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

The transition metal orthophosphates Na2 M 2Fe(PO4)3 (M = Co, Ni) crystallize in an alluaudite-type structure. The chains characterizing the alluaudite structure are then built up from [M 2O10] units alternating with [FeO6] octahedra.


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 transitionmetal 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 openframework 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], alluauditetype compounds exhibit electronic and/or ionic conductivity, as has been shown by Warner et al. (1993). In this context, we have explored A 2 O-MO-P 2 O 5 systems, where A is a monovalent cation and M a divalent cation. A new alluaudite structure of formula Na 2 Co 2 Fe(PO 4 ) 3 was synthesized by solid-state reaction. During our investigation of these systems, we characterized the following compounds: AgMg 3 (PO 4 )-(HPO 4 ) 2 (Assani et al., 2011a), Ag 2 Ni 3 (HPO 4 )(PO 4 ) 2 (Assani et al., 2011b) and Na 2 Ni 2 Fe(PO 4 ) 3 (Essehli et al., 2011). The present paper reports the solid-state synthesis and characterization of a new transition-metal phosphate, namely, Na 2 Co 2 Fe(PO 4 ) 3 .

Figure 2
A layer perpendicular to the b axis, resulting from the chains connected via the vertices of the PO 4 tetrahedra.

Figure 3
Polyhedral representation of Na 2 Co 2 Fe(PO 4 ) 3 showing the tunnels running along the [001] direction.

Synthesis and crystallization
Na 2 Co 2 Fe(PO 4 ) 3 was synthesized by a solid-state reaction by mixing the precursors of sodium (Na 2 CO 3 ), cobalt (CoCO 3 ), iron (Fe 2 O 3 ) 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 À1 . The resulting product contained brown crystals of a suitable size for the X-ray diffraction study.

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 F 2 against all reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on all data will be even larger.  (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)