RbZnFe(PO4)2: synthesis and crystal structure.

A new iron phosphate, rubidium zinc iron(III) phosphate, RbZnFe(PO4)2, has been synthesized as single crystals by the flux method. This compound is isostructural to the previously reported KCoAl(PO4)2 [Chen et al. (1997 ▸). Acta Cryst. C53,1754-1756]. Its structure consists of a three-dimensional framework built up from corner-sharing PO4 and (Zn,Fe)O4 tetra-hedra. This mode of linkage forms channels parallel to the [100], [010] and [001] directions in which the Rb(+) ions are located.

Chemical context

Phosphates with open-framework structures, similar to other porous materials such as zeolites, are inter­esting because of their wide industrial and environmental applications ranging from catalysis to ion-exchange and separation (Gier & Stucky, 1991 ▸; Maspoch et al., 2007 ▸). Among them, iron phosphates (Redrup & Weller, 2009 ▸; Lajmi et al., 2009 ▸) are particularly attractive because of their rich crystal chemistry (Moore, 1970 ▸; Gleitzer, 1991 ▸) and they present inter­esting and variable physical properties (Elbouaanani et al., 2002 ▸; Riou-Cavellec et al., 1999 ▸). Among the variety of iron orthophosphates synthesized and characterized over the past three decades, only two rubidium-containing compounds have been reported, namely Rb9Fe7(PO4)10 (Hidouri et al., 2010 ▸) and RbCuFe(PO4)2 (Badri et al., 2013 ▸). In this paper, we report the structure of a new rubidium iron orthophosphate, RbZnFe(PO4)2, synthesized during our investigation of the Rb3PO4–Zn3(PO4)2–FePO4 quasi-system. This compound is isostructural to KCoAl(PO4)2 (Chen et al., 1997 ▸) and KZnFe(PO4)2 (Badri et al., 2014 ▸).

Structural commentary

The structure is made up of a three-dimensional assemblage of MO4 (M = 0.5Zn + 0.5Fe) and PO4 tetra­hedra through corner-sharing. 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 MO4 tetra­hedron is linked to four PO4 tetra­hedra by sharing corners. In addition, it shows the presence of two kinds of rings through corner-sharing of MO4 and PO4 tetra­hedra (Fig. 2 ▸). The first presents an elliptical form and comprises four MO4 and four PO4 tetra­hedra, the second consists of two MO4 and two PO4 tetra­hedra and has a quasi-rectangular form. From an examination of the inter-atomic 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 Zn2+ ions in the zinc phosphate RbZnPO4 (Elammari & Elouadi, 1991 ▸) and 1.865 Å reported for the Fe3+ ions with the same coordination in the iron phosphate in FePO4 (Long et al., 1983 ▸). The P—O distances within the PO4 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.

Figure 1

A view of the crystal structure of RbZnFe(PO4)2 along [100]. Colour key: M(1)O4 tetrahedra are purple, M(2)O4 tetrahedra are red, P(1)O4 tetrahedra are dark grey, P(2)O4 tetrahedra are light grey and Rb+ cations are yellow spheres.

Figure 2

A view of the crystal structure of RbZnFe(PO4)2 along [001], showing the elliptical and quasi-rectangular forms of corner-sharing MO4 and PO4 tetrahedra (edge with green colour). The colour key is as in Fig. 1.

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.

Figure 3

The environment of the Rb cations, showing displacement ellipsoids drawn at the 50% probability level. Authors: Define symmetry operators (in the Figure) and codes (in the caption)

Synthesis and crystallization

Single crystals of RbZnFe(PO4)2 were grown in a flux of rubidium dimolybdate Rb2Mo2O7, in an atomic ratio P:Mo = 4:1. Appropriate amounts of Rb2CO3, Zn(NO3)2·6H2O, Fe(NO3)3·9H2O, (NH4)2HPO4 and MoO3 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−1 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(PO4)2.

Refinement

Crystal data, data collection and structure refinement details are summarized in Table 1 ▸. The application of direct methods revealed the Rb atoms and located two sites, labelled M(1) and M(2), statistically occupied by the Fe3+ and Zn2+ ions. This distribution was supported by the M(1)—O and M(2)—O distances which are between the classical pure Zn—O and Fe—O values. Succeeding difference Fourier syntheses led to the positions of all the remaining atoms.

Table 1

Experimental details

Crystal data
Chemical formula	RbZnFe(PO4)2
M r	396.63
Crystal system, space group	Monoclinic, C2/c
Temperature (K)	293
a, b, c (Å)	13.601 (4), 13.304 (5), 8.978 (9)
β (°)	100.76 (5)
V (Å3)	1596.0 (18)
Z	8
Radiation type	Mo Kα
μ (mm−1)	11.29
Crystal size (mm)	0.43 × 0.25 × 0.18
Data collection
Diffractometer	Enraf–Nonius TurboCAD-4
Absorption correction	Part of the refinement model (ΔF) (Walker & Stuart 1983 ▸)
T min, T max	0.054, 0.070
No. of measured, independent and observed [I > 2σ(I)] reflections	1409, 1409, 1227
R int	0.089
(sin θ/λ)max (Å−1)	0.594
Refinement
R[F 2 > 2σ(F 2)], wR(F 2), S	0.036, 0.110, 1.05
No. of reflections	1409
No. of parameters	118
	w = 1/[σ2(F o 2) + (0.0565P)2 + 31.2735P] where P = (F o 2 + 2F c 2)/3
Δρmax, Δρmin (e Å−3)	0.85, −0.76

Computer programs: CAD-4 EXPRESS (Enraf–Nonius, 1994 ▸), XCAD4 (Harms & Wocadlo, 1995 ▸), SIR92 (Altomare et al., 1993 ▸), SHELXL2014 (Sheldrick, 2015 ▸), DIAMOND (Brandenburg, 1999 ▸) and WinGX (Farrugia, 2012 ▸).

Despite several synthesis attempts, all the obtained crystals of RbZnFe(PO4)2 were of poor quality, resulting in the large discrepancies found in a number of reflections; hence in this study the refinement was performed using a filter of the reflections by [sin (θ)/λ]. The four reflections (85, 34, 85 and 75) were omitted as the difference between the observed and calculated structure factors was larger than 10σ.

Crystal structure: contains datablock(s) global, I. DOI: 10.1107/S205698901601046X/br2261sup1.cif

Structure factors: contains datablock(s) I. DOI: 10.1107/S205698901601046X/br2261Isup2.hkl

CCDC reference: 1488120

Additional supporting information: crystallographic information; 3D view; checkCIF report

RbZnFe(PO4)2	F(000) = 1496
Mr = 396.63	Dx = 3.301 Mg m−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
a = 13.601 (4) Å	Cell parameters from 25 reflections
b = 13.304 (5) Å	θ = 8.1–11.1°
c = 8.978 (9) Å	µ = 11.29 mm−1
β = 100.76 (5)°	T = 293 K
V = 1596.0 (18) Å3	Prism, brown
Z = 8	0.43 × 0.25 × 0.18 mm

Enraf–Nonius TurboCAD-4 diffractometer	Rint = 0.089
Radiation source: fine-focus sealed tube	θmax = 25.0°, θmin = 2.2°
non–profiled ω/2τ scans	h = −16→15
Absorption correction: part of the refinement model (ΔF) (Walker & Stuart 1983)	k = 0→15
Tmin = 0.054, Tmax = 0.070	l = 0→10
1409 measured reflections	2 standard reflections every 120 min
1409 independent reflections	intensity decay: 1%
1227 reflections with I > 2σ(I)

Refinement on F2	118 parameters
Least-squares matrix: full	0 restraints
R[F2 > 2σ(F2)] = 0.036	w = 1/[σ2(Fo2) + (0.0565P)2 + 31.2735P] where P = (Fo2 + 2Fc2)/3
wR(F2) = 0.110	(Δ/σ)max < 0.001
S = 1.05	Δρmax = 0.85 e Å−3
1409 reflections	Δρmin = −0.76 e Å−3

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.

	x	y	z	Uiso*/Ueq	Occ. (<1)
Rb	0.18260 (6)	0.24668 (6)	0.22827 (9)	0.0316 (3)
Zn1	0.87122 (6)	0.55912 (6)	0.11383 (9)	0.0169 (3)	0.5
Fe1	0.87122 (6)	0.55912 (6)	0.11383 (9)	0.0169 (3)	0.5
Zn2	0.92406 (6)	0.12098 (6)	−0.05652 (9)	0.0166 (3)	0.5
Fe2	0.92406 (6)	0.12098 (6)	−0.05652 (9)	0.0166 (3)	0.5
P1	0.14761 (12)	0.06205 (13)	−0.08572 (19)	0.0166 (4)
O11	0.1420 (4)	−0.0526 (4)	−0.0852 (6)	0.0295 (12)
O12	0.2450 (3)	0.1026 (4)	0.0096 (6)	0.0243 (11)
O13	0.3570 (5)	0.3996 (5)	0.2456 (6)	0.0397 (15)
O14	0.0638 (4)	0.1055 (5)	−0.0151 (7)	0.0385 (14)
P2	0.92645 (12)	0.36174 (12)	−0.03358 (18)	0.0144 (4)
O21	0.8903 (5)	0.2550 (4)	−0.0146 (7)	0.0323 (13)
O22	0.0389 (4)	0.3613 (4)	−0.0253 (6)	0.0269 (11)
O23	0.3731 (4)	0.0942 (5)	0.3168 (6)	0.0367 (14)
O24	0.8990 (4)	0.4217 (4)	0.0972 (6)	0.0252 (11)

	U11	U22	U33	U12	U13	U23
Rb	0.0383 (5)	0.0321 (5)	0.0259 (4)	0.0006 (3)	0.0097 (3)	−0.0044 (3)
Zn1	0.0197 (4)	0.0151 (5)	0.0149 (4)	0.0019 (3)	0.0006 (3)	−0.0023 (3)
Fe1	0.0197 (4)	0.0151 (5)	0.0149 (4)	0.0019 (3)	0.0006 (3)	−0.0023 (3)
Zn2	0.0194 (5)	0.0149 (5)	0.0149 (4)	−0.0013 (3)	0.0021 (3)	−0.0026 (3)
Fe2	0.0194 (5)	0.0149 (5)	0.0149 (4)	−0.0013 (3)	0.0021 (3)	−0.0026 (3)
P1	0.0191 (8)	0.0138 (8)	0.0156 (8)	−0.0026 (7)	−0.0004 (6)	0.0035 (6)
O11	0.043 (3)	0.014 (3)	0.033 (3)	−0.003 (2)	0.013 (2)	0.003 (2)
O12	0.019 (2)	0.023 (3)	0.028 (3)	−0.0014 (19)	−0.003 (2)	−0.003 (2)
O13	0.059 (4)	0.043 (3)	0.015 (3)	−0.011 (3)	0.002 (3)	0.011 (2)
O14	0.021 (3)	0.042 (3)	0.052 (4)	0.000 (2)	0.007 (3)	−0.017 (3)
P2	0.0211 (9)	0.0095 (8)	0.0127 (8)	−0.0020 (6)	0.0031 (6)	0.0001 (6)
O21	0.053 (4)	0.016 (3)	0.034 (3)	−0.010 (2)	0.024 (3)	−0.010 (2)
O22	0.023 (3)	0.023 (3)	0.037 (3)	0.000 (2)	0.010 (2)	0.006 (2)
O23	0.037 (3)	0.056 (4)	0.016 (3)	−0.006 (3)	0.004 (2)	−0.011 (2)
O24	0.040 (3)	0.014 (2)	0.023 (3)	0.005 (2)	0.009 (2)	−0.0051 (19)

Rb—O21i	2.925 (6)	Zn1—O22vi	1.900 (5)
Rb—O12	2.979 (5)	Zn2—O13vii	1.860 (6)
Rb—O14	3.098 (6)	Zn2—O14viii	1.878 (5)
Rb—O13	3.107 (6)	Zn2—O21	1.897 (5)
Rb—O22	3.109 (5)	Zn2—O11ix	1.919 (5)
Rb—O24i	3.123 (5)	P1—O13iii	1.514 (5)
Rb—O11ii	3.181 (5)	P1—O14	1.519 (6)
Rb—O12iii	3.215 (6)	P1—O11	1.527 (5)
Rb—O23	3.269 (6)	P1—O12	1.535 (5)
Rb—O21iv	3.298 (7)	P2—O22viii	1.517 (5)
Zn1—O23v	1.877 (5)	P2—O23vii	1.520 (5)
Zn1—O24	1.879 (5)	P2—O24	1.523 (5)
Zn1—O12v	1.886 (5)	P2—O21	1.522 (5)
O21i—Rb—O12	142.06 (14)	O12iii—Rb—O23	102.79 (14)
O21i—Rb—O14	115.16 (17)	O21i—Rb—O21iv	76.81 (19)
O12—Rb—O14	47.17 (13)	O12—Rb—O21iv	98.31 (14)
O21i—Rb—O13	108.19 (16)	O14—Rb—O21iv	139.07 (14)
O12—Rb—O13	98.37 (15)	O13—Rb—O21iv	54.77 (14)
O14—Rb—O13	136.49 (16)	O22—Rb—O21iv	148.78 (13)
O21i—Rb—O22	110.80 (16)	O24i—Rb—O21iv	89.55 (14)
O12—Rb—O22	92.90 (15)	O11ii—Rb—O21iv	80.58 (14)
O14—Rb—O22	66.86 (16)	O12iii—Rb—O21iv	98.09 (13)
O13—Rb—O22	94.89 (14)	O23—Rb—O21iv	44.69 (13)
O21i—Rb—O24i	47.19 (13)	O23v—Zn1—O24	110.7 (3)
O12—Rb—O24i	169.12 (13)	O23v—Zn1—O12v	104.6 (2)
O14—Rb—O24i	128.20 (14)	O24—Zn1—O12v	115.9 (2)
O13—Rb—O24i	79.99 (16)	O23v—Zn1—O22vi	112.0 (3)
O22—Rb—O24i	76.59 (15)	O24—Zn1—O22vi	110.8 (2)
O21i—Rb—O11ii	56.47 (13)	O12v—Zn1—O22vi	102.6 (2)
O12—Rb—O11ii	85.60 (14)	O13vii—Zn2—O14viii	118.1 (3)
O14—Rb—O11ii	76.11 (17)	O13vii—Zn2—O21	103.5 (3)
O13—Rb—O11ii	135.33 (15)	O14viii—Zn2—O21	109.7 (3)
O22—Rb—O11ii	129.51 (14)	O13vii—Zn2—O11ix	110.9 (3)
O24i—Rb—O11ii	103.19 (13)	O14viii—Zn2—O11ix	113.5 (3)
O21i—Rb—O12iii	139.13 (14)	O21—Zn2—O11ix	98.8 (2)
O12—Rb—O12iii	78.67 (16)	O13iii—P1—O14	111.5 (4)
O14—Rb—O12iii	95.38 (16)	O13iii—P1—O11	110.3 (3)
O13—Rb—O12iii	45.51 (13)	O14—P1—O11	109.7 (3)
O22—Rb—O12iii	55.68 (13)	O13iii—P1—O12	106.8 (3)
O24i—Rb—O12iii	92.85 (14)	O14—P1—O12	105.7 (3)
O11ii—Rb—O12iii	163.87 (13)	O11—P1—O12	112.8 (3)
O21i—Rb—O23	101.14 (15)	O22viii—P2—O23vii	110.8 (3)
O12—Rb—O23	56.67 (14)	O22viii—P2—O24	110.8 (3)
O14—Rb—O23	94.63 (15)	O23vii—P2—O24	109.5 (3)
O13—Rb—O23	80.27 (17)	O22viii—P2—O21	109.5 (3)
O22—Rb—O23	147.51 (14)	O23vii—P2—O21	110.3 (3)
O24i—Rb—O23	132.85 (14)	O24—P2—O21	105.7 (3)
O11ii—Rb—O23	64.93 (15)