Crystal structure of a silver-, cobalt- and iron-based phosphate with an alluaudite-like structure: Ag1.655Co1.64Fe1.36(PO4)3

The transition metal orthophosphate Ag1.655Co1.64Fe1.36(PO4)3 crystallizes in an alluaudite-type structure. The chains characterizing the alluaudite structure are built up from edge-sharing [CoO6] and [FeO6] octahedra linked together by PO4 tetrahedra. The Ag+ cations are located in two types of channels in the resulting framework.

The new silver-, cobalt-and iron-based phosphate, silver cobalt iron tris(orthophosphate), Ag 1.655 Co 1.64 Fe 1.36 (PO 4 ) 3 , was synthesized by solid-state reactions. Its structure is isotypic to that of Na 2 Co 2 Fe(PO 4 ) 3 , and belongs to the alluaudite family, with a partial cationic disorder, the Ag I atoms being located on an inversion centre and twofold rotation axis sites (Wyckoff positions 4a and 4e), with partial occupancies of 0.885 (2) and 0.7688 (19), respectively. One of the two P atoms in the asymmetric unit completely fills one 4e site while the Co and Fe atoms fill another 4e site, with partial occupancies of 0.86 (5) and 0.14 (5), respectively. The remaining Co 2+ and Fe 3+ cations are distributed on a general position, 8f, in a 0.39 (4):0.61 (4) ratio. All O atoms and the other P atoms are in general positions. The structure is built up from zigzag chains of edge-sharing [MO 6 ] (M = Fe/Co) octahedra stacked parallel to [101]. These chains are linked together through PO 4 tetrahedra, forming polyhedral sheets perpendicular to [010]. The resulting framework displays two types of channels running along [001], in which the Ag I atoms (coordination number eight) are located.

Chemical context
Compounds belonging to the large alluaudite structural family (Moore, 1971;Moore & Ito, 1979;Hatert et al., 2000Hatert et al., , 2004 have been of continuing interest owing to their open-framework architecture, with hexagonal-shaped channels, and their physical properties. This fact is amply justified by their practical applications, for example as corrosion inhibitors, passivators of metal surfaces, and catalysts (Korzenski et al., 1999). In addition, interest in alluaudite phosphates with monovalent cations has continued to grow in the electrochemical field, where they have applications as positive electrodes in lithium and sodium batteries (Trad et al., 2010). Accordingly, our attention is mostly focused on the elaboration and structural characterization of new alluaudite-type phosphates within the A 2 O-MO-P 2 O 5 systems (A = monovalent cation M = divalent cation). For instance, most recently, the hydrothermal investigation of the Na 2 O-MO-P 2 O 5 pseudo-ternary system has allowed the isolation of the sodium-and magnesium-based alluaudite phosphate NaMg 3 (PO 4 )(HPO 4 ) 2 (Ould Saleck et al., 2015). On the other hand, within the Na 2 O-CoO-Fe 2 O 3 -P 2 O 5 and Na 2 O-ZnO-Fe 2 O 3 -P 2 O 5 pseudo-quaternary systems, solid-state synthesis has allowed Na 2 Co 2 Fe(PO 4 ) 3 (Bouraima et al., 2015) and Na 1.67 Zn 1.67 Fe 1.33 (PO 4 ) 3 (Khmiyas et al., 2015) to be obtained. With the same objective, a new ISSN 2056-9890 silver-, cobalt-and iron-based alluaudite-type phosphate, namely Ag 1.655 Co 1.64 Fe 1.36 (PO 4 ) 3 , has been synthesized by means of solid-state reactions and characterized by single crystal X-ray diffraction.

Structural commentary
In the new isolated compound, either cobalt or iron atoms are distributed in the two octahedral sites while the phosphorus atoms are tetrahedrally coordinated, as shown in Fig. 1 (Fig. 2). The junction between these chains is ensured by sharing vertices with the PO 4 tetrahedra so as to form an open layer perpendicular to [010] (Fig. 3). The three-dimensional framework resulting from the stacking of the sheets along the b-axis direction delimits channels parallel to [001] in which the Ag + cations are accommodated, as shown in Fig. 4.

Comparison with a related structure
It is worth mentioning that the distribution of metallic cations observed in the case of the silver-cobalt-iron-based phosphate is not encountered in the sodium homologue. Hence, in the title silver-based phosphate, the octahedral M1 site (Wyckoff position 8f) is occupied to 60% by Fe1 and to 40% by Co1. The octahedrally surrounded M2 site (Wyckoff position 4e) is essentially occupied by Fe2 atoms (43%) along with a small amount of Co2 (7%). However, in the Na 2 Co 2-Fe(PO 4 ) 3 phosphate, the M1 and M2 sites are entirely occupied by Fe and Co atoms, respectively. For the mixed sites, the occupancy rate was refined without any constraint. The results     The principal building units in the structure of the title compound. Displacement ellipsoids are drawn at the 50% probability level.

Synthesis and crystallization
The title compound was isolated from solid-state reactions in air by mixing nitrates of silver, cobalt and iron with phosphoric acid. The various precursors are taken in the molar ratio Ag:Co:Fe:P = 2:2:1:3. The mixture was stirred at room temperature overnight. After different heat treatments in a platinum crucible at up to 873 K, the reaction mixture was heated to the melting temperature of 1221 K. The molten product was then cooled to room temperature at a rate of 5 K h À1 . Brown homogeneous crystals corresponding to the title compound of a suitable size for X-ray diffraction were obtained.

Refinement
Crystal data, data collection and structure refinement details are summarized in

Silver cobalt iron 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.