Redetermination of Fe2[BP3O12]

Explorations of phases in the quaternary FeIII–BIII–PV–O system prepared by the high temperature solution growth (HTSG) method led to single-crystal growth of anhydrous diiron(III) borotriphosphate, Fe2[BP3O12]. This phase has been synthesized previously as a microcrystalline material and its structure refined in space group P3 from powder X-ray diffraction data using the Rietveld method [Chen et al. (2004 ▶). J. Inorg. Mater. 19, 429-432]. In the current single-crystal study, it was shown that the correct space group is P63/m. The three-dimensional structure of the title compound is built up from FeO6 octahedra (3.. symmetry), trigonal–planar BO3 groups ( symmetry) and PO4 tetrahedra (m.. symmetry). Two FeO6 octahedra form Fe2O9 dimers via face-sharing, while the anionic BO3 and PO4 groups are connected via corner-sharing to build up the [BP3O12]6− anion. Both units are interconnected via corner-sharing.


Related literature
Reviews on the crystal chemistry of borophosphates were given by Kniep et al. (1998) and Ewald et al. (2007). For the previous powder study of Fe 2 [BP 3 O 12 ], see: Chen et al. (2004). For the structure of a related borophosphate, see: Zhao et al. (2009). Meisel et al. (2004) have reported the structure of V 2 [BP 3 O 12 ] and Mi et al. (2000) Table 1 Selected bond lengths (Å ).

Comment
The systematic development of borophosphates has led to a broad spectrum of new borophosphate compounds with quite different anionic partial structures, such as oligomeric units, chains, ribbons, layers, and three-dimensional frameworks.
Most of the borophosphate compounds were synthesized under hydrothermal conditions; hence, their structures usually incorporate water molecules, hydroxy groups or organic templates. There are considerably less anhydrous borophosphate compounds known, which might have better chemical and thermal stability than the hydrous or templated phases to ensure the feasibility of industrial applications. Herein, we report the redetermined structure of the anhydrous diiron(III) borotri- The basic building units of the three-dimensional structure of the title compound are FeO 6 octahedra (3.. symmetry), trigonal-planar BO 3 groups (6 symmetry) and PO 4 tetrahedra (m.. symmetry) (Fig. 1). Two neighboring FeO 6 octahedra are connected via their faces to form Fe 2 O 9 dimers. Trigonal-planar BO 3 units and PO 4 tetrahedra are isolated. Each BO 3 triangle connects three PO 4 tetrahedra via corner-sharing O atoms and each PO 4 connects three Fe 2 O 9 groups and one BO 3 group also via corner sharing. As shown in Fig on the current findings, a space group change from P3 to P6 3 /m seems to be most likely for the Cr compound but has to be evidenced experimentally. Meisel et al. (2004) have reported the analogous vanadium(III) compound V 2 [BP 3 O 12 ] in space group P6 3 /m, but with a tripled unit cell (a of the V compound ≈ 3 1/2 × a of the Fe and Cr compounds). However, a comparison of the three structures shows very similar frameworks.

Experimental
Single crystals of Fe 2 [BP 3 O 12 ] have been prepared by the high temperature solution growth (HTSG) method in air. A powder mixture of Fe 2 O 3 , B 2 O 3 and NaPO 3 at the molar ratio of Fe: B: Na: P = 1:5:10:10 was first ground in an agate mortar and then transferred to a platinum crucible. The sample was gradually heated in air at 1173 K for 24 h. In this stage, the reagents were completely melted. After that, the intermediate product was slowly cooled to 673 K at the rate of 2 K h -1 supplementary materials sup-2 and then quenched to room temperature. The obtained crystals were light-red and of prismatic shape. The dimensions of the used sample were typical for the grown crystals in this batch.

Refinement
Chen et al. (2004) have refined the structure of Fe 2 [BP 3 O 12 ] using the Rietved method from powder X-ray data and determined the space group to be P3, in analogy with the chromium compound Cr 2 [BP 3 0 12 ] (Mi et al., 2000). However, in our study we determined the structure from single-crystal X-ray diffraction data in the centrosymmetric space group P6 3 /m.
In the progress of the space group determination using XPREP (Sheldrick, 2008), the mean |E*E-1| statistics gave a value of 0.948 revealing that the structure is centrosymmetric; the CFOM (combined figure-of-merit) value for each space group determination were P3 (16.06), P3 (7.16), P6 3 (7.56), P6 3 /m (1.75). So we selected the latter space group to solve the structure. The final refinement converged with satisfactory results (R 1 (gt) = 0.0348). Furthermore, the final refined model was checked with the ADDSYM algorithm using the program PLATON (Spek, 2009), and no higher symmetry was found.
Hence, our final structure model is considered to be reasonable and corrects the previous model by Chen et al. (2004).
The highest peak in the difference electron density map is located at a distance of 1.41 Å from the Fe1 site while the deepest hole is at a distance of 0.83 Å from the same site. Fig. 1. Section of the structure of Fe 2 [BP 3 0 12 ] with the atom labelling scheme. The displacement ellipsoids are drawn at the 50% probability level. [Symmetry codes:   (6)