The iron phosphate NaBaFe2(PO4)3

A new iron phosphate, sodium barium diiron tris(phosphate), NaBaFe2(PO4)3, has been synthesized by the flux method and shown to exhibit the well known langbeinite type structure. The Na, Ba and Fe atoms all lie on threefold axes, while the P and O atoms occupy general positions, one of the O atoms being disordered over two positions, with site occupancy factors of ca 0.7 and 0.3. The [Fe2(PO4)3]∞ framework consists of FeO6 octahedra sharing all their corners with the PO4 tetrahedra. The Na+ and Ba2+ cations are almost equally distributed over two distinct cavities, in which they occupy slightly different positions.

A new iron phosphate, sodium barium diiron tris(phosphate), NaBaFe 2 (PO 4 ) 3 , has been synthesized by the flux method and shown to exhibit the well known langbeinite type structure. The Na, Ba and Fe atoms all lie on threefold axes, while the P and O atoms occupy general positions, one of the O atoms being disordered over two positions, with site occupancy factors of ca 0.7 and 0.3. The [Fe 2 (PO 4 ) 3 ] 1 framework consists of FeO 6 octahedra sharing all their corners with the PO 4 tetrahedra. The Na + and Ba 2+ cations are almost equally distributed over two distinct cavities, in which they occupy slightly different positions.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: BR2076).
The iron phosphate NaBaFe 2 (PO 4 ) 3 M. Hidouri, H. Jerbi and M. Ben Amara Comment Iron phosphates are of increasing interst because of their potential applications in various fields ranging from catalysis (Moffat, 1978) to ionic conductivity (Padhi et al., 1997). Moreover, these materials are very attractive in terms of basic reasearch because they exhibit a rich structural chemistry owing to the possible (+2/+3) mixed valence of iron and its tendency to exhibit various coordination polyhedra.
The title cmpound, sodium barium diiron phosphate NaBaFe 2 (PO 4 ) 3 was isolated during a systematic investigation of the Na 2 O-MO-Fe 2 O 3 -P 2 O 5 systems where M is a divalent cation. Its structure (Fig. 1) exhibits a three-dimensional [Fe 2 (PO 4 ) 3 ] ∞ framework built up from corner-sharing FeO 6 octahedra and PO 4 tetrahedra. Each octahedron is linked to six adjacent tetrahedra and reciprocally each tetrahedron is connected to four neighboring octahedra. This framework delimits two sorts of large cavities, statistically occupied by the Na + and Ba 2+ cations.
The two symmetry distinct FeO 6 octahedra contained in this structure are somewhat distorted as indicated by the Fe-O distances ranging from 1.963 (5) to 1.991 (4) Å. The average <Fe-O> distances of 1.986 Å for Fe(1) and 1.973 Å for Fe (2) are slightly lower than the value 2.03 Å predicted by Shannon for octahedral Fe 3+ ions (Shannon, 1976).
The as-described structure is closely related to the langbeinite-like phosphates KBaM 2 (PO 4 ) 3 (M = Fe, Cr) (Battle et al., 1986(Battle et al., , 1988. However, it differs by the fact that the atom O4, which occupies a single site in the potassium phosphates, is, in the title compound, statistically occupying two distinct positions, O4A and O4B which exhibit partial occupancies of 0.7 and 0.3, respectively. These different values can be explained by the fact that the O4A site is occupied if it is bonded to Na(1), Ba(1) or Ba(2) whereas the O4B site is occupied if it is bonded to Na(1) or Ba(1).

Experimental
Single crystals of the title compund were grown in a flux of sodium dimolybdate Na 2 Mo 2 O 7 with an atomic ratio P:Mo = 6:1. A starting mixture of 1.071 g of Na 2 CO 3 , 1.993 g of BaCO 3 , 8.162 g of Fe(NO 3 ) 3 .9H 2 O, 4.002 g of (NH 4 ) 2 HPO 4 supplementary materials sup-2 and 1.454 g of MoO 3 was dissolved in nitric acid and the obtained solution was evaporated to dryness. The dry residue was transferred into a platinum crucible and then heated up 600°C to decompose H 2 O and NH 3 . In a second step, the sample was melted for 1 h at 900°C and then cooled down to room temperature with a 10° h -1 rate. The final product, obtained after washing with warm water to dissolve the flux is essentially composed of pink and prismatic shaped crystals. Their qualitative elemental analysis using electron microprobe analysis indicated the presence of Na, Ba, Fe and P and no impurity elements have been detected.

Refinement
The Ba and Fe atoms were located by direct methods and the remaining atoms were found by successive difference Fourier maps. All atomic positions were refined anisotropically.