1. Chemical Context
The interest shown by researchers in recent years in the structural study of phosphates is of paramount importance in the sense or they are engaged in the development of new materials based on transition metal phosphates. Indeed the tetrahedral character phosphates related to octahedral transition metals led to open structures having structural characteristics favorable to the mobility of ions. Very often, the presence of transition metal cations in the structure gives rise to remarkable magnetic properties, such as ferro , antiferro  or ferromagnetic behavior . Moreover, one of the axes developed by our team in our laboratory is the search for new materials based phosphate belonging to the family of alluaudite  or α-CrPO4  type structure owing to their outstanding potentialities, particularly, as a new promising cathode material for batteries devices   . The crystal structures belonging to α-CrPO4 family shows a great similarity with of the alluaudite structure, can thus be represented by the general formula A(2)A(1)M(1)M(2)2(PO4)3. Hence, the octahedral M(1) and M(2) sites, accommodating di- or trivalent cations, are connected to the PO4 groups leading to an open framework. The resulting configuration let appear tunnels where the mono- and/or bivalent cations are localized. The cations corresponding positions in tunnels are symbolized by the A(2) and A(1) sites.
In the same concept, our team recently isolated, and then characterize the crystal structures belonging to these different family such as Na2Co2Fe (PO4)3 , Na1, 67Zn1, 67Fe1, 33(PO4)3  and Ag1.655Co1.64Fe1.36(PO4)3  belonging to the alluaudite type structure. Besides, bivalent and trivalent based phosphates, namely SrNi2Fe(PO4)3 , SrCo2Fe(PO4)3  and MMnII2MnIII(PO4)3 (M = Pb, Sr, Ba)   , with rarely observed mixed-valent MnII/III cations, are isolated and their structures are isotype with α-CrPO4. Continuing our investigation of the ternary system MO-M'O-P2O5 (M = bivalent cation and M’ = metal of transition), we had success to synthesize a new transition metal phosphate compound named BaCo2Fe (PO4)3. The present work reports synthesis and crystal structure of the new Baryum cobalt iron phosphate, isolated within the BaO-CoO-Fe2O3-P2O5 quaternary, belonging to theα-CrPO4 type structure.
2. Structural Commentary
The crystal data for title compound show that they are isostructural, crystallize with the α-CrPO4 structure. In this structure, all atoms are in special positions, except two oxygen atoms (O1, O2) which are in general position of the Imma space group. The asymmetric unit contains six cationic sites including two for Ba, two for P and two M(1) and M(2)2 containing the cobalt and iron atoms. This asymmetric unit formed by a sequence octahedral and tetrahedral giving a three-dimensional structure constructed on the basis of PO4 tetrahedra, FeO6 and CoO6 octahedra. The connection different between polyhedral produces two chain types running along . The first chain is built up from two edge-sharing CoO6 octahedra leading to the formation of Co2O10 dimers which are connected to two PO4 tetrahedra by a common edge. The second one is formed by alternating FeO6 octahedra and PO4 tetrahedra sharing vertices (Figure 1). The junction of both chains, by sharing common vertices of tetrahedra PO4, gives arise an open three-dimensional framework that delimits two types of tunnels parallel to  and  where the BaII cations are accommodated as shown in Figure 2(a) and Figure 2(b). The new structure namely BaCo2Fe (PO4)3 is characterized by a single type of baryum atom surrounded by eight oxygen atoms with Ba-O bond length varies between 2.7290 Å and 2.7621 Å. Moreover, the bond valence sums for the different crystallographic site calculated using the empirical parameters  is 2.28, 2.95, 1.949, 4.847, 4.969 for BaII+, FeIII+, CoII+, P1V+, P2V+ respectively. The bond valence sum is very close to the ideal value, thus confirming the non-disordered distribution of FeIII+ and CoII+ on the M(1) and M(2) sites.
Figure 1. View of two types channels along c axis .
Figure 2. (a) View along a axis polyhedral of SrCo2Fe(PO4)3 of tunnels parallel to the (010) plane. (b) Polyhedral representation of SrCo2Fe(PO4)3 with channels running parallel to .
3. Synthesis and Crystallization
Commercially available Fe(NO)3, 9H2O, (NH4)2HPO4, Ba(NO3)2, and (CH3COO)2Co, 4H2O were used as starting for the synthesis of new compound, BaCo2Fe(PO4)3. This phase has been synthesized by solid-state reaction by mixing a stochiometric amount of the starting precursors. The mixture is finely ground in an agate mortar for a sufficient time then heat treatments, realized in platinum crucible, up to 873 K, the reaction mixture is heated in a crystallization furnace to the melting point, situated at 1246.150 K. The molten product was then cooled to room temperature at 5˚K\h rate. The final product contains brown crystals with a suitable size for the X-ray diffraction corresponding to the title compound.
We have been able to synthesize, by diffusion in the solid state, the new phosphate of formula. Its crystalline structure was determined from the X-ray diffraction data. It crystallizes in orthorhombic mesh with the Imma space group, a = 10,5067(2)Å, b = 13,3155(3)Å and c = 6,6471(2)Å as the mesh parameters. The particularity of this new phosphate is that it is similar toα-CrPO4, its structure is formed to tetrahedrons and octahedra CoO6 and FeO6. The combination of these different coordination polyhedra leads to the formation of a sheet, the latter are linked together by chains to build a three-dimensional network releasing tunnels in which are housed the Ba2+ cations.
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