Crystal structure of disodium dicobalt(II) iron(III) tris-(orthophosphate) with an alluaudite-like structure.

The title compound, Na2Co2Fe(PO4)3, was synthesized by a solid-state reaction. This new stoichiometric phase crystallizes in an alluaudite-like structure. In this structure, all atoms are in general positions except for four atoms which are located at the special positions of the C2/c space group. One Co atom, one P and one Na atom are all located on Wyckoff position 4e (2), while the second Na atom is located on an inversion centre 4a (-1). The other Co and Fe atoms occupy a general position with a statistical distribution. The open framework results from [(Co,Fe)2O10] units of edge-sharing [(Co,Fe)O6] octa-hedra, which alternate with [CoO6] octa-hedra that form infinite chains running along the [10-1] direction. These chains are linked together through PO4 tetra-hedra by the sharing of vertices so as to build layers perpendicular to [010]. The three-dimensional framework is accomplished by the stacking of these layers, leading to the formation of two types of tunnels parallel to [010] in which the Na(+) cations are located, each cation being surrounded by eight O atoms.

Chemical context

A particular focus of ours concerns compounds with alluaudite-type structures, and we set the objective of synthesising new transition-metal-based phosphates within the well-known alluaudite family. We are inter­ested in this because transition-metal phosphates are of great inter­est with applications in several fields. Compounds belonging to the large structural family of derivatives (Moore, 1971 ▸) have been of continuing inter­est due to their structural properties, such as their open-framework architecture and their physical properties. Moreover, the flexibility of the alluaudite structure will, no doubt, permit the use of alluaudite-type phosphates for practical applications, such as corrosion inhibition, passivation of metal surfaces and catalysis (Korzenski et al., 1999 ▸). These materials abound in magnetic properties of metallic phosphate. Transition metals can play an important role in microporous skeletons by supplying an active catalytic site keeping the selectivity of frames (Weil et al., 2009 ▸). Metallic phosphates present a multitude of structural wealth which are the object of studies of catalysts (Viter & Nagornyi, 2009 ▸; Gao & Gao, 2005 ▸), ion exchange (Clearfield, 1988 ▸) and the positive electrode in lithium and sodium batteries (Trad et al., 2010 ▸). As a result of the presence of channels parallel to [100], alluaudite-type compounds exhibit electronic and/or ionic conductivity, as has been shown by Warner et al. (1993 ▸). In this context, we have explored A 2O–MO–P2O5 systems, where A is a monovalent cation and M a divalent cation. A new alluaudite structure of formula Na2Co2Fe(PO4)3 was synthesized by solid-state reaction. During our investigation of these systems, we characterized the following compounds: AgMg3(PO4)(HPO4)2 (Assani et al., 2011a ▸), Ag2Ni3(HPO4)(PO4)2 (Assani et al., 2011b ▸) and Na2Ni2Fe(PO4)3 (Essehli et al., 2011 ▸). The present paper reports the solid-state synthesis and characterization of a new transition-metal phosphate, namely, Na2Co2Fe(PO4)3.

Structural commentary

In the refinement of the first model of this structure, we placed the Fe atom in Wyckoff position 4e and Co in the general position 8f. The results of the refinement of this model are acceptable if we disregard the high weight values. However, bond-valence-sum calculations (Brown & Altermatt, 1985 ▸) are not in favor of this model and, consequently, the examination of all possible models led to the best one in which half of the Co, Na, and P atoms are in Wyckoff position 4e, and the second Na atom is in position 4a of the C2/c space group, the remaining Co and Fe fulfilling the 8f site. In this case, bond-valence-sum calculations for Co22+, Co12+, Fe12+, Na1+, Na2+, P15+ and P25+ ions are as expected, viz 1.78, 2.02, 2.81, 1.25, 0.94, 4.98 and 4.99 valence units, respectively.

The new phase of formula Na2Co2Fe(PO4)3 crystallizes in the alluaudite type. The structure of this compound is built up from two edge-sharing [(Co,Fe)O6] octa­hedra, leading to the formation of [(Co,Fe)2O10] dimers that are connected by a common edge to [CoO6] octa­hedra, as shown in Fig. 1 ▸. The linkage of alternating [CoO6] and [(Co,Fe)2O10] octa­hedra leads to infinite chains along the [10] direction. These chains held together via the vertices of the PO4 tetra­hedra in such a way as to build layers perpendicular to [010] (Fig. 2 ▸). The junction of different octa­hedra by common vertices of PO4 tetra­hedra form an open three-dimensional framework that delimits two types of tunnels parallel to [100] and [001] accommodating the Na+ cations, as shown in Fig. 3 ▸. In the tunnels, each sodium atom is surrounded by eight oxygen atoms with Na1—O and Na2—O bond lengths varying between 2.2895 (9) and 2.8754 (10) Å) and between 2.3940 (9) and 2.8513 (16) Å, respectively.

Figure 1

The principal building units in the structure of the title compound. Displacement ellipsoids are drawn at the 50% probability level. Symmetry codes: (i) −x + , y + , −z + ; (ii) −x + , −y + , −z + 1; (iii) −x + 1, −y + 1, −z; (iv) −x + , −y + , −z; (v) −x + 1, y, −z + ; (vi) x − , −y + , z − ; (vii) x, −y + 1, z + ; (viii) x, −y + 1, z − ; (ix) −x + 2, y, −z + ; (x) −x + 2, −y + 1, −z + 1; (xi) x + , −y + , z + ; (xii) −x + , −y + , −z + 1.

Figure 2

A layer perpendicular to the b axis, resulting from the chains connected via the vertices of the PO4 tetra­hedra.

Figure 3

Polyhedral representation of Na2Co2Fe(PO4)3 showing the tunnels running along the [001] direction.

Synthesis and crystallization

Na2Co2Fe(PO4)3 was synthesized by a solid-state reaction by mixing the precursors of sodium (Na2CO3), cobalt (CoCO3), iron (Fe2O3) and phospho­ric acid 85% wt. The various precursors were taken in the molar ratio Na:Co:Fe:P = 2:2:1:3.

After different heat treatments in a platinum crucible up to 873 K, the reaction mixture was heated to the melting point of 1000 K. The molten product was then cooled to room temperature at a rate of 5 K h−1. The resulting product contained brown crystals of a suitable size for the X-ray diffraction study.

Refinement

Crystal data, data collection and structure refinement details are summarized in Table 1 ▸. The same x, y and z parameters and anisotropic displacement parameters are used for Co1 and Fe1 sharing the same site. Three reflections, (042), (110) and (42), probably affected by the beam-stop, were removed during the last refinement cycle. The highest peak and the deepest hole in the final Fourier map are at 0.40 Å and 0.42 Å from Na1 and Na2, respectively.

Table 1

Experimental details

Crystal data
Chemical formula	Na2Co2Fe(PO4)3
M r	504.60
Crystal system, space group	Monoclinic, C2/c
Temperature (K)	296
a, b, c ()	11.7106(6), 12.4083(7), 6.4285(3)
()	113.959(2)
V (3)	853.63(8)
Z	4
Radiation type	Mo K
(mm1)	6.26
Crystal size (mm)	0.31 0.25 0.19
Data collection
Diffractometer	Bruker X8 APEX
Absorption correction	Multi-scan (SADABS; Bruker, 2009 ▸)
T min, T max	0.504, 0.748
No. of measured, independent and observed [I > 2(I)] reflections	15289, 1882, 1807
R int	0.030
(sin /)max (1)	0.806
Refinement
R[F 2 > 2(F 2)], wR(F 2), S	0.016, 0.046, 1.10
No. of reflections	1879
No. of parameters	95
max, min (e 3)	0.70, 0.92

Computer programs: APEX2 and SAINT (Bruker, 2009 ▸), SHELXS97 and SHELXL97 (Sheldrick, 2008 ▸), ORTEP-3 for Windows (Farrugia, 2012 ▸), DIAMOND (Brandenburg, 2006 ▸) and publCIF (Westrip, 2010 ▸).

Crystal structure: contains datablock(s) I. DOI: 10.1107/S2056989015007926/br2248sup1.cif

Structure factors: contains datablock(s) I. DOI: 10.1107/S2056989015007926/br2248Isup2.hkl

CCDC reference: 1060932

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

Na2Co2Fe(PO4)3	F(000) = 972
Mr = 504.60	Dx = 3.926 Mg m−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -c 2yc	Cell parameters from 1882 reflections
a = 11.7106 (6) Å	θ = 2.5–34.9°
b = 12.4083 (7) Å	µ = 6.26 mm−1
c = 6.4285 (3) Å	T = 296 K
β = 113.959 (2)°	Block, brown
V = 853.63 (8) Å3	0.31 × 0.25 × 0.19 mm
Z = 4

Bruker X8 APEX diffractometer	1882 independent reflections
Radiation source: fine-focus sealed tube	1807 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.030
φ and ω scans	θmax = 34.9°, θmin = 2.5°
Absorption correction: multi-scan (SADABS; Bruker, 2009)	h = −18→18
Tmin = 0.504, Tmax = 0.748	k = −20→20
15289 measured reflections	l = −9→10

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)] = 0.016	w = 1/[σ2(Fo2) + (0.026P)2 + 1.033P] where P = (Fo2 + 2Fc2)/3
wR(F2) = 0.046	(Δ/σ)max = 0.002
S = 1.10	Δρmax = 0.70 e Å−3
1879 reflections	Δρmin = −0.92 e Å−3
95 parameters	Extinction correction: SHELXL97 (Sheldrick, 2008), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
0 restraints	Extinction coefficient: 0.0049 (3)

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.

	x	y	z	Uiso*/Ueq	Occ. (<1)
Fe1	0.717910 (14)	0.842242 (12)	0.13094 (2)	0.00487 (5)	0.50
Co1	0.717910 (14)	0.842242 (12)	0.13094 (2)	0.00487 (5)	0.50
Co2	0.5000	0.731171 (18)	0.2500	0.00645 (5)
P1	0.76483 (3)	0.60996 (2)	0.37608 (4)	0.00406 (6)
P2	0.5000	0.29046 (3)	0.2500	0.00393 (7)
Na1	0.5000	0.5000	0.0000	0.01623 (17)
Na2	1.0000	0.48710 (12)	0.7500	0.0343 (3)
O1	0.77862 (8)	0.67773 (7)	0.18650 (14)	0.00805 (14)
O2	0.83828 (8)	0.66574 (7)	0.60820 (14)	0.00755 (14)
O3	0.82670 (9)	0.49990 (7)	0.38761 (16)	0.00954 (15)
O4	0.62676 (8)	0.60150 (7)	0.32737 (15)	0.00871 (14)
O5	0.60276 (8)	0.36657 (7)	0.25279 (15)	0.00938 (15)
O6	0.45930 (8)	0.21880 (7)	0.03503 (13)	0.00683 (13)

	U11	U22	U33	U12	U13	U23
Fe1	0.00449 (7)	0.00506 (7)	0.00514 (7)	−0.00034 (4)	0.00203 (5)	−0.00037 (4)
Co1	0.00449 (7)	0.00506 (7)	0.00514 (7)	−0.00034 (4)	0.00203 (5)	−0.00037 (4)
Co2	0.00659 (9)	0.00646 (9)	0.00728 (9)	0.000	0.00381 (7)	0.000
P1	0.00506 (11)	0.00343 (11)	0.00348 (10)	−0.00004 (8)	0.00150 (8)	−0.00003 (7)
P2	0.00357 (15)	0.00423 (15)	0.00352 (14)	0.000	0.00094 (11)	0.000
Na1	0.0262 (4)	0.0032 (3)	0.0068 (3)	0.0017 (3)	−0.0061 (3)	0.0008 (2)
Na2	0.0187 (5)	0.0569 (8)	0.0218 (5)	0.000	0.0026 (4)	0.000
O1	0.0094 (3)	0.0097 (3)	0.0052 (3)	−0.0011 (3)	0.0031 (3)	0.0014 (3)
O2	0.0097 (3)	0.0073 (3)	0.0049 (3)	−0.0018 (3)	0.0021 (3)	−0.0015 (2)
O3	0.0113 (4)	0.0061 (3)	0.0109 (3)	0.0026 (3)	0.0043 (3)	−0.0012 (3)
O4	0.0057 (3)	0.0080 (3)	0.0124 (3)	−0.0002 (3)	0.0037 (3)	0.0002 (3)
O5	0.0063 (3)	0.0081 (3)	0.0124 (3)	−0.0022 (3)	0.0024 (3)	0.0024 (3)
O6	0.0065 (3)	0.0089 (3)	0.0047 (3)	−0.0008 (3)	0.0019 (2)	−0.0019 (2)

Fe1—O5i	1.9456 (9)	P2—O6v	1.5468 (8)
Fe1—O3i	2.0158 (9)	P2—O6	1.5468 (8)
Fe1—O2ii	2.0374 (9)	Na1—O5iii	2.2895 (9)
Fe1—O6iii	2.0543 (9)	Na1—O5	2.2895 (9)
Fe1—O1iv	2.0724 (8)	Na1—O4iii	2.3835 (9)
Fe1—O1	2.1434 (9)	Na1—O4	2.3836 (9)
Co2—O4v	2.1072 (9)	Na1—O4viii	2.5217 (9)
Co2—O4	2.1072 (9)	Na1—O4v	2.5218 (9)
Co2—O2ii	2.1567 (9)	Na1—O5v	2.8754 (10)
Co2—O2vi	2.1568 (9)	Na1—O5viii	2.8754 (10)
Co2—O6vii	2.1632 (8)	Na2—O3ix	2.3940 (9)
Co2—O6iii	2.1632 (8)	Na2—O3	2.3940 (9)
P1—O4	1.5206 (9)	Na2—O3x	2.5269 (10)
P1—O3	1.5336 (9)	Na2—O3vii	2.5269 (10)
P1—O1	1.5422 (9)	Na2—O2ix	2.8158 (15)
P1—O2	1.5502 (9)	Na2—O2	2.8158 (15)
P2—O5v	1.5240 (9)	Na2—O6xi	2.8513 (16)
P2—O5	1.5241 (9)	Na2—O6xii	2.8513 (16)
O5i—Fe1—O3i	94.91 (4)	O5—Na1—O4viii	73.59 (3)
O5i—Fe1—O2ii	110.51 (4)	O4iii—Na1—O4viii	67.32 (4)
O3i—Fe1—O2ii	86.18 (4)	O4—Na1—O4viii	112.68 (4)
O5i—Fe1—O6iii	164.12 (4)	O5iii—Na1—O4v	73.59 (3)
O3i—Fe1—O6iii	98.29 (4)	O5—Na1—O4v	106.41 (3)
O2ii—Fe1—O6iii	79.27 (3)	O4iii—Na1—O4v	112.68 (4)
O5i—Fe1—O1iv	86.97 (4)	O4—Na1—O4v	67.32 (4)
O3i—Fe1—O1iv	99.56 (4)	O4viii—Na1—O4v	180.0
O2ii—Fe1—O1iv	161.21 (4)	O5iii—Na1—O5v	126.25 (4)
O6iii—Fe1—O1iv	82.19 (3)	O5—Na1—O5v	53.75 (4)
O5i—Fe1—O1	81.40 (4)	O4iii—Na1—O5v	86.18 (3)
O3i—Fe1—O1	174.04 (4)	O4—Na1—O5v	93.82 (3)
O2ii—Fe1—O1	90.73 (3)	O4viii—Na1—O5v	114.14 (3)
O6iii—Fe1—O1	86.11 (3)	O4v—Na1—O5v	65.86 (3)
O1iv—Fe1—O1	84.97 (3)	O5iii—Na1—O5viii	53.75 (4)
O4v—Co2—O4	80.44 (5)	O5—Na1—O5viii	126.25 (4)
O4v—Co2—O2ii	165.05 (3)	O4iii—Na1—O5viii	93.82 (3)
O4—Co2—O2ii	86.54 (3)	O4—Na1—O5viii	86.18 (3)
O4v—Co2—O2vi	86.54 (3)	O4viii—Na1—O5viii	65.86 (3)
O4—Co2—O2vi	165.05 (3)	O4v—Na1—O5viii	114.14 (3)
O2ii—Co2—O2vi	107.25 (5)	O5v—Na1—O5viii	180.0
O4v—Co2—O6vii	92.44 (3)	O3ix—Na2—O3	172.39 (8)
O4—Co2—O6vii	113.30 (3)	O3ix—Na2—O3x	81.52 (3)
O2ii—Co2—O6vii	85.96 (3)	O3—Na2—O3x	97.99 (3)
O2vi—Co2—O6vii	74.34 (3)	O3ix—Na2—O3vii	97.99 (3)
O4v—Co2—O6iii	113.30 (3)	O3—Na2—O3vii	81.52 (3)
O4—Co2—O6iii	92.44 (3)	O3x—Na2—O3vii	172.68 (8)
O2ii—Co2—O6iii	74.34 (3)	O3ix—Na2—O2ix	55.93 (3)
O2vi—Co2—O6iii	85.96 (3)	O3—Na2—O2ix	117.11 (5)
O6vii—Co2—O6iii	146.65 (5)	O3x—Na2—O2ix	62.15 (3)
O4—P1—O3	112.98 (5)	O3vii—Na2—O2ix	111.51 (5)
O4—P1—O1	108.59 (5)	O3ix—Na2—O2	117.11 (5)
O3—P1—O1	108.95 (5)	O3—Na2—O2	55.93 (3)
O4—P1—O2	110.92 (5)	O3x—Na2—O2	111.51 (5)
O3—P1—O2	106.53 (5)	O3vii—Na2—O2	62.15 (3)
O1—P1—O2	108.78 (5)	O2ix—Na2—O2	76.15 (5)
O5v—P2—O5	103.42 (7)	O3ix—Na2—O6xi	116.10 (5)
O5v—P2—O6v	108.79 (5)	O3—Na2—O6xi	71.28 (4)
O5—P2—O6v	112.97 (5)	O3x—Na2—O6xi	83.54 (4)
O5v—P2—O6	112.97 (5)	O3vii—Na2—O6xi	103.11 (4)
O5—P2—O6	108.79 (5)	O2ix—Na2—O6xi	145.12 (3)
O6v—P2—O6	109.82 (7)	O2—Na2—O6xi	126.19 (2)
O5iii—Na1—O5	180.0	O3ix—Na2—O6xii	71.28 (4)
O5iii—Na1—O4iii	78.23 (3)	O3—Na2—O6xii	116.10 (5)
O5—Na1—O4iii	101.77 (3)	O3x—Na2—O6xii	103.11 (4)
O5iii—Na1—O4	101.77 (3)	O3vii—Na2—O6xii	83.54 (4)
O5—Na1—O4	78.23 (3)	O2ix—Na2—O6xii	126.19 (2)
O4iii—Na1—O4	180.0	O2—Na2—O6xii	145.12 (3)
O5iii—Na1—O4viii	106.41 (3)	O6xi—Na2—O6xii	52.71 (4)