RbCuFe(PO4)2

A new iron phosphate, rubidium copper(II) iron(III) bis(phosphate), RbCuFe(PO4)2, has been synthesized as single crystals by the flux method. This compound is isostructural with KCuFe(PO4)2 [Badri et al. (2011 ▶), J. Solid State Chem. 184, 937–944]. Its structure is built up from Cu2O8 units of edge-sharing CuO5 polyhedra, interconnected by FeO6 octahedra through common corners to form undulating chains that extend infinitely along the [011] and [01-1] directions. The linkage of such chains is ensured by the PO4 tetraedra and the resulting three-dimensional framework forms quasi-elliptic tunnels parallel to the [101] direction in which the Rb+ cations are located.

A new iron phosphate, rubidium copper(II) iron(III) bis-(phosphate), RbCuFe(PO 4 ) 2 , has been synthesized as single crystals by the flux method. This compound is isostructural with KCuFe(PO 4 ) 2 [Badri et al. (2011), J. Solid State Chem. 184, 937-944]. Its structure is built up from Cu 2 O 8 units of edge-sharing CuO 5 polyhedra, interconnected by FeO 6 octahedra through common corners to form undulating chains that extend infinitely along the [011] and [011] directions. The linkage of such chains is ensured by the PO 4 tetraedra and the resulting three-dimensional framework forms quasi-elliptic tunnels parallel to the [101] direction in which the Rb + cations are located.

Related literature
For the physical properties of iron phosphates, see: Elbouaanani et al. (2002). For the structural chemistry of iron phosphates, see: Moore (1970); Gleitzer (1991). For rubidium iron phosphates, see: Hidouri et al. (2010) Iron phosphates are the subject of growing interest because of their attractive physical properties (Elbouaanani et al., 2002) as well as for their structural diversity (Moore, 1970;Gleitzer, 1991). Among the variety of iron monophosphates synthesized and characterized over the past three decades, only one rubidium-containing compound has been reported, namely Rb 9 Fe 7 (PO 4 ) 10 (Hidouri et al., 2010). In this paper, we report the structure of a new rubidium iron monophosphate RbCuFe(PO 4 ) 2 synthesized during our investigation of the Rb 3 PO 4 -Cu 3 (PO 4 ) 2 -FePO 4 quasi system.
Due to the poor quality of the crystal, anisotropic structure refinement systematically led to non-positive definite thermal parameters. For this reason, all the atoms were refined istropically. [011] directions (Fig. 1). These chains are linked by the PO 4 tetrahedra, giving rise to a three-dimensional framework.
The environments of the different coordination polyhehdra are shown in Fig. 2. Each CuO 5 polyhedron is linked to four PO 4 tetrahedra, by sharing three corners with three tetrahedra and one edge with the fourth, the two O atoms forming the shared edge being also shared by two FeO 6 octahedra (Fig. 2a). Each FeO 6 octahedron is corner-linked to six PO 4 tetrahedra, two of its corners being also shared with two CuO 5 polyhedra (Fig. 2b). Each P1O 4 tetrahedron shares one edge with one CuO 5 polyhedron and four corners, one with a CuO 5 polyhedron and three with three FeO 6 octahedra ( Fig.   2c). Each P2O 4 tetrahdron shares three corners with three FeO 6 octahedra,the remaining corner being doubly shared with two CuO 5 polyhedra (Fig. 2d). The anionic framework induced by this mode of connectivity forms quasi-elliptic tunnels along the [101] direction (Fig. 3).
From an examination of the cation-oxygen distances, the CuO 5 polyhedron is highly deformed, with one longer Cu-O distance of 2.174 (3) Å compared to the four others [from 1.916 (3) to 2.030 (3) Å]. Such deformation has already been observed in KCuFe(PO 4 ) 2 and it can be attributed to the Jahn-Teller effect accentuated for the Cu 2+ ions with d 9 configuration. The FeO 6 octahedron is also highly distorted, whith Fe-O distances varying between 1.937 (4) and 2.187 (3) Å. Corresponding mean <Fe-O> distance of 2.018 Å is close to 2.03 Å predicted by Shannon for octahedral Fe 3+ ions (Shannon, 1976). The P-O distances within the PO 4 tetrahedra are in the range 1.518 (3) and 1.576 (3) Å, with an overall distances of 1.540 (3) Å consistent with 1.537 Å calculated by Baur for the monophosphate groups (Baur, 1974). The Rb + ions occupy a single distinct site. Their environment was determined assuming all cation-oxygen distances shorter than the shortest distance between Rb + and its nearest cation. This environment (Fig. 4)

Experimental
Single crystals of RbCuFe(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 (Fluka, 99%), Cu(NO 3 ) 2 .6H 2 O (Acros, 99%), Fe(NO 3 ) 3 .9H 2 O (Fisher, 98.6%), (NH 4 ) 2 HPO 4 (Merck, 99%) and MoO 3 (Acros, 99%) were dissolved in aqueous nitric acid and the obtained solution was evaporated to dryness. The obtained residue was homogenized by grinding and then gradually heated up to 873 K in a platinum crucible. After being reground, the mixture was melted for 1 h at 1173 K and subsequently cooled at a rate of 10 K h -1 down to 673 K, after which the furnace was turned off. The crystals extracted from the flux by washing with warm water, are essentially composed by dark-green plate crystals of RbCuFe(PO 4 ) 2 .

Refinement
Due to the poor quality of the crystal, anisotropic structure refinement led to non-positive definite thermal parameters for the Fe, P1, P2, O11, O13, O21 and O23 atoms. Then, all the atoms were refined istropically. The obtained largest positive and negative difference electron densities are closest to the Rb atoms.   The environments of the CuO 5 (a), FeO 6 (b), P1O 4 (c) and P2O 4 (d) polyhedra.  The environment of the Rb cations showing the thermal ellipsoids drawn at the 50% probability level.

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. 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.