Synthesis and crystal structure of NaMgFe(MoO4)3.

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 octa-hedra which are linked to each other through the common corners of [MoO4] tetra-hedra. The resulting anionic three-dimensional framework leads to the formation of channels along the [101] direction in which the Na(+) cations are located.

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 c (Kotova & Kozhevnikova, 2003 ▸; Kozhevnikova & Imekhenova, 2009 ▸) to the phase of variable composition Na(1− M (1−Fe(1+(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.

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 [10] (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 [10] 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.

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.

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
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[F 2 > 2σ(F 2)], wR(F 2), S	0.025, 0.068, 1.19
No. of reflections	2983
No. of parameters	168
No. of restraints	4
Δρmax, Δρmin (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 ▸).

Crystal structure: contains datablock(s) global, I. DOI: 10.1107/S205698901600829X/br2259sup1.cif

Structure factors: contains datablock(s) I. DOI: 10.1107/S205698901600829X/br2259Isup2.hkl

CCDC reference: 1481125

Additional supporting information: crystallographic information; 3D view; checkCIF report

FeMgMo3NaO12	Z = 2
Mr = 582.97	F(000) = 542
Triclinic, P1	Dx = 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

Enraf–Nonius TurboCAD-4 diffractometer	Rint = 0.014
Radiation source: fine-focus sealed tube	θmax = 30.0°, θmin = 3.0°
non–profiled ω/2τ scans	h = −9→9
Absorption correction: ψ scan (North et al., 1968)	k = −9→9
Tmin = 0.478, Tmax = 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 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(Fo2) + (0.0308P)2 + 2.3858P] where P = (Fo2 + 2Fc2)/3
wR(F2) = 0.068	(Δ/σ)max = 0.001
S = 1.19	Δρmax = 1.47 e Å−3
2983 reflections	Δρmin = −1.60 e Å−3
168 parameters	Extinction correction: SHELXL2014/7 (Sheldrick 2015), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
4 restraints	Extinction coefficient: 0.0074 (5)

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.

	x	y	z	Uiso*/Ueq	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)

	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)

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—O11vii	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—O31iv	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—O22v	2.010 (3)	Mo3—O31	1.801 (2)
Mg2—O11vi	2.012 (3)
O21i—Na—O12	106.29 (15)	O24—Mg2—O11vi	90.58 (11)
O21i—Na—O11	92.34 (14)	O22v—Mg2—O11vi	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)	O22v—Mg2—O32i	93.77 (12)
O11—Na—O24ii	81.96 (12)	O11vi—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)	O22v—Mg2—O12ii	91.08 (12)
O24ii—Na—O23ii	63.66 (10)	O11vi—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—O11vii	106.05 (13)
O14—Mg1—O34i	90.26 (12)	O13—Mo1—O11vii	111.66 (13)
O33—Mg1—O31iii	177.80 (12)	O12—Mo1—O11vii	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—O31iv	98.59 (12)	O21—Mo2—O24	110.70 (14)
O23—Mg1—O31iv	88.75 (11)	O23—Mo2—O24	105.75 (13)
O14—Mg1—O31iv	87.53 (11)	O22—Mo2—O24	111.86 (13)
O34i—Mg1—O31iv	171.86 (11)	O33—Mo3—O32	108.10 (14)
O31iii—Mg1—O31iv	81.22 (11)	O33—Mo3—O34	110.71 (13)
O13i—Mg2—O24	88.40 (12)	O32—Mo3—O34	109.21 (14)
O13i—Mg2—O22v	90.10 (12)	O33—Mo3—O31	108.43 (13)
O24—Mg2—O22v	175.47 (12)	O32—Mo3—O31	109.96 (13)
O13i—Mg2—O11vi	92.52 (11)	O34—Mo3—O31	110.40 (12)