Synthesis and crystal structure of calcium dizinc iron(III) tris(orthophosphate), CaZn2Fe(PO4)3

The orthophosphate CaZn2Fe(PO4)3 crystallizes in the monoclinic system. The structure of this phosphate is built up from edge-sharing [ZnO5] polyhedra linked together by FeO6 octahedra and PO4 tetrahedra.

Single crystals of the title compound, CaZn 2 Fe(PO 4 ) 3 , were synthesized by conventional solid-state reaction. In the asymmetric unit, all atoms are located in fully occupied general positions of the P2 1 /c space group. The zinc atoms are located on two crystallographically independent sites with tetrahedral and distorted triangular-based bipyramidal geometries. Two edge-sharing triangular bipyramidal ZnO 5 units form a dimer, which is linked to slightly deformed FeO 6 octahedra via a common edge. The resulting chains are interconnected through PO 4 tetrahedra to form a layer perpendicular to the b axis. Moreover, the remaining PO 4 and ZnO 4 tetrahedra are linked together through common vertices to form tapes parallel to the c axis and surrounding a chain of Ca 2+ cations to build a sheet, also perpendicular to the b axis. The stacking of the two layers along the b axis leads to the resulting three-dimensional framework, which defines channels in which the Ca 2+ cations are located, each cation being coordinated by seven oxygen atoms.

Chemical context
Microporous compounds with an open anionic framework containing transition metals have been widely studied during recent years, especially iron phosphates, because of their potential applications in several fields such as gas sensing (Abdurahman et al., 2014), catalysis (Ai, 1999), as cathode materials for rechargeable lithium batteries (Masquelier et al., 1998), biocompatibility of glass fibres for tissue engineering (Ahmed et al., 2004), and immobilization of spent nuclear fuel (Mesko & Day, 1999). Metal phosphates with an open framework can exhibit different architectures such as linearchain, layered and three-dimensional structures with channels or cavities where a variety of cations with different sizes, ratio and charges are accommodated. The occupancy of the allowed sites by cations can provide different properties such as remarkable flexibility, fast ionic conduction and low thermal expansion, mainly observed in the compounds belonging to the NASICON family with the general formula MM 0 2 P 3 O 12 (where M = alkali metal, alkaline-earth metal or a vacant site and M 0 = Zr, Ti, Hf, etc.; Senbhagaraman et al., 1993). In our previous hydrothermal investigations, a variety of compounds have been synthesized and characterized with different ratios of alkaline earth metal:P, viz. Sr 2 Mn 3 (HPO 4 ) 2 (PO 4 ) 2 (Khmiyas et al., 2013), BaMn II 2 Mn III (PO 4 ) 3 , Mg 7 (PO 4 ) 2 (HPO 4 ) 4 (Assani et al., 2011). In this context, our interest is focused on the synthesis of new iron orthophosphates with an open-framework structure. Accordingly, we have succeeded in synthesizing and structurally characterizing a new calcium, zinc and iron-based open-framework phosphate, namely CaZn 2 Fe(PO 4 ) 3 .

Structural commentary
All atoms in asymmetric unit of the title compound occupy general positions of the P2 1 /c space group. The refinement of this model was very easy and lead to an ordered structure in which the zinc cations occupy two sites with different environments. The coordination numbers of all cations were confirmed by bond-valence-sum calculations (Brown & Altermatt, 1985). The obtained values for Ca II+ , Zn II+ , Fe III+ and P V+ are as expected, viz. Ca1 (1.93), Zn1 (2.00), Zn2 (1.91), Fe1 (3.04), P1 (5.11), P2 (4.97) and P3 (4.94). The crystal structure is build up from PO 4 and Zn1O 4 tetrahedra, distorted triangular-based bipyramidal Zn2O 5 and FeO 6 octahedra, as shown in Fig. 1. The FeO 6 octahedra are slightly deformed with Fe-O distances varying from 1.8908 (8) to 2.1318 (8) Å and share a common edge with the highly distorted [(Zn2) 2 O 8 ] dimer resulting from the edge-sharing of two triangular-based bipyramidal Zn2O 5 units. Sequences of these polyhedra build chains interconnected by PO 4 tetrahedra, forming a layer perpendicular to the b axis, as shown in Fig. 2. The remaining Zn1O 4 tetrahedra are linked to irregular PO 4 groups via common corners, forming tapes parallel to the c axis, which are linked together by Ca 2+ cations in sheets perpendicular to the b axis (see Fig. 3). The obtained threedimensional framework shows one type of channel running along the [001] direction in which the Ca 2+ cations are located, each being coordinated by seven oxygen atoms (Fig. 4).

Database Survey
The formula of the title compound, CaZn 2 Fe(PO 4 ) 3 , is similar to some compounds with alluaudite structures, space group C2/c or the -CrPO 4 structure, space group Imma. However, its structure is different and to our knowledge there is no known isotypic structure. The principal building units in the structure of the title compound. Displacement ellipsoids are drawn at the 50% probability level.

Figure 2
Edge-sharing triangular bipyramidal ZnO 5 units linked to FeO 6 octahedra and to PO 4 tetrahedra, forming a layer perpendicular to the b axis.

Figure 3
A layer perpendicular to the b axis, resulting from the chains connected via vertices of the ZnO 4 and PO 4 tetrahedra.
Co, Ni, Cu) compounds, which are predicted to have the same structures or isotypes are in preparation, while the structures of SrM 2 Fe(PO 4 ) 3 (M = Co, Ni) compounds are isotypic with -CrPO 4 (Bouraima et al., 2016;Ouaatta et al., 2015). Mention may also be made of other similar compounds, for example the phosphates Na 2 Co 2 Fe(PO 4 ) 3 , NaCr 2 Zn(PO 4 ) 3 and Na 1.66 Zn 1.66 Fe 1.34 (PO 4 ) 3 (Bouraima et al., 2015;Souiwa et al., 2015;Khmiyas et al., 2015) adopting the alluaudite structure type. In conclusion, we can say that the structure of this phosphate is similar to the alluaudite structure but with lower symmetry.

Synthesis and crystallization
Single crystals of CaZn 2 Fe(PO 4 ) 3 were synthesized by a conventional solid-state method. Appropriate amounts of metal nitrate reagents, in the presence of H 3 PO 4 85 wt%, were first dissolved in deionized water in the molar ratio Ca:Zn:Fe:P = 2:2:1:3 for 24 h. Then, the resulting solution was evaporated to dryness. The powder residue was ground in an agate mortar and progressively heated in a platinum crucible at a heating rate of 141 K h À1 until melting occurred at 1283 K. The melted product was cooled down at a rate of 5 K h À1 . As result of the reaction, we obtained transparent crystals corresponding to the title compound CaZn 2 Fe(PO 4 ) 3 .

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. The reflections (202) and (330) probably affected by the beam-stop were omitted from the refinement. The maximum and minimum electron densities in the final Fourier map are at 0.56 and 0.44 Å from Ca1 and Zn2, respectively.

Crystal data
CaZn 2  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.