supporting information

Acta Cryst. (2013). E69, i52    [ doi:10.1107/S1600536813019569 ]


A. Badri, M. Hidouri and M. Ben Amara

Abstract top

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.

Comment top

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 Rb9Fe7(PO4)10 (Hidouri et al., 2010). In this paper, we report the structure of a new rubidium iron monophosphate RbCuFe(PO4)2 synthesized during our investigation of the Rb3PO4—Cu3(PO4)2-FePO4 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.

This compound is isostructural with KCuFe(PO4)2 (Badri et al., 2011) and K(Fe,Mg)(PO4)2 (Yatskin et al., 2012). The structure consists of a three-dimensional assemblage of Cu2O8 units of edge-sharing CuO5 trigonal bipyramids, linked to each other by FeO6 octahedra through common corners to form crossing zigzag chains that run parallel to the [011] and [011] directions (Fig. 1). These chains are linked by the PO4 tetrahedra, giving rise to a three-dimensional framework.

The environments of the different coordination polyhehdra are shown in Fig. 2. Each CuO5 polyhedron is linked to four PO4 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 FeO6 octahedra (Fig. 2a). Each FeO6 octahedron is corner-linked to six PO4 tetrahedra, two of its corners being also shared with two CuO5 polyhedra (Fig. 2b). Each P1O4 tetrahedron shares one edge with one CuO5 polyhedron and four corners, one with a CuO5 polyhedron and three with three FeO6 octahedra (Fig. 2c). Each P2O4 tetrahdron shares three corners with three FeO6 octahedra,the remaining corner being doubly shared with two CuO5 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 CuO5 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(PO4)2 and it can be attributed to the Jahn–Teller effect accentuated for the Cu2+ ions with d9 configuration. The FeO6 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 Fe3+ ions (Shannon, 1976). The P—O distances within the PO4 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) is then constituted by nine O atoms, with Rb—O distances ranging from 2.887 (3) to 3.123 (3) Å.

Related literature top

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). This compound is isostructural with KCuFe(PO4)2 (Badri et al., 2011) and K(Fe,Mg)(PO4)2 (Yatskin et al., 2012). For P—O distances in monophosphate groups, see: Baur (1974). For ionic radii, see: Shannon (1976).

Experimental top

Single crystals of RbCuFe(PO4)2 were grown in a flux of rubidium dimolybdate, Rb2Mo2O7, in an atomic ratio P:Mo = 4:1. Appropriate amounts of Rb2CO3 (Fluka, 99%), Cu(NO3)2.6H2O (Acros, 99%), Fe(NO3)3.9H2O (Fisher, 98.6%), (NH4)2HPO4 (Merck, 99%) and MoO3 (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(PO4)2.

Refinement top

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.

Computing details top

Data collection: COLLECT (Nonius, 1997); cell refinement: COLLECT (Nonius, 1997); data reduction: SCALEPACK and DENZO (Otwinowski & Minor, 1997); program(s) used to solve structure: SIR92 (Altomare et al., 1993); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 1999); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).

Figures top
[Figure 1] Fig. 1. The crossing chains of the alternating Cu2O8 units and FeO6 octahedra. The FeO6 octahedra and the CuO5 trigonal bipyramids are represented by purple and green polyhedra, respectively.
[Figure 2] Fig. 2. The environments of the CuO5 (a), FeO6 (b), P1O4 (c) and P2O4 (d) polyhedra.
[Figure 3] Fig. 3. A projection along the [101] direction of the structure showing the large tunnels occupied by the Rb+ cations. FeO6, CuO5, P1O4 and P2O4 are represented by purple, green, solid grey and hatched grey polyhedra, respectively. The Rb atoms are illustrated by cyan circles.
[Figure 4] Fig. 4. The environment of the Rb cations showing the thermal ellipsoids drawn at the 50% probability level.
Rubidium copper iron bis(phosphate) top
Crystal data top
RbCuFe(PO4)2F(000) = 744
Mr = 394.80Dx = 3.983 Mg m3
Monoclinic, P21/nMo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ynCell parameters from 25 reflections
a = 8.054 (1) Åθ = 2.8–26.9°
b = 9.906 (3) ŵ = 13.28 mm1
c = 9.140 (1) ÅT = 293 K
β = 115.47 (1)°Plate, green
V = 658.3 (2) Å30.25 × 0.21 × 0.14 mm
Z = 4
Data collection top
Enraf–Nonius CAD4
1236 reflections with I > \2(I)
Radiation source: fine-focus sealed tubeRint = 0.046
Graphite monochromatorθmax = 26.9°, θmin = 2.8°
ω/2θ scansh = 110
Absorption correction: part of the refinement model (ΔF)
(SHELXL97; Sheldrick 2008)
k = 112
Tmin = 0.060, Tmax = 0.119l = 1110
1939 measured reflections2 standard reflections every 120 min
1437 independent reflections intensity decay: 1%
Refinement top
Refinement on F2Primary atom site location: structure-invariant direct methods
Least-squares matrix: fullSecondary atom site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.046 w = 1/[σ2(Fo2) + (0.0642P)2 + 8.1747P]
where P = (Fo2 + 2Fc2)/3
wR(F2) = 0.126(Δ/σ)max = 0.001
S = 1.08Δρmax = 3.25 e Å3
1437 reflectionsΔρmin = 2.25 e Å3
54 parametersExtinction correction: SHELXL97 (Sheldrick, 2008), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
0 restraintsExtinction coefficient: 0.056 (3)
Crystal data top
RbCuFe(PO4)2V = 658.3 (2) Å3
Mr = 394.80Z = 4
Monoclinic, P21/nMo Kα radiation
a = 8.054 (1) ŵ = 13.28 mm1
b = 9.906 (3) ÅT = 293 K
c = 9.140 (1) Å0.25 × 0.21 × 0.14 mm
β = 115.47 (1)°
Data collection top
Enraf–Nonius CAD4
1236 reflections with I > \2(I)
Absorption correction: part of the refinement model (ΔF)
(SHELXL97; Sheldrick 2008)
Rint = 0.046
Tmin = 0.060, Tmax = 0.1192 standard reflections every 120 min
1939 measured reflections intensity decay: 1%
1437 independent reflections
Refinement top
R[F2 > 2σ(F2)] = 0.04654 parameters
wR(F2) = 0.1260 restraints
S = 1.08Δρmax = 3.25 e Å3
1437 reflectionsΔρmin = 2.25 e Å3
Special details top

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 F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
Rb0.08146 (10)0.36746 (7)0.92434 (9)0.0187 (3)*
Cu0.36913 (11)0.11858 (8)0.94182 (10)0.0103 (3)*
Fe0.98385 (13)0.12391 (9)0.75520 (11)0.0084 (3)*
P11.2630 (2)0.08995 (18)0.1423 (2)0.0082 (4)*
O110.4087 (7)0.0231 (5)0.0958 (6)0.0116 (10)*
O120.2147 (7)0.0101 (5)0.2460 (6)0.0116 (10)*
O130.8515 (7)0.2895 (5)0.7505 (6)0.0123 (10)*
O140.0942 (7)0.1317 (5)0.9900 (6)0.0115 (10)*
P20.1321 (2)0.15855 (18)0.6387 (2)0.0087 (4)*
O210.4575 (7)0.2610 (5)0.1010 (6)0.0138 (10)*
O220.1462 (7)0.1108 (5)0.4869 (6)0.0119 (10)*
O230.3040 (7)0.2436 (5)0.7543 (6)0.0118 (10)*
O240.1483 (7)0.0378 (5)0.7545 (6)0.0112 (10)*
Geometric parameters (Å, º) top
Rb—O13i2.888 (5)Fe—O14vii1.940 (5)
Rb—O21ii2.947 (5)Fe—O131.946 (5)
Rb—O12iii2.952 (5)Fe—O12vi1.952 (5)
Rb—O21iii2.969 (5)Fe—O22vi2.005 (5)
Rb—O14iv3.003 (5)Fe—O24vii2.081 (5)
Rb—O12v3.085 (5)Fe—O23viii2.185 (5)
Rb—O233.087 (5)P1—O13ix1.519 (5)
Rb—O11iii3.120 (5)P1—O14x1.526 (5)
Rb—O22v3.122 (5)P1—O12vii1.533 (5)
Cu—O11ii1.915 (5)P1—O11vii1.557 (5)
Cu—O21ii1.930 (5)P2—O221.515 (5)
Cu—O231.993 (5)P2—O21iii1.521 (5)
Cu—O242.028 (5)P2—O241.565 (5)
Cu—O11vi2.178 (5)P2—O231.576 (5)
O13i—Rb—O21ii69.35 (14)O11ii—Cu—O23170.6 (2)
O13i—Rb—O12iii112.20 (14)O21ii—Cu—O2394.0 (2)
O21ii—Rb—O12iii176.53 (14)O11ii—Cu—O2498.0 (2)
O13i—Rb—O21iii138.05 (14)O21ii—Cu—O24146.1 (2)
O21ii—Rb—O21iii102.61 (12)O23—Cu—O2473.0 (2)
O12iii—Rb—O21iii78.36 (14)O11ii—Cu—O11vi84.8 (2)
O13i—Rb—O14iv54.85 (14)O21ii—Cu—O11vi112.0 (2)
O21ii—Rb—O14iv93.87 (14)O23—Cu—O11vi93.67 (19)
O12iii—Rb—O14iv89.52 (14)O24—Cu—O11vi100.20 (19)
O21iii—Rb—O14iv86.08 (14)O14vii—Fe—O1388.7 (2)
O13i—Rb—O12v48.09 (13)O14vii—Fe—O12vi90.8 (2)
O21ii—Rb—O12v68.83 (13)O13—Fe—O12vi92.7 (2)
O12iii—Rb—O12v109.70 (11)O14vii—Fe—O22vi176.0 (2)
O21iii—Rb—O12v168.12 (14)O13—Fe—O22vi90.5 (2)
O14iv—Rb—O12v102.37 (13)O12vi—Fe—O22vi85.3 (2)
O13i—Rb—O23121.31 (14)O14vii—Fe—O24vii92.6 (2)
O21ii—Rb—O2356.71 (14)O13—Fe—O24vii172.8 (2)
O12iii—Rb—O23123.04 (13)O12vi—Fe—O24vii94.4 (2)
O21iii—Rb—O2349.30 (14)O22vi—Fe—O24vii88.73 (19)
O14iv—Rb—O23105.24 (13)O14vii—Fe—O23viii91.5 (2)
O12v—Rb—O23119.56 (13)O13—Fe—O23viii85.59 (19)
O13i—Rb—O11iii158.60 (14)O12vi—Fe—O23viii177.11 (19)
O21ii—Rb—O11iii129.19 (14)O22vi—Fe—O23viii92.38 (19)
O12iii—Rb—O11iii48.65 (13)O24vii—Fe—O23viii87.29 (19)
O21iii—Rb—O11iii55.53 (14)O13ix—P1—O14x111.4 (3)
O14iv—Rb—O11iii124.74 (13)O13ix—P1—O12vii106.2 (3)
O12v—Rb—O11iii122.91 (13)O14x—P1—O12vii112.1 (3)
O23—Rb—O11iii80.05 (13)O13ix—P1—O11vii108.3 (3)
O13i—Rb—O22v98.03 (14)O14x—P1—O11vii110.3 (3)
O21ii—Rb—O22v72.07 (14)O12vii—P1—O11vii108.4 (3)
O12iii—Rb—O22v104.52 (14)O22—P2—O21iii112.5 (3)
O21iii—Rb—O22v119.17 (14)O22—P2—O24111.3 (3)
O14iv—Rb—O22v152.82 (13)O21iii—P2—O24110.7 (3)
O12v—Rb—O22v51.19 (13)O22—P2—O23113.0 (3)
O23—Rb—O22v86.70 (13)O21iii—P2—O23109.4 (3)
O11iii—Rb—O22v80.87 (13)O24—P2—O2399.3 (3)
O11ii—Cu—O21ii95.2 (2)
Symmetry codes: (i) x+1, y, z+2; (ii) x, y, z+1; (iii) x1/2, y1/2, z+1/2; (iv) x, y, z+2; (v) x+1/2, y1/2, z+3/2; (vi) x+1, y, z+1; (vii) x+1, y, z; (viii) x+3/2, y+1/2, z+3/2; (ix) x+1/2, y+1/2, z1/2; (x) x+1, y, z1.

Experimental details

Crystal data
Chemical formulaRbCuFe(PO4)2
Crystal system, space groupMonoclinic, P21/n
Temperature (K)293
a, b, c (Å)8.054 (1), 9.906 (3), 9.140 (1)
β (°) 115.47 (1)
V3)658.3 (2)
Radiation typeMo Kα
µ (mm1)13.28
Crystal size (mm)0.25 × 0.21 × 0.14
Data collection
DiffractometerEnraf–Nonius CAD4
Absorption correctionPart of the refinement model (ΔF)
(SHELXL97; Sheldrick 2008)
Tmin, Tmax0.060, 0.119
No. of measured, independent and
observed [I > \2(I)] reflections
1939, 1437, 1236
(sin θ/λ)max1)0.637
R[F2 > 2σ(F2)], wR(F2), S 0.046, 0.126, 1.08
No. of reflections1437
No. of parameters54
Δρmax, Δρmin (e Å3)3.25, 2.25

Computer programs: COLLECT (Nonius, 1997), SCALEPACK and DENZO (Otwinowski & Minor, 1997), SIR92 (Altomare et al., 1993), SHELXL97 (Sheldrick, 2008), DIAMOND (Brandenburg, 1999).