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.

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). The title 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

Crystal data

• RbCuFe(PO₄)₂

• M_r = 394.80

• Monoclinic, P 2₁ /n

• a = 8.054 (1) Å

• b = 9.906 (3) Å

• c = 9.140 (1) Å

• β = 115.47 (1)°

• V = 658.3 (2) ų

• Z = 4

• Mo Kα radiation

• μ = 13.28 mm⁻¹

• T = 293 K

• 0.25 × 0.21 × 0.14 mm

Data collection

• Enraf–Nonius CAD4 diffractometer

• Absorption correction: part of the refinement model (ΔF) (SHELXLA; Sheldrick 2008) T_min = 0.060, T_max = 0.119

• 1939 measured reflections

• 1437 independent reflections

• 1236 reflections with I > \2(I)

• R_int = 0.046

• 2 standard reflections every 120 min intensity decay: 1%

Refinement

• R[F² > 2σ(F²)] = 0.046

• wR(F²) = 0.126

• S = 1.08

• 1437 reflections

• 54 parameters

• Δρ_max = 3.25 e Å⁻³

• Δρ_min = −2.25 e Å⁻³

Data collection: CAD-4 EXPRESS (Enraf–Nonius, 1994); cell refinement: CAD-4 EXPRESS; data reduction: XCAD4 (Harms & Wocadlo, 1995); 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.

Supporting information

S1. Comment

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

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).

S2. Experimental

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.

S3. 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.

Computing details

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

	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.
	Fig. 2. The environments of the CuO5 (a), FeO6 (b), P1O4 (c) and P2O4 (d) polyhedra.
	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.
	Fig. 4. The environment of the Rb cations showing the thermal ellipsoids drawn at the 50% probability level.

Rubidium copper iron bis(phosphate)

Crystal data

RbCuFe(PO4)2	F(000) = 744
Mr = 394.80	Dx = 3.983 Mg m^−3
Monoclinic, P21/n	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2yn	Cell parameters from 25 reflections
a = 8.054 (1) Å	θ = 2.8–26.9°
b = 9.906 (3) Å	µ = 13.28 mm^−1
c = 9.140 (1) Å	T = 293 K
β = 115.47 (1)°	Plate, green
V = 658.3 (2) Å^3	0.25 × 0.21 × 0.14 mm
Z = 4

Data collection

Enraf–Nonius CAD4 diffractometer	1236 reflections with I > \2(I)
Radiation source: fine-focus sealed tube	Rint = 0.046
Graphite monochromator	θmax = 26.9°, θmin = 2.8°
ω/2θ scans	h = −1→10
Absorption correction: part of the refinement model (ΔF) (SHELXL97; Sheldrick 2008)	k = −1→12
Tmin = 0.060, Tmax = 0.119	l = −11→10
1939 measured reflections	2 standard reflections every 120 min
1437 independent reflections	intensity decay: 1%

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.046	w = 1/[σ^2(Fo^2) + (0.0642P)^2 + 8.1747P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.126	(Δ/σ)max = 0.001
S = 1.08	Δρmax = 3.25 e Å^−3
1437 reflections	Δρmin = −2.25 e Å^−3
54 parameters	Extinction correction: SHELXL97 (Sheldrick, 2008), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.056 (3)

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 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 (Ų)

	x	y	z	Uiso*/Ueq
Rb	0.08146 (10)	−0.36746 (7)	0.92434 (9)	0.0187 (3)*
Cu	0.36913 (11)	−0.11858 (8)	0.94182 (10)	0.0103 (3)*
Fe	0.98385 (13)	0.12391 (9)	0.75520 (11)	0.0084 (3)*
P1	1.2630 (2)	0.08995 (18)	0.1423 (2)	0.0082 (4)*
O11	0.4087 (7)	0.0231 (5)	0.0958 (6)	0.0116 (10)*
O12	0.2147 (7)	−0.0101 (5)	0.2460 (6)	0.0116 (10)*
O13	0.8515 (7)	0.2895 (5)	0.7505 (6)	0.0123 (10)*
O14	0.0942 (7)	0.1317 (5)	0.9900 (6)	0.0115 (10)*
P2	0.1321 (2)	−0.15855 (18)	0.6387 (2)	0.0087 (4)*
O21	0.4575 (7)	−0.2610 (5)	0.1010 (6)	0.0138 (10)*
O22	0.1462 (7)	−0.1108 (5)	0.4869 (6)	0.0119 (10)*
O23	0.3040 (7)	−0.2436 (5)	0.7543 (6)	0.0118 (10)*
O24	0.1483 (7)	−0.0378 (5)	0.7545 (6)	0.0112 (10)*

Geometric parameters (Å, º)

Rb—O13^i	2.888 (5)	Fe—O14^vii	1.940 (5)
Rb—O21^ii	2.947 (5)	Fe—O13	1.946 (5)
Rb—O12^iii	2.952 (5)	Fe—O12^vi	1.952 (5)
Rb—O21^iii	2.969 (5)	Fe—O22^vi	2.005 (5)
Rb—O14^iv	3.003 (5)	Fe—O24^vii	2.081 (5)
Rb—O12^v	3.085 (5)	Fe—O23^viii	2.185 (5)
Rb—O23	3.087 (5)	P1—O13^ix	1.519 (5)
Rb—O11^iii	3.120 (5)	P1—O14^x	1.526 (5)
Rb—O22^v	3.122 (5)	P1—O12^vii	1.533 (5)
Cu—O11^ii	1.915 (5)	P1—O11^vii	1.557 (5)
Cu—O21^ii	1.930 (5)	P2—O22	1.515 (5)
Cu—O23	1.993 (5)	P2—O21^iii	1.521 (5)
Cu—O24	2.028 (5)	P2—O24	1.565 (5)
Cu—O11^vi	2.178 (5)	P2—O23	1.576 (5)
O13^i—Rb—O21^ii	69.35 (14)	O11^ii—Cu—O23	170.6 (2)
O13^i—Rb—O12^iii	112.20 (14)	O21^ii—Cu—O23	94.0 (2)
O21^ii—Rb—O12^iii	176.53 (14)	O11^ii—Cu—O24	98.0 (2)
O13^i—Rb—O21^iii	138.05 (14)	O21^ii—Cu—O24	146.1 (2)
O21^ii—Rb—O21^iii	102.61 (12)	O23—Cu—O24	73.0 (2)
O12^iii—Rb—O21^iii	78.36 (14)	O11^ii—Cu—O11^vi	84.8 (2)
O13^i—Rb—O14^iv	54.85 (14)	O21^ii—Cu—O11^vi	112.0 (2)
O21^ii—Rb—O14^iv	93.87 (14)	O23—Cu—O11^vi	93.67 (19)
O12^iii—Rb—O14^iv	89.52 (14)	O24—Cu—O11^vi	100.20 (19)
O21^iii—Rb—O14^iv	86.08 (14)	O14^vii—Fe—O13	88.7 (2)
O13^i—Rb—O12^v	48.09 (13)	O14^vii—Fe—O12^vi	90.8 (2)
O21^ii—Rb—O12^v	68.83 (13)	O13—Fe—O12^vi	92.7 (2)
O12^iii—Rb—O12^v	109.70 (11)	O14^vii—Fe—O22^vi	176.0 (2)
O21^iii—Rb—O12^v	168.12 (14)	O13—Fe—O22^vi	90.5 (2)
O14^iv—Rb—O12^v	102.37 (13)	O12^vi—Fe—O22^vi	85.3 (2)
O13^i—Rb—O23	121.31 (14)	O14^vii—Fe—O24^vii	92.6 (2)
O21^ii—Rb—O23	56.71 (14)	O13—Fe—O24^vii	172.8 (2)
O12^iii—Rb—O23	123.04 (13)	O12^vi—Fe—O24^vii	94.4 (2)
O21^iii—Rb—O23	49.30 (14)	O22^vi—Fe—O24^vii	88.73 (19)
O14^iv—Rb—O23	105.24 (13)	O14^vii—Fe—O23^viii	91.5 (2)
O12^v—Rb—O23	119.56 (13)	O13—Fe—O23^viii	85.59 (19)
O13^i—Rb—O11^iii	158.60 (14)	O12^vi—Fe—O23^viii	177.11 (19)
O21^ii—Rb—O11^iii	129.19 (14)	O22^vi—Fe—O23^viii	92.38 (19)
O12^iii—Rb—O11^iii	48.65 (13)	O24^vii—Fe—O23^viii	87.29 (19)
O21^iii—Rb—O11^iii	55.53 (14)	O13^ix—P1—O14^x	111.4 (3)
O14^iv—Rb—O11^iii	124.74 (13)	O13^ix—P1—O12^vii	106.2 (3)
O12^v—Rb—O11^iii	122.91 (13)	O14^x—P1—O12^vii	112.1 (3)
O23—Rb—O11^iii	80.05 (13)	O13^ix—P1—O11^vii	108.3 (3)
O13^i—Rb—O22^v	98.03 (14)	O14^x—P1—O11^vii	110.3 (3)
O21^ii—Rb—O22^v	72.07 (14)	O12^vii—P1—O11^vii	108.4 (3)
O12^iii—Rb—O22^v	104.52 (14)	O22—P2—O21^iii	112.5 (3)
O21^iii—Rb—O22^v	119.17 (14)	O22—P2—O24	111.3 (3)
O14^iv—Rb—O22^v	152.82 (13)	O21^iii—P2—O24	110.7 (3)
O12^v—Rb—O22^v	51.19 (13)	O22—P2—O23	113.0 (3)
O23—Rb—O22^v	86.70 (13)	O21^iii—P2—O23	109.4 (3)
O11^iii—Rb—O22^v	80.87 (13)	O24—P2—O23	99.3 (3)
O11^ii—Cu—O21^ii	95.2 (2)

Symmetry codes: (i) −x+1, −y, −z+2; (ii) x, y, z+1; (iii) x−1/2, −y−1/2, z+1/2; (iv) −x, −y, −z+2; (v) −x+1/2, y−1/2, −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, z−1/2; (x) x+1, y, z−1.