RbZnFe(PO4)2: synthesis and crystal structure

Badri, A.; Ben Amara, M.

doi:10.1107/S205698901601046X

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Volume 72| Part 8| August 2016| Pages 1074-1076

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RbZnFe(PO₄)₂: synthesis and crystal structure

Abdessalem Badri ^a ^* and Mongi Ben Amara ^a

^aUnité de recherche, Matériaux Inorganiques, Faculté des Sciences, Université de Monastir, 5019 Monastir, Tunisia
^*Correspondence e-mail: badri_abdessalem@yahoo.fr

Edited by I. D. Brown, McMaster University, Canada (Received 6 June 2016; accepted 28 June 2016; online 7 July 2016)

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 tetrahedra. This mode of linkage forms channels parallel to the [100], [010] and [001] directions in which the Rb⁺ ions are located.

Keywords: crystal structure; iron phosphate; rubidium; open-framework structure.

CCDC reference: 1488120

1. 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 ).

2. 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)

3. 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.

4. 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[F² > 2σ(F²)], wR(F²), S	0.036, 0.110, 1.05
No. of reflections	1409
No. of parameters	118
	w = 1/[σ²(F_o²) + (0.0565P)² + 31.2735P] where P = (F_o² + 2F_c²)/3
Δρ_max, Δρ_min (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 ( $[\overline{6}]$ 85, $[\overline{9}]$ 34, $[\overline{8}]$ 85 and $[\overline{3}]$ 75) were omitted as the difference between the observed and calculated structure factors was larger than 10σ.

Supporting information

CCDC reference: 1488120

Crystal structure: contains datablocks global, I. DOI: https://doi.org/10.1107/S205698901601046X/br2261sup1.cif

Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S205698901601046X/br2261Isup2.hkl

checkCIF report

Computing details top

Cell refinement: CAD-4 EXPRESS (Enraf–Nonius, 1994); data reduction: XCAD4 (Harms & Wocadlo, 1995); program(s) used to solve structure: SIR92 (Altomare et al., 1993); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: DIAMOND (Brandenburg, 1999); software used to prepare material for publication: WinGX (Farrugia, 2012).

Rubidium zinc Iron(III) phosphate top

Crystal data top

RbZnFe(PO₄)₂	F(000) = 1496
M_r = 396.63	D_x = 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

Data collection top

Enraf–Nonius TurboCAD-4 diffractometer	R_int = 0.089
Radiation source: fine-focus sealed tube	θ_max = 25.0°, θ_min = 2.2°
non–profiled ω/2τ scans	h = −16→15
Absorption correction: part of the refinement model (ΔF) (Walker & Stuart 1983)	k = 0→15
T_min = 0.054, T_max = 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 top

Refinement on F²	118 parameters
Least-squares matrix: full	0 restraints
R[F² > 2σ(F²)] = 0.036	w = 1/[σ²(F_o²) + (0.0565P)² + 31.2735P] where P = (F_o² + 2F_c²)/3
wR(F²) = 0.110	(Δ/σ)_max < 0.001
S = 1.05	Δρ_max = 0.85 e Å⁻³
1409 reflections	Δρ_min = −0.76 e Å⁻³

Special details top

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 (Å²) top

	x	y	z	U_iso*/U_eq	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)

Atomic displacement parameters (Å²) top

	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)

Geometric parameters (Å, º) top

Rb—O21ⁱ	2.925 (6)	Zn1—O22^vi	1.900 (5)
Rb—O12	2.979 (5)	Zn2—O13^vii	1.860 (6)
Rb—O14	3.098 (6)	Zn2—O14^viii	1.878 (5)
Rb—O13	3.107 (6)	Zn2—O21	1.897 (5)
Rb—O22	3.109 (5)	Zn2—O11^ix	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—O21^iv	3.298 (7)	P2—O22^viii	1.517 (5)
Zn1—O23^v	1.877 (5)	P2—O23^vii	1.520 (5)
Zn1—O24	1.879 (5)	P2—O24	1.523 (5)
Zn1—O12^v	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—O21^iv	76.81 (19)
O12—Rb—O14	47.17 (13)	O12—Rb—O21^iv	98.31 (14)
O21ⁱ—Rb—O13	108.19 (16)	O14—Rb—O21^iv	139.07 (14)
O12—Rb—O13	98.37 (15)	O13—Rb—O21^iv	54.77 (14)
O14—Rb—O13	136.49 (16)	O22—Rb—O21^iv	148.78 (13)
O21ⁱ—Rb—O22	110.80 (16)	O24ⁱ—Rb—O21^iv	89.55 (14)
O12—Rb—O22	92.90 (15)	O11ⁱⁱ—Rb—O21^iv	80.58 (14)
O14—Rb—O22	66.86 (16)	O12ⁱⁱⁱ—Rb—O21^iv	98.09 (13)
O13—Rb—O22	94.89 (14)	O23—Rb—O21^iv	44.69 (13)
O21ⁱ—Rb—O24ⁱ	47.19 (13)	O23^v—Zn1—O24	110.7 (3)
O12—Rb—O24ⁱ	169.12 (13)	O23^v—Zn1—O12^v	104.6 (2)
O14—Rb—O24ⁱ	128.20 (14)	O24—Zn1—O12^v	115.9 (2)
O13—Rb—O24ⁱ	79.99 (16)	O23^v—Zn1—O22^vi	112.0 (3)
O22—Rb—O24ⁱ	76.59 (15)	O24—Zn1—O22^vi	110.8 (2)
O21ⁱ—Rb—O11ⁱⁱ	56.47 (13)	O12^v—Zn1—O22^vi	102.6 (2)
O12—Rb—O11ⁱⁱ	85.60 (14)	O13^vii—Zn2—O14^viii	118.1 (3)
O14—Rb—O11ⁱⁱ	76.11 (17)	O13^vii—Zn2—O21	103.5 (3)
O13—Rb—O11ⁱⁱ	135.33 (15)	O14^viii—Zn2—O21	109.7 (3)
O22—Rb—O11ⁱⁱ	129.51 (14)	O13^vii—Zn2—O11^ix	110.9 (3)
O24ⁱ—Rb—O11ⁱⁱ	103.19 (13)	O14^viii—Zn2—O11^ix	113.5 (3)
O21ⁱ—Rb—O12ⁱⁱⁱ	139.13 (14)	O21—Zn2—O11^ix	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)	O22^viii—P2—O23^vii	110.8 (3)
O12—Rb—O23	56.67 (14)	O22^viii—P2—O24	110.8 (3)
O14—Rb—O23	94.63 (15)	O23^vii—P2—O24	109.5 (3)
O13—Rb—O23	80.27 (17)	O22^viii—P2—O21	109.5 (3)
O22—Rb—O23	147.51 (14)	O23^vii—P2—O21	110.3 (3)
O24ⁱ—Rb—O23	132.85 (14)	O24—P2—O21	105.7 (3)
O11ⁱⁱ—Rb—O23	64.93 (15)

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

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