Crystal structure of (Na0.70)(Na0.70,Mn0.30)(Fe3+,Fe2+)2Fe2+(VO4)3, a sodium-, iron- and manganese-based vanadate with the alluaudite-type structure

The title transition metal vanadate crystallizes in an alluaudite-type structure. The chains characterizing the alluaudite structure are built up from edge-sharing [FeO6] octahedra linked together by VO4 tetrahedra.


Chemical context
Over recent decades, the synthesis and structural characterization of transition-metal-based functional materials adopting layered or channel structures has been the focus of much scientific work. In accordance with widespread studies devoted to the improvement of those materials, we have contributed to the search for new functional materials by undertaking synthesis and structural characterization of new transition and alkali metal phosphates exhibiting channel structures and belonging to the well-known alluaudite structure type (Moore, 1971) that can be represented by the general formula A(1)A(2)M(1)M(2) 2 (XO 4 ) 3 . The M(1) and M(2) sites accommodate di-or trivalent cations in an octahedral environment and are connected to the tetrahedral XO 4 groups, leading to an open-framework structure. Alluauditetype phosphates are of special interest as positive electrode materials in lithium and sodium batteries. For instance, the alluaudite-type lithium manganese phosphate Li 0.78 Na 0.22 MnPO 4 is proposed by Kim et al. (2014) as a promising new positive electrode for Li rechargeable batteries. Furthermore, in the more active alluaudite-type cathode material for sodium-ion batteries, Na 2 Fe 3-x Mn x (PO 4 ) 3 , the electrochemical performance is associated either with morphology or with the electronic and crystalline structure (Huang et al., 2015).

Structural commentary
The preparation of this compound by melting a mixture of three metal oxide precursors in addition to vanadium oxide forced us to explore several crystallographic models. 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 basic building units of the structure are shown in Fig. 1 The principal building units in the structure of the title compound. Displacement ellipsoids are drawn at the 50% probability level.

Figure 3
A layer perpendicular to [010], resulting from the connection of chains via vertices of VO 4 tetrahedra.

Figure 4
Polyhedral representation of (Na located on an inversion centre (4b), and Na2 + is present at a site on a twofold rotation axis (4e) with 70% occupancy. All other sites are fully occupied. Nearly the same cationic distribution was reported by Yakubovich et al. (1977) for the alluaudite-type phosphate Na 2 (Fe 3+ ,Fe 2+ ) 2 Fe 2+ (PO 4 ) 3 . The crystal structure of the title compound is built up from edge-sharing [FeO 6 ] octahedra, leading to the formation of kinked chains running along [101] (Fig. 2). These chains are held together through the vertices of VO 4 tetrahedra, generating layers perpendicular to [010] (Fig. 3). Thereby an open three-dimensional framework is formed that delimits two types of channels parallel to [001] in which the disordered (Na1 + /Mn1 2+ ) and statistically occupied Na2 + cations are accommodated (Fig. 4). The (Na1 + ,Mn1 2+ ) site has a distorted octahedral oxygen environment, with (Na1 + ,Mn1 2+ )-O bond lengths between 2.4181 (16) and 2.5115 (15) Å . The Na2 + cation is coordinated by eight oxygen atoms with Na2-O distances in the range 2.4879 (18) to 2.982 (3) Å . The disorder of Na + in the channels might admit ionic mobility for this material.

Synthesis and crystallization
The title compound was prepared by solid-state reactions in air. Sodium nitrate, metallic manganese and iron were mixed with vanadium oxide in proportions corresponding to the molar ratios Na:Mn:Fe:V = 2:2:1:3. The reaction mixture underwent several heat treatments in a platinum crucible until the melting temperature situated at about 1030 K was reached. Each thermal treatment was interspersed with grinding in an agate mortar. The resulting product contained black single crystals crystals of a suitable size for the X-ray diffraction study.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. For the (Na1 + ,Mn1 2+ ) site, full occupation was assumed, with the sum of the site occupation factors constrained to be 1. The site-occupation factor of Na2 + was refined freely. In the last step of the refinement, the site occupation factors were fixed to fulfill electro-neutrality. Reflection (1 5 0) was probably affected by the beam-stop and was omitted from the refinement. The remaining maximum and minimum electron density peaks are located 0.59 and 0.41 Å from Fe2 and V2, respectively.  (Farrugia, 2012), DIAMOND (Brandenburg, 2006) and publCIF (Westrip, 2010 program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015b); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012) and DIAMOND (Brandenburg, 2006); software used to prepare material for publication: publCIF (Westrip, 2010).