Synthesis and crystal structure of a new alluaudite-like iron phosphate Na2CaMnFe(PO4)3

Disodium calcium manganese iron tris(phosphate), Na2CaMnFe(PO4)3, crystallizes in the monoclinic space group C2/c. The structure belongs to the alluaudite structural type and thus, it obeys the X(2)X(1)M(1)M(2)2(PO4)3 general formula. Both the X(2) and X(1) sites are fully occupied by sodium, while M(1) is occupied by calcium and M(2) exhibits a statistical distribution of iron and manganese.


Chemical context
A promising line of research in the materials science field is the creation of materials based on inorganic phosphates, which have considerable potential for use in laser engineering, optics and electronics owing to their non-linear optical, electrical and luminescent properties. In recent years, iron monophosphates have assumed great importance for their promising applications in several fields such as catalysis (Moffat, 1978), corrosion inhibition (Meisel et al., 1983) and electrochemistry as a positive electrode for lithium ion batteries (Padhi et al., 1997;Ravet et al.,2005;Trad et al., 2010). The physical properties of inorganic materials are related to their structure. A large number of iron phosphates belong to the alluaudite structure type (Yakubovich et al., 1977;Corbin et al., 1986;Korzenski et al., 1998;Hatert et al., 2003;Strutynska et al., 2013) discovered for the first time from natural minerals by Fisher (1955). The term alluaudite refers to a large family of natural or synthetic compounds with the general formula proposed by Moore (1971) of X(2)X(1)M(1)M(2) 2 (PO 4 ) 3 with X and M being cationic sites ranked in descending order of size. The M sites are fully occupied while the X sites can be empty or partially occupied. In this paper, we report a structural study of a new composition of alluaudite-like iron phosphate Na 2 CaMn-Fe(PO 4 ) 3 . In this compound the M(1) and M(2) sites are occupied by Ca and (0.5Mn + 0.5Fe), respectively, while the X(1) and X(2) sites are fully occupied by Na atoms.
In iron phosphates adopting the alluaudite-type structure, the M(2) site is often preferentially occupied by iron with oxidation state +III. Consequently, and on basis of the Mö ssbauer spectroscopy results observed in similar compounds, the presence of Fe II and Mn III in the M(2) site was not considered in the Na 2 CaMnFe(PO 4 ) 3 compound. Indeed, in Na 2 Mn 2 Fe(PO 4 ) 3 (Hidouri et al., 2011), iron and manganese adopt exclusively the oxidation states +III and +II, respectively, whereas in NaMnFe 2 (PO 4 ) 3 (Trad et al., 2010), Mn III and Fe II were observed in very low amounts, leading to a Mn/ Fe ratio close to 1.

Structural commentary
The structure of the title compound consists of infinite chains ( Fig. 1) formed by a succession of pairs of M(2)O 6 octahedra linked together by common edges and sharing edges with a strongly distorted M(1)O 8 polyhedron. Connected equivalent chains through the PO 4 tetrahedra lead to the formation of sheets stacked parallel to the ac plane ( Fig. 2) and interconnected along the b axis by PO 4 tetrahedra. The resulting three-dimensional anionic framework exhibits two kinds of tunnels parallel to the c axis situated at (1/2, 0, z) and (0, 0, z) ( Fig. 3) and occupied by the Na + ions.  (8) , respectively. In the P1O 4 and P2O 4 tetrahedra, the P-O distances vary between 1.521 (2) and 1.547 (2) Å . Their mean distances hP1-Oi= 1.538 (2) Å and hP2-Oi= 1.537 (2) Å are in a good accordance with the value of 1.537 Å calculated by Baur (1974) for monophosphate groups.
Assuming sodium-oxygen distances below 3.0, both the Na1 and Na2 sites are surrounded by six oxygen atoms. Their environments approximate strongly distorted octahedra (Fig. 5). Note that in the ideal alluaudite-type structure, both X(2) and X(1) sites are eightfold coordinated, such as for example in Na 2 Mn 2 Fe(PO 4 ) 3 and Na 2 Cd 2 Fe(PO 4 ) 3 (Hidouri et al., 2011). However, in Na 4 CaFe 4 (PO 4 ) 6 (Hidouri et al., 2004), the coordination numbers of the X(1) and X(2) sites are eight and six, respectively. The decrease of the X(2) coordination number seems to be related to the presence of calcium (0.5 Ca + 0.5 Na) in the M(1) site. In the title compound, the decrease View of a chain showing the distorted octahedral sites M(1) (orange polyhedra) and M(2) (cyan polyhedra).

Figure 2
View showing a sheet made of MO 6 octahedra and PO 4 tetrahedra (light grey).

Figure 3
View of the alluaudite structure in the ab plane. The polyhedra represent a chain of MO 6 octahedra parallel to [101]; Tunnel 1 (light-green atoms) and Tunnel 2 (dark-green atoms). of the coordination numbers from eight to six for both the X(1) and X(2) sites is probably related to the increase of the calcium content in the M(1) site, which becomes exclusively occupied by calcium.

Synthesis and crystallization
Single crystals of the title compound were obtained in a flux of sodium dimolybdate Na 2 Mo 2 O 7 . A starting mixture of appropriate amounts of Fe(NO 3 ) 3 Á9H 2 O (3.999 g); Mn(NO 3 ) 2 Á6H 2 O (2.472 g); CaCO 3 (0.985 g); (NH 4 ) 2 HPO 4 (3.921 g); Na 2 CO 3 (1.845 g) and MoO 3 (2.148 g) was dissolved in nitric acid and then dried for 24 h at 353 K. The dry residue was well ground in an agate mortar and was gradually heated up to 873 K in a platinum crucible to evacuate the decomposition products NH 3 , CO 2 and H 2 O. Then, the obtained product was melted for 1 h at 1073 K and was cooled slowly to 473 K at a rate of 10 K h À1 . Finally, hexagonally shaped brown crystals of Na 2 CaMnFe(PO 4 ) 3 were obtained after washing the mixture with boiling water. The environment of cations (a) Na1 and (b) Na2. Computer programs: CAD-4 EXPRESS (Enraf-Nonius, 1994), XCAD4 (Harms & Wocadlo, 1995), SIR92 (Altomare et al., 1993), SHELXL2014 (Sheldrick, 2015), DIAMOND (Brandenburg, 1999) and WinGX (Farrugia, 2012

Crystal data
Na 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.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )