RbZnFe(PO4)2: synthesis and crystal structure

Rubidium zinc iron(III) phosphate is isostructural to KCoAl(PO4)2. Its structure consists of a three-dimensional framework built up from corner-sharing PO4 and (Zn,Fe)O4 tetrahedra. This mode of linkage forms tunnels parallel to the [100], [010] and [001] directions in which the Rb+ ions are located.


Structural commentary
The structure is made up of a three-dimensional assemblage of MO 4 (M = 0.5Zn + 0.5Fe) and PO 4 tetrahedra through cornersharing. This framework delimits crossing channels along the [100] and [001] directions, in which the Rb + ions are located ( Figs. 1 and 2). A projection of the structure along [001] direction reveals that each MO 4 tetrahedron is linked to four PO 4 tetrahedra by sharing corners. In addition, it shows the presence of two kinds of rings through corner-sharing of MO 4 and PO 4 tetrahedra (Fig. 2). The first presents an elliptical form and comprises four MO 4 and four PO 4 tetrahedra, the second consists of two MO 4 and two PO 4 tetrahedra and has a quasi-rectangular form. From an examination of the interatomic distances (cation-oxygen), the M(1) and M(2) sites exhibit similar regular tetrahedral environments, as seen in the cation-oxygen distances which vary from 1.877 (5) to 1.900 (5) Å for M(1) and from 1.860 (6) to 1.919 (5) Å for M(2). The average distances of 1.885 (2) and 1.888 (2) Å are between the values of 1.926 (2) Å observed for tetrahedrally coordinated Zn 2+ ions in the zinc phosphate RbZnPO 4 (Elammari & Elouadi, 1991) and 1.865 Å reported for the Fe 3+ ions with the same coordination in the iron phosphate in FePO 4 (Long et al., 1983). The P-O distances within the PO 4 tetrahedra are between 1.514 (5) and 1.535 (5) Å and with mean distances of 1.523 (9) Å for P(1) and 1.520 (3) Å for P(2), consistent with the value of 1.537 Å calculated by Baur (1974) for orthophosphate groups.
The Rb + ions occupy a single site at the intersection of the crossing tunnels. Their environment was determined assuming all cation-oxygen distances to be shorter than the shortest distance between Rb + and its nearest cation. This environment ( Fig. 3) then consists of ten O atoms with Rb-O distances ranging from 2.925 (6) to 3.298 (7) Å .

Synthesis and crystallization
Single crystals of RbZnFe(PO 4 ) 2 were grown in a flux of rubidium dimolybdate Rb 2 Mo 2 O 7 , in an atomic ratio P:Mo = 4:1. Appropriate amounts of Rb 2 CO 3 , Zn(NO 3 ) 2 Á6H 2 O, Fe(NO 3 ) 3 Á9H 2 O, (NH 4 ) 2 HPO 4 and MoO 3 were used. All of the chemicals were analytically pure from commercial sources and used without further purification. The reagents were weighted in the atomic ratio P:Mo = 2:1 and dissolved in nitric acid and then dried for 24 h at 353 K. The dry residue was gradually heated to 873 K in a platinum crucible to remove the decomposition products. In a second step, the mixture was ground, melted for 1 h at 1173 K and subsequently cooled at a rate of 10 K h À1 to 773 K, after which the furnace was turned off. The crystals obtained by washing the final product with warm water in order to dissolve the flux are essentially comprised of beige hexagonally shaped crystals of RbZnFe(PO 4 ) 2 .

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1 A view of the crystal structure of RbZnFe(PO 4 ) 2 along [001], showing the elliptical and quasi-rectangular forms of corner-sharing MO 4 and PO 4 tetrahedra (edge with green colour). The colour key is as in Fig. 1.

Crystal data
RbZnFe ( Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )