NASICON-related Na3.4Mn0.4Fe1.6(PO4)3

The solid solution, sodium [iron(III)/manganese(II)] tris(orthophosphate), Na3.4Mn0.4Fe1.6(PO4)3, was obtained using a flux method. Its crystal structure is related to that of NASICON-type compounds. The [(Mn/Fe)2(PO4)3] framework is built up from an (Mn/Fe)O6 octahedron (site symmetry 3.), with a mixed Mn/Fe occupancy, and a PO4 tetrahedron (site symmetry .2). The Na+ cations are distributed over two partially occupied sites in the cavities of the framework. One Na+ cation (site symmetry -3.) is surrounded by six O atoms, whereas the other Na+ cation (site symmetry .2) is surrounded by eight O atoms.

The solid solution, sodium [iron(III)/manganese(II)] tris-(orthophosphate), Na 3.4 Mn 0.4 Fe 1.6 (PO 4 ) 3 , was obtained using a flux method. Its crystal structure is related to that of NASICON-type compounds. The [(Mn/Fe) 2 (PO 4 ) 3 ] framework is built up from an (Mn/Fe)O 6 octahedron (site symmetry 3.), with a mixed Mn/Fe occupancy, and a PO 4 tetrahedron (site symmetry .2). The Na + cations are distributed over two partially occupied sites in the cavities of the framework. One Na + cation (site symmetry 3.) is surrounded by six O atoms, whereas the other Na + cation (site symmetry .2) is surrounded by eight O atoms.
There are two Na sites (Wyckoff positions 6b and 18e), one mixed occupied Mn/Fe site (12c), one P site (18e) and two O sites (36f) in the asymmetric unit of (I) (Fig. 1). The basic building block of the structure is the [(Mn/Fe) 2 (PO 4 ) 3 ] unit, which consists of two (Mn/Fe)O 6 polyhedra interlinked by three bridging PO 4 -tetrahedra (Fig. 2). These fragments alternate with Na1O 6 -polyhedra along [001] forming ribbons, which in turn are interconnected by PO 4 -tetrahedra forming a three-dimensional framework (Fig. 2)

Experimental
The title compound was obtained during investigation of the melting system Na 2 O-P 2 O 5 -Fe 2 O 3 -MnO. A mixture of NaPO 3 (12.24 g), Na 2 CO 3 (1.908 g), Fe 2 O 3 (2.4 g) and MnCO 3 . Mn(OH) 2 (3.8 g) was ground in an agate mortar, placed in a platinum crucible and heated up to 1273 K. The melt was kept at this temperature for 3 h. After that, the temperature was cooled down to 973 K at a rate of 10 K/h. The light-violet crystals of (I) were recovered using hot water. The chemical composition of single-crystal was verified using EDX analysis.

Refinement
For refinement of the Fe/Mn ratio and the Na-content, SUMP instructions in SHELXL (Sheldrick, 2008) were employed, assuming full occupancy of the (Fe/Mn) site and an average charge of the (Fe/Mn) and Na sites of +9. The refined composition is close to that determined by EDX measurements. The highest remaining peak in the final difference Fourier map is 0.76 A from P1 and the deepest hole is 1.09 Å from the same atom.

Figure 1
The asymmetric unit of (I), showing displacement ellipsoids at the 50% probability level.  Elementary fragments and three-dimensional framework in the title compound. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.