Crystal structure of a sodium, zinc and iron(III)-based non-stoichiometric phosphate with an alluaudite-like structure: Na1.67Zn1.67Fe1.33(PO4)3

The transition-metal orthophosphate Na1.67Zn1.67Fe1.33(PO4)3 cristallizes in an alluaudite-type structure. The chains characterizing the alluaudite structure are then built up from [M 2O10] (M = Fe/Zn) units alternating with [ZnO6] octahedra. This structure is characterized by a cationic disorder in one tunnel and in the general position.

The new title compound, disodium dizinc iron(III) tris(phosphate), Na 1.67 Zn 1.67 Fe 1.33 (PO 4 ) 3 , which belongs to the alluaudite family, has been synthesized by solid-state reactions. In this structure, all atoms are in general positions except for four, which are located on special positions of the C2/c space group. This structure is characterized by cation substitutional disorder at two sites, one situated on the special position 4e (2) and the other on the general position 8f. The 4e site is partially occupied by Na + [0.332 (3)], whereas the 8f site is entirely filled by a mixture of Fe and Zn. The full-occupancy sodium and zinc atoms are located at the Wyckoff positions on the inversion center 4a (1) and on the twofold rotation axis 4e, respectively. Refinement of the occupancy ratios, bond-valence analysis and the electrical neutrality requirement of the structure lead to the given composition for the title compound. The threedimensional framework of this structure consists of kinked chains of edgesharing octahedra stacked parallel to [101]. The chains are formed by a succession of trimers based on [ZnO 6 ] octahedra and the mixed-cation Fe III /Zn II [(Fe/Zn)O 6 ] octahedra [Fe III :Zn III ratio 0.668 (3)/0.332 (3)]. Continuous chains are held together by PO 4 phosphate groups, forming polyhedral sheets perpendicular to [010]. The stacked sheets delimit two types of tunnels parallel to the c axis in which the sodium cations are located. Each Na + cation is coordinated by eight O atoms. The disorder of Na in the tunnel might presage ionic mobility for this material.

Chemical context
Alkali transition-metal phosphates belonging to the alluaudite family constitute one of the most diverse and rich classes of minerals, and have been studied intensively over the last few years. Owing to their outstanding physico-chemical properties, these compounds have many potential applications in various fields, such as catalytic activity (Kacimi et al., 2005) and as promising cathodes for sodium-ion batteries through the presence of mobile cations located in the tunnels of the open three-dimensional framework (Huang et al., 2015). In their recent study, Huang et al. (2015) point out that the electrochemical performance is not only associated with morphology, but also with the electronic and crystalline structure.
Accordingly, a large number of alluaudite phases with alkali cations in the tunnels have been reported. Nevertheless, the presence of alkali metals in the tunnels of synthetic alluaudite phases is frequently accompanied by cationic distributions that lead to non-stoichiometric compositions, such as: (Na 0.38 ,Ca 0.31 )MgFe 2 (PO 4 ) 3 (Zid et al., 2005); NaFe 3.67 (PO 4 ) 3 (Korzenski et al., 1998) (Hermann et al., 2002). As part of our study on alluaudite-related phosphates (Bouraima et al., 2015;Assani et al., 2011), we report the synthesis and the crystal structure of a new sodium, zinc and iron-based non-stoichiometric phosphate, namely Na 1.67 Zn 1.67 Fe 1.33 (PO 4 ) 3 .

Structural commentary
The alluaudite structure of the title compound crystallizes in the monoclinic space group C2/c, with Z = 4. The principal building units of the crystal structure are represented in Fig. 1.
Refinement of the occupancy fractions, bond-valence analysis based on the formula proposed by Brown & Altermatt (1985) and the required electrical neutrality of the structure lead to the formula Na 1.67 Zn 1.67 Fe 1.33 (PO 4 ) 3 for the title compound. The mixed Fe1 and Zn1 atoms are located at the general position 8f with Fe 3+ /Zn 2+ occupancy fractions of 0.668 (3)/ 0.332 (3), and form a highly distorted [(Fe1/Zn1)O 6 ] octahedral group, with Fe 3+ /Zn 2+ -O bond lengths ranging from 1.951 (1) to 2.209 (1)Å . The Zn2 atom is surrounded by six oxygen atoms, building a slightly distorted octahedron with an average Zn2-O bond length of 2.153 (1) Å .
The crystal structure of this phosphate compound consists of infinite kinked chains of two edge-sharing [Fe1/Zn1O 6 ] octahedra leading to the formation of [(Fe1,Zn1) 2 O 10 ] dimers that are connected by a common edge to [Zn2O 6 ] octahedra, as shown in Fig. 2. These chains are linked by PO 4 tetrahedral groups, forming a stack of sheets perpendicular to [010] and alternating with sodium layers, as shown in Fig. 3, which reveal small tunnels along the [201] direction. The three-dimensional framework also encloses two types of large tunnels, in which the Na + cations reside, as shown in Fig. 4. The site 4e centred on the first tunnel is partially occupied by Na1 [0.332 (3)], whereas Na2 occupies site 4a centred on the second tunnel. Each sodium atom is surrounded by eight oxygen atoms with Na1-O and Na2-O bond lengths in the ranges 2.448 (1)-2.908 (2) Å , and 2.324 (1)-2.901 (1) Å , respectively. The displacement ellipsoids of the partially occupied atom Na1 are rather larger than those of the rest of the atoms. Most probably this is due to the size of the channels, which allows atom The principal building units in the structure of the title compound. Displacement ellipsoids are drawn at the 50% probability level.

Figure 2
A view along the b axis of a sheet resulting from chains connected by vertices of PO 4 tetrahedra. Na1 to have more freedom. The disorder of Na in the tunnel may presage ionic mobility for this material.

Synthesis and crystallization
Single crystals of Na 1.67 Zn 1.67 Fe 1.33 (PO 4 ) 3 were synthesised by conventional solid-state reaction (Girolami et al., 1999). The nitrate-based sodium, zinc and iron precursors, in addition to the 85 wt% H 3 PO 4 were taken in proportions corresponding to the molar ratio Na:Zn:Fe:P = 2:2:1:3. The resulting reaction mixture was ground in an agate mortar and progressively heated in a platinum crucible to the melting temperature of 1135 K. The melted product was cooled at a rate of 5 K/h. The product was obtained as transparent brown crystals corresponding to the title phosphate.