Synthesis and crystal structure of NaMgFe(MoO4)3

Mhiri, M.; Badri, A.; Ben Amara, M.

doi:10.1107/S205698901600829X

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Volume 72| Part 6| June 2016| Pages 864-867

https://doi.org/10.1107/S205698901600829X

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Synthesis and crystal structure of NaMgFe(MoO₄)₃

Manel Mhiri,^a ^* Abdessalem Badri ^a and Mongi Ben Amara ^a

^aUnité de Recherche, Matériaux Inorganiques, Faculté des Sciences, Université de Monastir, 5019 Monastir, Tunisia
^*Correspondence e-mail: mhirimanel@yahoo.fr

Edited by I. D. Brown, McMaster University, Canada (Received 14 May 2016; accepted 20 May 2016; online 27 May 2016)

The iron molybdate NaMgFe(MoO₄)₃ {sodium magnesium iron(III) tris[molybdate(VI)]} has been synthesized by the flux method. This compound is isostructural with α-NaFe₂(MoO₄)₃ and crystallizes in the triclinic space group P-1. Its structure is built up from [Mg,Fe]₂O₁₀ units of edge-sharing [Mg,Fe]O₆ octahedra which are linked to each other through the common corners of [MoO₄] tetrahedra. The resulting anionic three-dimensional framework leads to the formation of channels along the [101] direction in which the Na⁺ cations are located.

Keywords: crystal structure; iron molybdate; anionic framework; Na–Fe–Mo–O system.

CCDC reference: 1481125

1. Chemical context

Iron molybdates have been subject to very intensive research as a result of their numerous applications including as catalysts (Tian et al., 2011 ), multiferroic properties and more recently as a possible positive electrode in rechargeable batteries (Sinyakov et al., 1978 ; Mączka et al., 2011 ; Devi & Varadaraju, 2012 ). In these materials, the anionic framework is constructed from MoO₄ tetrahedra linked to the iron coordination polyhedra, leading to a large variety of crystal structures with a high capacity for cationic and anionic substitutions.

Until now, a total of six orthomolybdate compounds have been reported in the Na–Fe–Mo–O system: Na₉Fe(MoO₄)₆ (Savina et al., 2013 ); NaFe(MoO₄)₂ (Klevtsova, 1975 ); α-NaFe₂(MoO₄)₃, β-NaFe₂(MoO₄)₃ and Na₃Fe₂(MoO₄)₃ (Muessig et al., 2003 ); NaFe₄(MoO₄)₅ (Ehrenberg et al., 2006 ). Their structures are described in terms of three-dimensional networks of isolated [MoO₄] tetrahedra and [FeO₆] octahedra. The sodium and mixed-valence iron molybdate NaFe₂(MoO₄)₃ exhibits two polymorphs, both crystallizing in the triclinic system. The low-temperature α-phase changes irreversibly at high temperature into a β-phase. In addition to these orthomolybdate compounds, another phase with the formula Na₃Fe₂Mo₅O₁₆ and with layers of Mo₃O₁₃ units consisting of [MoO₆] octahedra has been synthesized and characterized (Bramnik et al., 2003 ). In addition, Kozhevnikova & Imekhenova (2009 ) have investigated the Na₂MoO₄–MMoO₄–Fe₂(MoO₄)₃ system (M = Mg, Mn, Ni, Co) and have attributed the Nasicon-type structure with space group R $[\overline{3}]$ c (Kotova & Kozhevnikova, 2003 ; Kozhevnikova & Imekhenova, 2009) to the phase of variable composition Na_(1−x)M_(1−x)Fe_(1+x)(MoO₄)₃. More recently, NaNiFe(MoO₄)₃ and NaZnFe(MoO₄)₃ (Mhiri et al., 2015 ) were found to be isostructural to β-NaFe₂(MoO₄)₃ and to have a good ionic conductivity with low activation energy, close to those of Nasicon-type compounds with similar formula such as AZr₂(PO₄)₃ (A = Na, Li). As an extension of the previous work, we report here on the synthesis and characterization by X-ray diffraction of a new compound, NaMgFe(MoO₄)₃, which is isostructural with α-NaFe₂(MoO₄)₃.

2. Structural commentary

The title NaMgFe(MoO₄)₃ structure is based on a three-dimensional framework of [Mg,Fe]₂O₁₀ units of edge-sharing [Mg,Fe]O₆ octahedra, connected to each other through the common corners of [MoO₄] tetrahedra. All [Mg,Fe]₂O₁₀ units are parallel to [1 $[\overline{1}]$ 0] (Fig. 1). In this structure, two types of layers (A and B), similar to those observed in α-NaFe₂(MoO₄)₃, are aligned parallel to (110) with the sequence –A–B–B′–A–B–B′– and stacked along [001]. B′ layers are obtained from B by an inversion centre located on the A planes (Fig. 2). The resulting anionic three-dimensional framework leads to the formation of channels along [101] in which the sodium ions are located (Fig. 3).

Figure 1
[Mg,Fe]₂O₁₀ units parallel to [1 $[\overline{1}]$ 0] in NaMgFe(MoO₄)₃ structure. [Mg,Fe]₂O₁₀ dimers are shown in blue and MoO₄ tetrahedra in purple.

Figure 2
Projection of the NaMgFe(MoO₄)₃ structure along the b axis. [Mg,Fe]₂O₁₀ dimers are shown in blue; MoO₄ tetrahedra in purple and Na⁺ cations as green spheres.

Figure 3
Channels along [101] in the structure of NaMgFe(MoO₄)₃. [Mg,Fe]₂O₁₀ dimers are shown in blue, MoO₄ tetrahedra in purple and Na⁺ cations as green spheres.

In the title structure, all atoms are located in general positions. The three crystallographically different molybdenum atoms have a tetrahedral coordination with Mo—O distances between 1.715 (3) and 1.801 (2) Å. The mean distances (Mo1—O = 1.762, Mo2—O = 1.766 and Mo3—O = 1.760 Å) are in good accordance with those usually observed in molybdates (Abrahams et al., 1967 ; Harrison & Cheetham, 1989 ; Smit et al., 2006 ). The [Mg,Fe]—O distances and the cis O—[Mg,Fe]—O angles in the [Mg,Fe]₂O₁₀ units range from 2.003 (3) to 2.099 (3) Å and from 81.2 (1) to 177.8 (1)°, respectively. This dispersion reflects a slight distortion of the [Mg,Fe]O₆ octahedra. The average distances [Mg,Fe]1—O = 2.059 and [Mg,Fe]2—O = 2.013 Å lie between the values of 1.990 Å observed for six-coordinated Fe³⁺ in LiFe(MoO₄)₂ (van der Lee et al. 2008 ) and 2.072 Å reported for Mg²⁺ with the same coordination in NaMg₃Al(MoO₄)₅ (Hermanowicz et al., 2006 ). This result is related to the disordered distribution of Fe³⁺ and Mg²⁺ in both sites. Assuming sodium–oxygen distances below 3.13 Å (Donnay & Allmann, 1970 ), the Na site is surrounded by five oxygen atoms (Fig. 4).

Figure 4
The environment of the Na⁺ cation showing displacement ellipsoids drawn at the 50% probability level.

3. Synthesis and crystallization

Crystals of the title compound were grown in a flux of sodium dimolybdate Na₂Mo₂O₇ with an atomic ratio Na:Mg:Fe:Mo = 5:1:1:7. Appropriate amounts of the starting reactants NaNO₃, Mg(NO₃)₂·6H₂O, Fe(NO₃)₃·9H₂O and (NH₄)₆Mo₇O₂₄·4H₂O were dissolved in nitric acid and the resulting solution was evaporated to dryness. The dry residue was then placed in a platinum crucible and slowly heated in air up to 673 K for 24 h to remove H₂O and NH₃. The mixture was ground in an agate mortar, melted for 2 h at 1123 K and then cooled to room temperature at a rate of 5 K h⁻¹. Crystals without regular shape were separated from the flux by washing in boiling water.

4. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 1. The application of the direct methods revealed two sites, labeled M(1) and M(2), statistically occupied by the Fe³⁺ and Mg²⁺ ions. This distribution was supported by the M1—O and M2—O distances which are between the classical values for pure Mg—O and Fe—O bonds. Succeeding difference Fourier synthesis led to the positions of all the remaining atoms.

Table 1
Experimental details

Crystal data
Chemical formula	NaMgFe(MoO₄)₃
M_r	582.97
Crystal system, space group	Triclinic, P $[\overline{1}]$
Temperature (K)	293
a, b, c (Å)	6.900 (4), 6.928 (1), 11.055 (1)
α, β, γ (°)	80.24 (1), 83.55 (2), 80.22 (3)
V (Å³)	511.3 (3)
Z	2
Radiation type	Mo Kα
μ (mm⁻¹)	5.15
Crystal size (mm)	0.28 × 0.14 × 0.07

Data collection
Diffractometer	Enraf–Nonius TurboCAD-4
Absorption correction	ψ scan (North et al., 1968 )
T_min, T_max	0.478, 0.695
No. of measured, independent and observed [I > 2σ(I)] reflections	3429, 2983, 2850
R_int	0.014
(sin θ/λ)_max (Å⁻¹)	0.703

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.025, 0.068, 1.19
No. of reflections	2983
No. of parameters	168
No. of restraints	4
Δρ_max, Δρ_min (e Å⁻³)	1.47, −1.60
Computer programs: CAD-4 EXPRESS (Enraf–Nonius, 1994 ), XCAD4 (Harms & Wocadlo, 1995 ), SIR92 (Altomare et al., 1993 ), SHELXL2014/7 (Sheldrick, 201 ), DIAMOND (Brandenburg & Putz, 1999 ) and WinGX publication routines (Farrugia, 2012 ).

Supporting information

CCDC reference: 1481125

Crystal structure: contains datablocks global, I. DOI: https://doi.org/10.1107/S205698901600829X/br2259sup1.cif

Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S205698901600829X/br2259Isup2.hkl

checkCIF report

Computing details top

Data collection: CAD-4 EXPRESS (Enraf–Nonius, 1994); cell refinement: CAD-4 EXPRESS (Enraf–Nonius, 1994); data reduction: XCAD4 (Harms & Wocadlo, 1995); program(s) used to solve structure: SIR92 (Altomare et al., 1993); program(s) used to refine structure: SHELXL2014/7 (Sheldrick, 201); molecular graphics: DIAMOND (Brandenburg & Putz, 1999); software used to prepare material for publication: WinGX publication routines (Farrugia, 2012).

(I) top

Crystal data top

FeMgMo₃NaO₁₂	Z = 2
M_r = 582.97	F(000) = 542
Triclinic, P1	D_x = 3.786 Mg m⁻³
a = 6.900 (4) Å	Mo Kα radiation, λ = 0.71073 Å
b = 6.928 (1) Å	Cell parameters from 25 reflections
c = 11.055 (1) Å	θ = 9.1–11.4°
α = 80.24 (1)°	µ = 5.15 mm⁻¹
β = 83.55 (2)°	T = 293 K
γ = 80.22 (3)°	Prism, brown
V = 511.3 (3) Å³	0.28 × 0.14 × 0.07 mm

Data collection top

Enraf–Nonius TurboCAD-4 diffractometer	R_int = 0.014
Radiation source: fine-focus sealed tube	θ_max = 30.0°, θ_min = 3.0°
non–profiled ω/2τ scans	h = −9→9
Absorption correction: ψ scan (North et al., 1968)	k = −9→9
T_min = 0.478, T_max = 0.695	l = −1→15
3429 measured reflections	2 standard reflections every 120 min
2983 independent reflections	intensity decay: 1.1%
2850 reflections with I > 2σ(I)

Refinement top

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)] = 0.025	w = 1/[σ²(F_o²) + (0.0308P)² + 2.3858P] where P = (F_o² + 2F_c²)/3
wR(F²) = 0.068	(Δ/σ)_max = 0.001
S = 1.19	Δρ_max = 1.47 e Å⁻³
2983 reflections	Δρ_min = −1.60 e Å⁻³
168 parameters	Extinction correction: SHELXL-2014/7 (Sheldrick 2015), Fc^*=kFc[1+0.001xFc²λ³/sin(2θ)]^-1/4
4 restraints	Extinction coefficient: 0.0074 (5)

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.

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 > 2sigma(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 (Å²) top

	x	y	z	U_iso*/U_eq	Occ. (<1)
Na	0.8586 (4)	0.5914 (4)	0.8148 (4)	0.0757 (12)
Mg1	0.8152 (1)	0.1703 (1)	0.50854 (8)	0.00851 (16)	0.7558 (7)
Fe1	0.8152 (1)	0.1703 (1)	0.50854 (8)	0.00851 (16)	0.2442 (7)
Mg2	0.77528 (8)	0.77491 (8)	0.11025 (5)	0.00785 (12)	0.2442 (7)
Fe2	0.77528 (8)	0.77491 (8)	0.11025 (5)	0.00785 (12)	0.7558 (7)
Mo1	0.75910 (4)	0.10066 (4)	0.85110 (2)	0.00799 (8)
O11	0.8166 (4)	0.8508 (4)	0.9264 (2)	0.0125 (5)
O12	0.9297 (4)	0.2547 (4)	0.8737 (3)	0.0146 (5)
O13	0.5170 (4)	0.2053 (4)	0.8938 (3)	0.0158 (5)
O14	0.7784 (5)	0.0889 (4)	0.6953 (2)	0.0185 (5)
Mo2	0.70522 (4)	0.28318 (4)	0.18835 (3)	0.00950 (8)
O21	0.4579 (4)	0.3458 (5)	0.2289 (3)	0.0232 (6)
O22	0.7436 (4)	0.0675 (4)	0.1148 (3)	0.0185 (5)
O23	0.8372 (4)	0.2322 (4)	0.3205 (2)	0.0173 (5)
O24	0.8015 (4)	0.4878 (4)	0.0918 (2)	0.0148 (5)
Mo3	0.27372 (4)	0.29658 (4)	0.54507 (2)	0.00732 (8)
O31	0.1224 (4)	0.1328 (4)	0.5056 (2)	0.0113 (4)
O32	0.2458 (5)	0.2976 (4)	0.7045 (2)	0.0194 (5)
O33	0.5183 (4)	0.2083 (4)	0.5042 (3)	0.0172 (5)
O34	0.2067 (4)	0.5383 (4)	0.4690 (3)	0.0153 (5)

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Na	0.0293 (12)	0.0255 (11)	0.186 (4)	0.0089 (9)	−0.0496 (18)	−0.0429 (17)
Mg1	0.0079 (4)	0.0081 (4)	0.0091 (4)	−0.0007 (3)	−0.0018 (3)	0.0001 (3)
Fe1	0.0079 (4)	0.0081 (4)	0.0091 (4)	−0.0007 (3)	−0.0018 (3)	0.0001 (3)
Mg2	0.0080 (2)	0.0073 (2)	0.0079 (2)	−0.00174 (18)	−0.00177 (18)	0.00103 (18)
Fe2	0.0080 (2)	0.0073 (2)	0.0079 (2)	−0.00174 (18)	−0.00177 (18)	0.00103 (18)
Mo1	0.00697 (13)	0.00870 (13)	0.00770 (13)	−0.00175 (9)	−0.00095 (9)	0.00122 (9)
O11	0.0163 (12)	0.0106 (11)	0.0094 (11)	−0.0013 (9)	−0.0021 (9)	0.0019 (8)
O12	0.0112 (11)	0.0152 (12)	0.0182 (12)	−0.0052 (9)	−0.0040 (9)	0.0003 (10)
O13	0.0090 (11)	0.0178 (12)	0.0194 (13)	−0.0009 (9)	−0.0016 (9)	−0.0005 (10)
O14	0.0258 (14)	0.0185 (13)	0.0092 (11)	−0.0021 (11)	−0.0017 (10)	0.0018 (10)
Mo2	0.01041 (14)	0.00802 (13)	0.00985 (13)	−0.00235 (9)	−0.00173 (9)	0.00076 (9)
O21	0.0133 (12)	0.0295 (16)	0.0261 (15)	−0.0028 (11)	−0.0017 (11)	−0.0026 (12)
O22	0.0226 (14)	0.0110 (12)	0.0229 (14)	−0.0050 (10)	−0.0032 (11)	−0.0017 (10)
O23	0.0192 (13)	0.0188 (13)	0.0125 (12)	−0.0009 (10)	−0.0033 (10)	0.0005 (10)
O24	0.0203 (13)	0.0100 (11)	0.0129 (11)	−0.0016 (9)	0.0019 (9)	−0.0012 (9)
Mo3	0.00771 (13)	0.00787 (13)	0.00674 (13)	−0.00290 (9)	−0.00105 (9)	−0.00015 (9)
O31	0.0093 (10)	0.0089 (10)	0.0167 (12)	−0.0027 (8)	−0.0026 (9)	−0.0027 (9)
O32	0.0266 (15)	0.0239 (14)	0.0088 (11)	−0.0081 (11)	−0.0019 (10)	−0.0010 (10)
O33	0.0107 (11)	0.0198 (13)	0.0212 (13)	−0.0025 (10)	−0.0018 (10)	−0.0025 (10)
O34	0.0189 (13)	0.0086 (11)	0.0179 (12)	−0.0027 (9)	−0.0027 (10)	0.0008 (9)

Geometric parameters (Å, º) top

Na—O21ⁱ	2.244 (4)	Mg2—O32ⁱ	2.019 (3)
Na—O12	2.296 (4)	Mg2—O12ⁱⁱ	2.036 (3)
Na—O11	2.308 (4)	Mo1—O14	1.727 (3)
Na—O24ⁱⁱ	2.604 (4)	Mo1—O13	1.751 (3)
Na—O23ⁱⁱ	2.772 (5)	Mo1—O12	1.780 (3)
Mg1—O33	2.025 (3)	Mo1—O11^vii	1.789 (3)
Mg1—O23	2.044 (3)	Mo2—O21	1.715 (3)
Mg1—O14	2.045 (3)	Mo2—O23	1.761 (3)
Mg1—O34ⁱ	2.054 (3)	Mo2—O22	1.787 (3)
Mg1—O31ⁱⁱⁱ	2.089 (3)	Mo2—O24	1.799 (3)
Mg1—O31^iv	2.099 (3)	Mo3—O33	1.731 (3)
Mg2—O13ⁱ	2.003 (3)	Mo3—O32	1.753 (3)
Mg2—O24	2.009 (3)	Mo3—O34	1.753 (3)
Mg2—O22^v	2.010 (3)	Mo3—O31	1.801 (2)
Mg2—O11^vi	2.012 (3)

O21ⁱ—Na—O12	106.29 (15)	O24—Mg2—O11^vi	90.58 (11)
O21ⁱ—Na—O11	92.34 (14)	O22^v—Mg2—O11^vi	85.22 (11)
O12—Na—O11	131.5 (2)	O13ⁱ—Mg2—O32ⁱ	91.56 (12)
O21ⁱ—Na—O24ⁱⁱ	169.3 (2)	O24—Mg2—O32ⁱ	90.53 (12)
O12—Na—O24ⁱⁱ	71.63 (12)	O22^v—Mg2—O32ⁱ	93.77 (12)
O11—Na—O24ⁱⁱ	81.96 (12)	O11^vi—Mg2—O32ⁱ	175.79 (12)
O21ⁱ—Na—O23ⁱⁱ	125.19 (19)	O13ⁱ—Mg2—O12ⁱⁱ	176.14 (11)
O12—Na—O23ⁱⁱ	115.39 (14)	O24—Mg2—O12ⁱⁱ	90.70 (12)
O11—Na—O23ⁱⁱ	85.84 (12)	O22^v—Mg2—O12ⁱⁱ	91.08 (12)
O24ⁱⁱ—Na—O23ⁱⁱ	63.66 (10)	O11^vi—Mg2—O12ⁱⁱ	91.24 (11)
O33—Mg1—O23	88.07 (12)	O32ⁱ—Mg2—O12ⁱⁱ	84.69 (12)
O33—Mg1—O14	88.89 (12)	O14—Mo1—O13	108.16 (14)
O23—Mg1—O14	174.80 (12)	O14—Mo1—O12	106.87 (14)
O33—Mg1—O34ⁱ	89.20 (12)	O13—Mo1—O12	110.69 (13)
O23—Mg1—O34ⁱ	93.92 (12)	O14—Mo1—O11^vii	106.05 (13)
O14—Mg1—O34ⁱ	90.26 (12)	O13—Mo1—O11^vii	111.66 (13)
O33—Mg1—O31ⁱⁱⁱ	177.80 (12)	O12—Mo1—O11^vii	113.08 (12)
O23—Mg1—O31ⁱⁱⁱ	89.73 (11)	O21—Mo2—O23	110.07 (14)
O14—Mg1—O31ⁱⁱⁱ	93.30 (12)	O21—Mo2—O22	109.36 (15)
O34ⁱ—Mg1—O31ⁱⁱⁱ	91.09 (11)	O23—Mo2—O22	109.03 (13)
O33—Mg1—O31^iv	98.59 (12)	O21—Mo2—O24	110.70 (14)
O23—Mg1—O31^iv	88.75 (11)	O23—Mo2—O24	105.75 (13)
O14—Mg1—O31^iv	87.53 (11)	O22—Mo2—O24	111.86 (13)
O34ⁱ—Mg1—O31^iv	171.86 (11)	O33—Mo3—O32	108.10 (14)
O31ⁱⁱⁱ—Mg1—O31^iv	81.22 (11)	O33—Mo3—O34	110.71 (13)
O13ⁱ—Mg2—O24	88.40 (12)	O32—Mo3—O34	109.21 (14)
O13ⁱ—Mg2—O22^v	90.10 (12)	O33—Mo3—O31	108.43 (13)
O24—Mg2—O22^v	175.47 (12)	O32—Mo3—O31	109.96 (13)
O13ⁱ—Mg2—O11^vi	92.52 (11)	O34—Mo3—O31	110.40 (12)

Symmetry codes: (i) −x+1, −y+1, −z+1; (ii) −x+2, −y+1, −z+1; (iii) x+1, y, z; (iv) −x+1, −y, −z+1; (v) x, y+1, z; (vi) x, y, z−1; (vii) x, y−1, z.

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