmicheva et al. (2005 ▶); Li et al. (2006 ▶); Morozov et al. (2006 ▶); Stevens et al. (1991 ▶); Zhao et al. (2010 ▶). For the scheelite (CaWO₄) structure, see: Sillen & Nylander (1943 ▶).

Experimental

Crystal data

• NaEr(MoO₄)₂

• M _r = 510.13

• Tetragonal,

• a = 5.1816 (8) Å

• c = 11.288 (3) Å

• V = 303.07 (11) ų

• Z = 2

• Mo Kα radiation

• μ = 17.87 mm⁻¹

• T = 173 K

• 0.08 × 0.04 × 0.04 mm

Data collection

• Rigaku Saturn70 CCD diffractometer

• Absorption correction: multi-scan (rescaled SADABS; Sheldrick, 1997 ▶) T _min = 0.263, T _max = 0.489

• 520 measured reflections

• 172 independent reflections

• 106 reflections with I > 2σ(I)

• R _int = 0.026

Refinement

• R[F ² > 2σ(F ²)] = 0.032

• wR(F ²) = 0.089

• S = 0.84

• 172 reflections

• 15 parameters

• Δρ_max = 1.12 e Å⁻³

• Δρ_min = −1.15 e Å⁻³

Data collection: CrystalClear (Rigaku, 2004 ▶); cell refinement: CrystalClear; data reduction: CrystalClear; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008 ▶) and PLATON (Spek, 2009 ▶); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008 ▶); molecular graphics: DIAMOND (Brandenburg, 2004 ▶); software used to prepare material for publication: SHELXTL (Sheldrick, 2008 ▶).

Supplementary Material

Additional supplementary materials: crystallographic information; 3D view; checkCIF report

supplementary crystallographic information

Comment

Alkali rare-earth bis(molybdates) with the general formula A¹⁺RE³⁺(MO₄)₂ (A^I is an alkali-metal cation, RE³⁺ is a rare-earth metal cation, M is Mo⁶⁺ or W⁶⁺) have been the subject of interest for many decades, mainly due to their applications as suitable host materials for fluorescence (Kuzmicheva et al., 2005; Morozov et al., 2006; Li et al., 2006). Some of these crystals are isostructural to scheelite (CaWO₄, I4₁/a; Sillen & Nylander, 1943), such as NaLa(MoO₄)₂ (Stevens et al., 1991), LiNd(MoO₄)₂ (Kolitsch, 2001), LiNd(WO₄)₂ (Huang et al., 2006) and LiDy(WO₄)₂ (Zhao et al., 2010).

In difference to CaWO₄ with one cation species only, the cations A¹⁺ and RE³⁺ are statistically disordered. Within alkali rare-earth bis(molybdates), different structures from the scheelite type have also been reported, such as LiLa(MoO₄)₂ (Pbca; Klevtsova, 1975) and CsDy(MoO₄)₂ (Pccm; Klevtsova et al. 1972).

The X-ray diffraction analysis has shown that the title compound NaEr(MoO₄)₂ is isostructural with the scheelite. In the title structure, Na and Er atoms are disordered over the same 4a site while Mo atoms reside on 4b site. The structure of NaEr(MoO₄)₂ may be regarded as composed of [MoO₄]^2- tetrahedra and of [(Na/Er)O₈]^14- polyhedra (each in the form of a distorted tetragonal antiprism) that share the oxygens (Fig. 2). Each oxygen of the [MoO₄]^2- tetrahedron is shared by the different Na/Er polyhedron and each oxygen of the [(Na/Er)O₈]^14- polyhedron is shared by the different [MoO₄]^2- tetrahedron.

Experimental

Single crystals of NaEr(MoO₄)₂ have been prepared by the high temperature solution growth (HTSG) method in air. A powder mixture of Na₂CO₃ (0.4418 g), Er₂O₃ (0.2657 g) and MoO₃ (2.000 g) at the molar ratio of Na:Er:Mo = 6:1:10 was first ground in an agate mortar and then transferred to a platinum crucible. The sample was gradually heated in air at 1173 K for 24 h. In this stage, the reagents were completely melted. After that, the intermediate product was slowly cooled to 673 K at the rate of 2 Kh^-1. It was kept at 673 for another 10 h and then quenched to room temperature. The obtained crystals were light-red and of the prismatical shape. The dimensions of the used sample were typical for the grown crystals in this batch.

Refinement

The Na and Er atoms are in substitutional disorder in the crystal structure. The tentative refinement that included the corresponding occupancy factors for the disordered Na/Er yielded Na1 : Er1 = 0.501 (2) : 0.499 (2). (The atomic positional and anisotropic displacement parameters of Na1 and Er1 atoms were constrained to be identical by using EADP and EXYZ constraint instructions (SHELXL-97; Sheldrick, 2008).) Therefore the ratio of Na and Er was fixed to 1:1 in the final model with the constrained positional and the displacement parameters of na and Er as given above. The highest peak in the difference electron density map equals to 1.12 e/ų at the distance of 0.83 Å from Na1/Er1 site while the deepest hole equals to -1.15 e/ų at the distance of 1.39 Å from Na1/Er1 site, too.

Figures

Fig. 1.

Section of the structure of NaEr(MoO4)2 with the atom labelling scheme. The displacement ellipsoids are drawn at the 50% probability level.

Fig. 2.

View of the crystal structure of NaEr(MoO4)2. The [MoO4]2- tetrahedra are shown in green.

Crystal data

NaEr(MoO4)2	Dx = 5.590 Mg m−3
Mr = 510.13	Mo Kα radiation, λ = 0.71073 Å
Tetragonal, I41/a	Cell parameters from 365 reflections
Hall symbol: -I 4ad	θ = 4.3–27.3°
a = 5.1816 (8) Å	µ = 17.87 mm−1
c = 11.288 (3) Å	T = 173 K
V = 303.07 (11) Å3	Prism, red
Z = 2	0.08 × 0.04 × 0.04 mm
F(000) = 454

Data collection

Rigaku Saturn70 CCD diffractometer	172 independent reflections
Radiation source: fine-focus sealed tube	106 reflections with I > 2σ(I)
confocal	Rint = 0.026
Detector resolution: 28.5714 pixels mm-1	θmax = 27.5°, θmin = 4.3°
ω scans	h = −2→6
Absorption correction: multi-scan (rescaled SADABS; Sheldrick, 1997)	k = −5→6
Tmin = 0.263, Tmax = 0.489	l = −14→14
520 measured reflections

Refinement

Refinement on F2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.032	w = 1/[σ^2^(Fo^2^) + (0.0639P)^2^] where P = (Fo^2^ + 2Fc^2^)/3
wR(F2) = 0.089	(Δ/σ)max < 0.001
S = 0.84	Δρmax = 1.12 e Å−3
172 reflections	Δρmin = −1.14 e Å−3
15 parameters	Extinction correction: SHELXL97 (Sheldrick, 2008), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
0 restraints	Extinction coefficient: 0.055 (5)
9 constraints

Special details

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.

Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq	Occ. (<1)
Er1	0.0000	0.2500	0.1250	0.0081 (5)	0.50
Na1	0.0000	0.2500	0.1250	0.0081 (5)	0.50
Mo1	0.5000	0.7500	0.1250	0.0086 (5)
O1	0.2568 (6)	0.5968 (6)	0.0397 (3)	0.0204 (12)

Atomic displacement parameters (Ų)

	U11	U22	U33	U12	U13	U23
Er1	0.0070 (6)	0.0070 (6)	0.0102 (8)	0.000	0.000	0.000
Na1	0.0070 (6)	0.0070 (6)	0.0102 (8)	0.000	0.000	0.000
Mo1	0.0067 (6)	0.0067 (6)	0.0124 (8)	0.000	0.000	0.000
O1	0.025 (2)	0.017 (2)	0.019 (2)	0.0012 (15)	−0.0045 (14)	−0.0007 (17)

Geometric parameters (Å, °)

Er1—O1i	2.420 (4)	Er1—O1vii	2.435 (3)
Er1—O1ii	2.420 (4)	Mo1—O1viii	1.774 (4)
Er1—O1iii	2.420 (4)	Mo1—O1ix	1.774 (4)
Er1—O1iv	2.420 (4)	Mo1—O1	1.774 (4)
Er1—O1v	2.435 (3)	Mo1—O1x	1.774 (4)
Er1—O1vi	2.435 (3)	O1—Na1iii	2.420 (4)
Er1—O1	2.435 (3)	O1—Er1iii	2.420 (4)
O1i—Er1—O1ii	79.63 (16)	O1v—Er1—O1vii	99.01 (7)
O1i—Er1—O1iii	126.16 (10)	O1vi—Er1—O1vii	99.01 (7)
O1ii—Er1—O1iii	126.16 (10)	O1—Er1—O1vii	133.38 (18)
O1i—Er1—O1iv	126.16 (10)	O1i—Er1—Er1xi	38.03 (7)
O1ii—Er1—O1iv	126.16 (10)	O1ii—Er1—Er1xi	69.88 (9)
O1iii—Er1—O1iv	79.62 (16)	O1iii—Er1—Er1xi	159.67 (8)
O1i—Er1—O1v	75.78 (12)	O1iv—Er1—Er1xi	101.19 (8)
O1ii—Er1—O1v	68.76 (7)	O1v—Er1—Er1xi	37.75 (8)
O1iii—Er1—O1v	152.76 (16)	O1vi—Er1—Er1xi	101.99 (9)
O1iv—Er1—O1v	73.67 (7)	O1—Er1—Er1xi	85.52 (9)
O1i—Er1—O1vi	68.76 (7)	O1vii—Er1—Er1xi	131.38 (8)
O1ii—Er1—O1vi	75.78 (12)	O1i—Er1—Na1xi	38.03 (7)
O1iii—Er1—O1vi	73.67 (7)	O1ii—Er1—Na1xi	69.88 (9)
O1iv—Er1—O1vi	152.76 (16)	O1iii—Er1—Na1xi	159.67 (8)
O1v—Er1—O1vi	133.38 (18)	O1iv—Er1—Na1xi	101.19 (8)
O1i—Er1—O1	73.67 (7)	O1v—Er1—Na1xi	37.75 (8)
O1ii—Er1—O1	152.76 (16)	O1vi—Er1—Na1xi	101.99 (9)
O1iii—Er1—O1	75.78 (12)	O1—Er1—Na1xi	85.52 (9)
O1iv—Er1—O1	68.76 (7)	O1vii—Er1—Na1xi	131.38 (8)
O1v—Er1—O1	99.01 (7)	O1viii—Mo1—O1ix	114.2 (2)
O1vi—Er1—O1	99.01 (7)	O1viii—Mo1—O1	107.15 (11)
O1i—Er1—O1vii	152.76 (16)	O1ix—Mo1—O1	107.15 (11)
O1ii—Er1—O1vii	73.67 (7)	O1viii—Mo1—O1x	107.15 (11)
O1iii—Er1—O1vii	68.76 (7)	O1ix—Mo1—O1x	107.15 (11)
O1iv—Er1—O1vii	75.78 (12)	O1—Mo1—O1x	114.2 (2)

Symmetry codes: (i) −y+3/4, x+1/4, z+1/4; (ii) y−3/4, −x+1/4, z+1/4; (iii) −x, −y+1, −z; (iv) x, y−1/2, −z; (v) y−1/4, −x+1/4, −z+1/4; (vi) −y+1/4, x+1/4, −z+1/4; (vii) −x, −y+1/2, z; (viii) y−1/4, −x+5/4, −z+1/4; (ix) −y+5/4, x+1/4, −z+1/4; (x) −x+1, −y+3/2, z; (xi) −x+1/2, −y+1/2, −z+1/2.