Crystal structure of strontium dicobalt iron(III) tris(orthophosphate): SrCo2Fe(PO4)3

The transition metal orthophosphate, SrCo2Fe(PO4)3, crystallizes in an alluaudite-type structure. The chains characterizing the alluaudite structure are built up from edge-sharing [CoO6] octahedra linked together by PO4 tetrahedra.

The title compound, SrCo 2 Fe(PO 4 ) 3 , has been synthesized by a solid-state reaction. It crystallizes with the -CrPO 4 type structure. In this structure, all atoms are on special positions of the Imma space group, except for two O atoms which are located on general positions. The three-dimensional network in the crystal structure is made up of two types of layers stacked normal to (100). The first layer is built from two edge-sharing CoO 6 octahedra, leading to the formation of Co 2 O 10 dimers that are connected to two PO 4 tetrahedra by a common edge and corners. The second layer results from apex-sharing FeO 6 octahedra and PO 4 tetrahedra, which form linear chains alternating with a zigzag chain of Sr II cations. These layers are linked together by common vertices of PO 4 tetrahedra and FeO 6 octahedra to form an open three-dimensional framework that delimits two types of channels parallel to [100] and [010] where the Sr II cations are located. Each Sr II cation is surrounded by eight O atoms.

Chemical context
The phosphate literature includes important works on the structural study of transition metal phosphates. The basic framework is built from tetrahedrally coordinated phosphorus linked to transition metals M in various environments, such as MO n (n = 4, 5 or 6). The manner in which polyhedra are interconnected generates important structure types with porous or lamellar set-ups that can exhibit interesting physical properties. Accordingly, widespread studies have been devoted to this family of compounds, stimulated by the wide range of potential and commercial applications of these materials. Examples include applications in catalysis, as ion exchangers and in the manufacture of lithium and sodium rechargeable batteries. One particular scientific area in our laboratory is focused on investigating new functional phosphates belonging to the alluaudite (Moore, 1971) or -CrPO 4 (Attfield et al., 1988) structure types, owing to their potential use as new cathode materials for battery devices (Trad et al., 2010;Kim et al., 2014;Huang et al., 2015).
In search of a new promising phosphate, a solid-state chemistry investigation of A 2 O-MO-M 0 2 O 3 -P 2 O 5 systems was undertaken. The present work reports on synthesis and crystal structure of the new strontium cobalt iron phosphate, SrCo 2-Fe(PO 4 ) 3 , which has the -CrPO 4 type structure.

Structural commentary
In the title phosphate, SrCo 2 Fe(PO 4 ) 3 , all atoms are on special positions, except two oxygen atoms (O3, O4) which are on general positions of the Imma space group. Its three-dimensional structure is constructed on the basis of PO 4 tetrahedra, FeO 6 and CoO 6 octahedra, as shown in Fig The principal building units in the structure of the title compound. Displacement ellipsoids are drawn at the 50% probability level.

Figure 3
A view along the a axis, showing a layer resulting from chains connected via vertices of PO 4 tetrahedra and FeO 6 octahedra, alternating with a zigzag chain of Sr atoms. between these polyhedra produces two types of layers stacked normal to (100). The first layer is built from two edge-sharing CoO 6 octahedra, leading to the formation of Co 2 O 10 dimers, which are connected to two PO 4 tetrahedra by a common edge and vertex, as shown in Fig. 2. The second layer is formed by alternating FeO 6 octahedra and PO 4 tetrahedra, which share corners, building linear chains that surround a zigzag chain of Sr II cations (see Fig. 3). The layers are joined by the apices of PO 4 tetrahedra and FeO 6 octahedra, giving rise to an open three-dimensional framework that delimits two types of channels parallel to [100] and [010] where the Sr II cations are located, as shown in Fig. 4 and Fig. 5. This structure is characterized by a stoichiometric composition in which the Sr atom is surrounded by eight oxygen atoms with Sr-O bond lengths that vary between 2.6561 (13) and 2.6690 (9)Å . The same Sr environment is observed in the manganese homologue phosphates, namely MMn II 2 Mn III (PO 4 ) 3 (M = Pb, Sr, Ba).

Synthesis and crystallization
The title phosphate, SrCo 2 Fe(PO 4 ) 3 , was synthesized in a solid-state reaction by mixing nitrates of strontium, cobalt and iron along with NH 4 H 2 PO 4 , taken in the molar proportions Sr:Co:Fe:P = 1:2:1:3. After a series of heat treatments up to 873 K in a platinum crucible, interspersed with grinding, the reaction mixture was heated to the melt (1343 K). The molten product was then cooled to room temperature at 5 K/h. The resulting solid contained brown crystals of a suitable size for X-ray diffraction.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. The highest peak and the deepest hole in the final Fourier map are at 0.63 and 0.68 Å from Sr1 and P2, respectively. The distinction between cobalt and iron by X-ray diffraction is nearly impossible. Therefore we have examined several crystallographic models during the crystal structure refinements of the title compound. Based on the stoichiometric ratio of 1:2 for iron and cobalt in the starting materials, we assumed the same ratio in the crystal structures with oxidation states of +II for cobalt and and +III for iron. The best model is obtained with Fe1 and Co1 atoms in the Wyckoff positions 4a (2/m) and 8g (2), respectively. This cationic distribution in this model corresponds to the stoichiometry of the expected compound, in addition to the electric neutrality in the structure in reasonable agreement with the final model.  (Brandenburg, 2006) and publCIF (Westrip, 2010). program(s) used to solve structure: SHELXT2014 (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015b); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012) and DIAMOND (Brandenburg, 2006); software used to prepare material for publication: publCIF (Westrip, 2010).

Strontium dicobalt iron(III) tris(orthophosphate)
Crystal data 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.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )