Synthesis, crystal structure and charge-distribution validation of β-Na4Cu(MoO4)3 adopting the alluadite structure-type.

Single crystals of a new variety of tetra-sodium copper(II) tris-[molybdate(VI)], Na4Cu(MoO4)3, have been synthesized by solid-state reactions and characterized by single-crystal X-ray diffraction. This alluaudite structure-type is characterized by the presence of infinite layers of composition (Cu/Na)2Mo3O14 parallel to the (100) plane, which are linked by MoO4 tetra-hedra, forming a three-dimensional framework containing two types of hexa-gonal channels in which Na(+) cations reside. The Cu(2+) and Na(2+) cations are located at the same general site with occupancies of 0.5. All atoms are on general positions except for one Mo, two Na (site symmetry 2) and another Na (site symmetry -1) atom. One O atom is split into two separate positions with occupancies of 0.5. The title compound is isotypic with Na5Sc(MoO4)4 and Na3In2As3O12. The structure model is supported by bond-valence-sum (BVS) and charge-distribution CHARDI methods. β-Na4Cu(MoO4)3 is compared and discussed with the K4Cu(MoO4)3 and α-Na4Cu(MoO4)3 structures.

Chemical context

In recent years, a number of molybdates have received considerable attention due to their amazing properties and high application potential in various fields, such as photoluminescence (Shi et al., 2014 ▸) and Li-ion batteries (Reddy et al., 2013 ▸). For example, the copper molybdate Cu3Mo2O9 doped with lithium displays high Coulombic efficiency in lithium-ion batteries and excellent charge-discharge stability (Xia et al., 2015 ▸). Many new molybdate phases have been synthesized and structurally characterized by X-ray diffraction, among which a large number belong to the alluaudite type, such as Na25Cs8Fe5(MoO4)24, which presents a high electrical conductivity (Savina et al., 2014 ▸). The alluaudite-type structure was first determined on natural minerals by Fisher, who showed that alluaudite compounds crystallize in the monoclinic C2/c space group (Fisher, 1955 ▸). Moore proposed the general formula X(2)X(1)M(1)M(2)2(PO4)3, in which the X and M mono-, bi- or trivalent cations are written according to their size (X are large cations and M are distorted octahedrally coordinated atoms). It represents the parental structure-type of the group referred to as alluaudite-type (Moore, 1971 ▸). The size of the channel and the stability of the alluaudite network led to many phases belonging to this structural type. We can totally or partially replace not only the monovalent cations, but also the central atoms of the MO6 octa­hedra and TO4 tetra­hedra. It is also possible to make substitutions with cations in different oxidation states adopting the same type of coordination number (Mo6+, V5+, P5+ and As5+). Alluaudite molybdates usually have the general formula of X(2)X(1)M(1)M(2)2(MoO4)3 and adopt the C2/c space group, with a ≃ 12, b ≃ 13 and c ≃ 7 Å, examples being the K0.13Na3.87MgMo3O12 (Ennajeh et al., 2015 ▸), Na3Fe2(MoO4)3 (Muessig et al., 2003 ▸) and Na4Co(MoO4)3 (Nasri et al., 2014 ▸) compounds. A review of the literature also reveals the presence of other formulae, such as Na5Sc(MoO4)4, Na2Ni(MoO4)2 (Klevtsova et al., 1991 ▸) and Na2.2Zn0.9(MoO4)2 (Efremov et al., 1975 ▸), which crystallize in the space group C2/c with cell parameters of about a ≃ 12, b ≃ 13 and c ≃ 7 Å. All alluaudite-type compounds can be described by the general formula given by Moore (1971 ▸), but their structures can differ by the number of formula units per unit cell. They are characterized by a three-dimensional heteropolyhedral frame­work formed by TO4 tetra­hedra and MO6 octa­hedra, delimiting two types of channels running along the c axis. A new variety of β-Na4Cu(MoO4)3 formulation was obtained by a reaction in the solid state at 873 K.

Structural commentary

The structural unit in β-Na4Cu(MoO4)3 is formed by MO6 (M = Cu1/Na1) octa­hedra linked by sharing vertices with Mo1O4 tetra­hedra and two slightly different Mo2O4 tetra­hedra, with a partially occupied (0.5 occupancy) Mo2 site. Atom O4 is split into two separate positions, with occupancies of 0.5 for the O4 and O41 atoms. The charge compensation is provided by Na+ cations (Fig. 1 ▸). The essential building units of the structure are M 2O10 units obtained from two edge-sharing MO6 octa­hedra. These units are connected by Mo1O4 tetra­hedra through vertex-sharing via Mo—O—M mixed bridges. This results in M 2Mo2O16 units. Each unit is connected to six other identical units by the sharing of vertices, leading to an infinite layer of the M 2Mo3O14 type parallel to the (100) plane (Fig. 2 ▸). The linkage of these layers is ensured by the two slightly different Mo2O4 tetra­hedra, linking via corners. This results in a three-dimensional framework delimited by two kinds of channels running along the c axis at (, 0, z) and (0, 0, z). These channels are occupied by Na+ cations (Fig. 3 ▸). In the anionic framework, each Mo2O4 tetra­hedron shares its O atoms with four different M 2O10 dimers belonging to two adjacent layers. The Mo1O4 tetra­hedron shares only three O atoms with three M 2O10 units belonging to the same layer, the other O atom being free and pointing towards the channels where the Na3 cations are located (Fig. 4 ▸). There is some compositional flexibility in the alluaudite structure and the studied material is isostructural with Na5Sc(MoO4)4 (Klevtsova et al., 1975 ▸) and Na3In2As3O12 (Khorari et al., 1997 ▸). The two crystallographically independent Mo atoms have tetra­hedral coordination, with an average Mo—O distance of 1.761 Å for Mo1 and 1.777 Å for Mo2, which is in a good agreement with those typically observed in Rb2Cu2(MoO4)3 (Solodovnikov & Solodovnikova, 1997 ▸). The Na+ cations occupy three crystallographically independent sites with different O-atom environments. The Na2, Na3 and Na4 atoms are surrounded by four, eight and six O atoms, respectively (Table 1 ▸). The Cu1 and Na1 cations are located at the same general site, with occupancies of 0.5, and have an octa­hedral environment with an average distance of 2.214 Å. This distance presents a mean between the Na—O distances of Na2Cu(PO3)4 (Laügt et al., 1972 ▸) and the Cu—O distances encountered in Ag2Cu2(MoO4)3 (Tsyrenova et al., 2009 ▸). The proposed structural model is confirmed by two validation models: (i) the bond-valence approach using the empirical formula of Brown (Brown & Altermatt, 1985 ▸) and (ii) the charge-distribution method Chardi (Nespolo, 2015 ▸, 2016 ▸). The charge distribution method is the most recent development of Pauling’s concept of bond strength (Pauling, 1929 ▸). Instead of empirical parameters used in the bond-valence approach, it exploits the experimental bond lengths deduced from the structural study to compute a non-integer coordination number, ECoN (effective coordination number), around a PC-atom (atom placed at the center of a polyhedron, q > 0), which is coordinated by V atoms (atoms located at the vertices, q < 0); q is the formal oxidation number. ECoN takes into account not only the number of V atoms around a given PC atom, but also their weight in terms of relative distances. Calculated charges Q(i) and valences V(i) are in good agreement with the formal oxidation number (q) multiplied by occupancy rates. The dispersion factor MAPD, which measures the mean absolute percentage deviation, is 2.2% for the calculated cationic charges. The variation of the ECoN value to the traditional coordination indicates the degree of distortion. The two validation models results are summarized in Table 2 ▸. Comparing our structure with that of a similar formulation, i.e. K4Cu(MoO4)3 (Menard et al., 2011 ▸), we found a clear difference, on the one hand, in the crystal symmetry and, on the other hand, in the arrangement of polyhedra. K4Cu(MoO4)3 crystallizes in the Pnma space group. Its structure can be described as being composed of a distorted square-planar CuO4 polyhedron bound by shared vertices to two Mo1O4 tetra­hedra to form CuMo2O10-type units. These units are inter­connected, on the one hand, by insertion of two Mo2O4 tetra­hedra which share a face with a partial occupation (0.5 occupancy) of Mo2 atoms, and secondly by forming a mixed bridge of the Mo—O—Cu type. This forms ribbons arranged parallel to the [100] direction. This results in a one-dimensional structure in which K+ atoms reside in the inter-ribbon spaces (Fig. 5 ▸). The structure of our new variety β-Na4Cu(MoO4)3 is compared with the α variety. Indeed, α-Na4Cu(MoO4)3 (Klevtsova et al., 1991 ▸) crystallizes in the triclinic system, space group P , and its structure is formed by the same Cu2O10 dimers present in our structure (here present as mixed-occupied M 2O10 units). The latter connects via Mo—O—Cu double composite bridges with two bidentate tetra­hedra MoO4 and by Mo—O—Cu simple bridges with monodentate MoO4 tetra­hedra to form Cu2Mo4O20 units. The Cu2Mo4O20 units are connected by MoO4 tetra­hedra and the pooling of vertices to form ribbons arranged in the [010] direction. All the ribbons form a one-dimensional framework with inter-ribbon spaces containing monovalent Na+ cations (Fig. 6 ▸). This structure has the same arrangement of structural units found in the one-dimensional structure of K3Mn(MoO4)3 (Solodovnikov et al., 1998 ▸) (Fig. 7 ▸).

Figure 1

Representation of the coordination polyhedra in the structural unit of β-Na4Cu(MoO4)3, showing (a) full atomic, (b) polyhedral. All atoms are represented as displacement ellipsoids at the 50% probability level. [Symmetry codes: (i) x, y, z − 1; (ii) −x + 1, y, −z + ; (iii) x + , −y + , z + ; (iv) x, −y, z + ; (v) −x + , −y + , −z + 1.]

Figure 2

A projection of the polyhedral layers in the bc plane.

Figure 3

A projection of the β-Na4Cu(MoO4)3 structure, viewed normal to (001), showing the channels where monovalent cations are located.

Figure 4

The association modes of M 2O10-based units by the (a) Mo(1)O4 and (b) Mo(2)O4 tetra­hedra. For clarity, we present only one atom of molybdenum, Mo2.

Table 1

Selected bond lengths (Å)

Mo1—O1	1.746 (3)	Na2—O5iv	2.462 (3)
Mo1—O5	1.762 (3)	Na2—O5	2.549 (3)
Mo1—O2i	1.764 (3)	Na2—O5ii	2.549 (3)
Mo1—O6	1.774 (3)	Na3—O41vii	2.443 (7)
Mo2—Mo2ii	0.447 (2)	Na3—O41viii	2.443 (7)
Mo2—O41	1.456 (7)	Na3—O1ix	2.493 (3)
Mo2—O3ii	1.738 (3)	Na3—O1ii	2.493 (3)
Mo2—O41ii	1.740 (7)	Na3—O1vi	2.675 (3)
Mo2—O4	1.787 (7)	Na3—O1x	2.675 (3)
Mo2—O3	1.822 (3)	Na3—O4vii	2.749 (7)
Mo2—O4ii	2.150 (6)	Na3—O4viii	2.749 (7)
Cu1—O4iii	1.884 (6)	Na3—O41xi	3.008 (7)
Cu1—O3	2.098 (3)	Na3—O41xii	3.008 (7)
Cu1—O2	2.116 (3)	Na4—O3	2.337 (2)
Cu1—O6	2.152 (3)	Na4—O3xiii	2.337 (2)
Cu1—O5iv	2.317 (3)	Na4—O2	2.424 (3)
Cu1—O6v	2.464 (4)	Na4—O2xiii	2.424 (3)
Cu1—O41iii	2.467 (4)	Na4—O1v	2.490 (4)
Na2—O5vi	2.462 (3)	Na4—O1xiv	2.490 (4)

Symmetry codes: (i) ; (ii) ; (iii) ; (iv) ; (v) ; (vi) ; (vii) ; (viii) ; (ix) ; (x) ; (xi) ; (xii) ; (xiii) ; (xiv) .

Table 2

CHARDI and BVS analysis of cation polyhedra in β-Na4Cu(MoO4)3

Cation	q(i)·sof(i)	Q(i)	V(i).sof(i)	CN(i)	ECoN(i)	d ar	d med
Mo1	6.00	6.24	5.93	4	4.00	1.761	1.761
Mo2	3.00	2.48	3.06	4	3.52	1.777	1.776
M	1.50	1.75	1.64	6	4.97	2.214	2.214
Na2	1.00	0.98	0.79	4	4.49	2.505	2.703
Na3	1.00	0.83	0.85	8	8.19	2.587	2.670
Na4	1.00	1.24	1.11	6	5.86	2.417	2.417

Notes: M = Cu1/Na1, q(i) = formal oxidation number, sof(i) = site-occupation factor, Q(i) = calculated charges, CN = coordination number, ECoN = number of effective coordination, MAPD = 100/NΣi N.|qi - Qi/qi|, d ar = arithmetic average distance and d med = weighted average distance.

Figure 5

A projection of the K4Cu(MoO4)3 structure, viewed along the [001] direction.

Figure 6

A projection of the α-Na4Cu(MoO4)3 structure, viewed in the (010) plane.

Figure 7

A projection of the K4Mn(MoO4)3 structure, viewed normal to (010).

Synthesis and crystallization

β-Na4Cu(MoO4)3 crystals were obtained by a solid-state reaction from the following reagents: Na2CO3 (Prolabo, 70128), Cu(CO2CH3)·H2O (Sigma–Aldrich, C5893) and (NH4)6Mo7O24·4H2O (Sigma–Aldrich, 13301) with an Na:Cu:Mo molar ratio of 4:1:3. The resulting mixture was milled in an agate mortar, placed in a porcelain crucible and then preheated slowly in air at 623 K for 24 h, in order to eliminate volatile products. Thereafter, it was heated to a temperature close to that of the fusion at 873 K. It was left at this temperature for 20 d to induce nucleation and crystal growth. The final residue was first cooled slowly (5 K per half day) to 823 K and then rapidly (50 K h−1) to room temperature. Green crystals of sufficient size for the measurement of intensities were obtained.

Refinement

Crystal data, data collection and structure refinement details are summarized in Table 3 ▸. We used of EADP and EXYZ constraints within SHELXL2014 (Sheldrick, 2015 ▸) for Cu1/Na1 located at the same crystallographic site. Atom O4 was split over two sites (O4 and O41) as this displayed a very elongated displacement ellipsoid. The occupancies of O4 and O41 were set to 0.5 in line with the occupany of Mo2; the two separate O-atom sites (O4 and O41) correspond to two different orientations of the Mo2O4 tetra­hedron related by symmetry. Refining atomic occupancies leads to a value of 0.497 (4) for the Cu atom. For conditions of electrical neutrality, we set the occupancy of the Cu atom as 0.5. This leads to well-defined ellipsoids. The maximum and minimum electron densities in the final Fourier difference map are acceptable and located at 0.77 and 0.82 Å, respectively, from the Na2 and Mo1 atoms.

Table 3

Experimental details

Crystal data
Chemical formula	Na4Cu(MoO4)3
M r	635.32
Crystal system, space group	Monoclinic, C2/c
Temperature (K)	298
a, b, c (Å)	12.5318 (9), 13.8181 (9), 7.1159 (7)
β (°)	111.95 (2)
V (Å3)	1142.9 (2)
Z	4
Radiation type	Mo Kα
μ (mm−1)	5.26
Crystal size (mm)	0.28 × 0.22 × 0.18
Data collection
Diffractometer	Enraf–Nonius CAD-4
Absorption correction	ψ scan (North et al., 1968 ▸)
T min, T max	0.224, 0.387
No. of measured, independent and observed [I > 2σ(I)] reflections	2678, 1238, 1208
R int	0.030
(sin θ/λ)max (Å−1)	0.638
Refinement
R[F 2 > 2σ(F 2)], wR(F 2), S	0.023, 0.058, 1.17
No. of reflections	1238
No. of parameters	104
Δρmax, Δρmin (e Å−3)	0.80, −0.72

Computer programs: CAD-4 EXPRESS (Duisenberg, 1992 ▸), XCAD4 (Harms & Wocadlo, 1995 ▸), SHELXS97 (Sheldrick, 2008 ▸), SHELXL2014 (Sheldrick, 2015 ▸), DIAMOND (Brandenburg & Putz, 2001 ▸), WinGX (Farrugia, 2012 ▸) and publCIF (Westrip, 2010 ▸).

Crystal structure: contains datablock(s) I. DOI: 10.1107/S2056989016010367/pj2032sup1.cif

Structure factors: contains datablock(s) I. DOI: 10.1107/S2056989016010367/pj2032Isup2.hkl

CCDC reference: 1487800

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

Na4Cu(MoO4)3	F(000) = 1180
Mr = 635.32	Dx = 3.692 Mg m−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
a = 12.5318 (9) Å	Cell parameters from 25 reflections
b = 13.8181 (9) Å	θ = 10–15°
c = 7.1159 (7) Å	µ = 5.26 mm−1
β = 111.95 (2)°	T = 298 K
V = 1142.9 (2) Å3	Prism, green
Z = 4	0.28 × 0.22 × 0.18 mm

Enraf–Nonius CAD-4 diffractometer	1208 reflections with I > 2σ(I)
ω/2θ scans	Rint = 0.030
Absorption correction: ψ scan (North et al., 1968)	θmax = 27.0°, θmin = 2.3°
Tmin = 0.224, Tmax = 0.387	h = −15→15
2678 measured reflections	k = −1→17
1238 independent reflections	l = −9→9

Refinement on F2	0 restraints
Least-squares matrix: full	w = 1/[σ2(Fo2) + (0.0173P)2 + 5.2182P] where P = (Fo2 + 2Fc2)/3
R[F2 > 2σ(F2)] = 0.023	(Δ/σ)max = 0.001
wR(F2) = 0.058	Δρmax = 0.80 e Å−3
S = 1.17	Δρmin = −0.72 e Å−3
1238 reflections	Extinction correction: SHELXL2014 (Sheldrick, 2015), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
104 parameters	Extinction coefficient: 0.00081 (11)

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.

	x	y	z	Uiso*/Ueq	Occ. (<1)
Mo1	0.72812 (2)	0.10879 (2)	0.11920 (4)	0.01991 (12)
Mo2	0.48085 (7)	0.28610 (4)	0.2400 (3)	0.0133 (3)	0.5
Cu1	0.70643 (5)	0.16100 (5)	0.61691 (9)	0.01741 (16)	0.5
Na1	0.70643 (5)	0.16100 (5)	0.61691 (9)	0.01741 (16)	0.5
Na2	0.5000	0.0169 (3)	0.2500	0.0482 (8)
Na3	0.0000	0.0000	0.0000	0.0299 (5)
Na4	0.5000	0.26934 (16)	0.7500	0.0219 (4)
O1	0.8745 (2)	0.0849 (2)	0.1859 (4)	0.0292 (6)
O2	0.6715 (2)	0.1702 (2)	0.8852 (4)	0.0337 (6)
O3	0.5390 (2)	0.2147 (2)	0.4714 (4)	0.0247 (6)
O4	0.3657 (5)	0.3629 (6)	0.2376 (11)	0.0239 (10)	0.5
O41	0.4153 (5)	0.3707 (6)	0.2551 (11)	0.0239 (10)	0.5
O5	0.6504 (3)	−0.0001 (2)	0.0904 (5)	0.0346 (7)
O6	0.7095 (3)	0.1754 (2)	0.3178 (5)	0.0373 (7)

	U11	U22	U33	U12	U13	U23
Mo1	0.01593 (17)	0.02069 (18)	0.01973 (18)	0.00034 (10)	0.00275 (11)	−0.00104 (11)
Mo2	0.0147 (8)	0.0162 (2)	0.0079 (4)	0.0004 (2)	0.0032 (6)	0.0000 (3)
Cu1	0.0227 (3)	0.0219 (3)	0.0093 (3)	−0.0007 (3)	0.0079 (2)	0.0006 (2)
Na1	0.0227 (3)	0.0219 (3)	0.0093 (3)	−0.0007 (3)	0.0079 (2)	0.0006 (2)
Na2	0.0217 (11)	0.098 (3)	0.0203 (11)	0.000	0.0031 (9)	0.000
Na3	0.0397 (12)	0.0224 (10)	0.0179 (9)	−0.0013 (10)	−0.0004 (9)	−0.0006 (9)
Na4	0.0225 (9)	0.0270 (11)	0.0189 (9)	0.000	0.0110 (7)	0.000
O1	0.0187 (12)	0.0341 (15)	0.0307 (14)	0.0037 (11)	0.0044 (10)	0.0072 (12)
O2	0.0268 (14)	0.0330 (16)	0.0327 (15)	0.0083 (12)	0.0011 (12)	0.0017 (13)
O3	0.0275 (13)	0.0335 (15)	0.0160 (12)	−0.0003 (12)	0.0113 (10)	0.0011 (11)
O4	0.017 (3)	0.0250 (18)	0.0238 (18)	0.005 (3)	0.001 (3)	−0.0039 (16)
O41	0.017 (3)	0.0250 (18)	0.0238 (18)	0.005 (3)	0.001 (3)	−0.0039 (16)
O5	0.0357 (15)	0.0279 (14)	0.0410 (17)	−0.0059 (13)	0.0152 (13)	0.0005 (13)
O6	0.0305 (14)	0.0398 (17)	0.0385 (17)	0.0010 (14)	0.0095 (13)	−0.0084 (14)

Mo1—O1	1.746 (3)	Na2—O5iv	2.462 (3)
Mo1—O5	1.762 (3)	Na2—O5	2.549 (3)
Mo1—O2i	1.764 (3)	Na2—O5ii	2.549 (3)
Mo1—O6	1.774 (3)	Na3—O41vii	2.443 (7)
Mo2—Mo2ii	0.447 (2)	Na3—O41viii	2.443 (7)
Mo2—O41	1.456 (7)	Na3—O1ix	2.493 (3)
Mo2—O3ii	1.738 (3)	Na3—O1ii	2.493 (3)
Mo2—O41ii	1.740 (7)	Na3—O1vi	2.675 (3)
Mo2—O4	1.787 (7)	Na3—O1x	2.675 (3)
Mo2—O3	1.822 (3)	Na3—O4vii	2.749 (7)
Mo2—O4ii	2.150 (6)	Na3—O4viii	2.749 (7)
Cu1—O4iii	1.884 (6)	Na3—O41xi	3.008 (7)
Cu1—O3	2.098 (3)	Na3—O41xii	3.008 (7)
Cu1—O2	2.116 (3)	Na4—O3	2.337 (2)
Cu1—O6	2.152 (3)	Na4—O3xiii	2.337 (2)
Cu1—O5iv	2.317 (3)	Na4—O2	2.424 (3)
Cu1—O6v	2.464 (4)	Na4—O2xiii	2.424 (3)
Cu1—O41iii	2.467 (4)	Na4—O1v	2.490 (4)
Na2—O5vi	2.462 (3)	Na4—O1xiv	2.490 (4)
O1—Mo1—O5	110.42 (15)	O3—Cu1—O2	85.28 (10)
O1—Mo1—O2i	111.01 (14)	O4iii—Cu1—O6	93.4 (2)
O5—Mo1—O2i	106.95 (15)	O3—Cu1—O6	82.16 (11)
O1—Mo1—O6	108.63 (14)	O2—Cu1—O6	166.61 (12)
O5—Mo1—O6	107.69 (15)	O4iii—Cu1—O5iv	96.1 (2)
O2i—Mo1—O6	112.08 (15)	O3—Cu1—O5iv	94.73 (11)
O3ii—Mo2—O4	118.4 (2)	O2—Cu1—O5iv	88.43 (12)
O3ii—Mo2—O3	110.75 (17)	O6—Cu1—O5iv	97.18 (12)
O4—Mo2—O3	112.3 (3)	O6v—Cu1—O2	89.55 (14)
O3ii—Mo2—O4ii	100.3 (2)	O6v—Cu1—O4iii	76.78 (15)
O4—Mo2—O4ii	113.9 (4)	O6v—Cu1—O5iv	172.13 (15)
O3—Mo2—O4ii	99.0 (2)	O6v—Cu1—O6	86.42 (14)
O4iii—Cu1—O3	168.8 (2)	O6v—Cu1—O3	92.66 (12)
O4iii—Cu1—O2	98.1 (2)