The alluaudite-like phosphate Na_1.79Mg_1.79Fe_1.21(PO₄)₃

The title phosphate, Na_1.79Mg_1.79Fe_1.21(PO₄)₃ (sodium mag­nesium iron phosphate), has been prepared as single crystals and its structure determined from X-ray diffraction data. The structure belongs to the alluaudite structure type, characterized by the X2X1M1M2₂(PO₄)₃ general formula. The Na⁺ ions occupy the large X2 and X1 sites, the Fe³⁺ ions partially occupy the M2 site and the Mg²⁺ ions are distributed over the M1 and M2 sites.

Key indicators

• Single-crystal X-ray study
• T = 293 K
• Mean [sigma](Mg-O) = 0.002 Å
• Disorder in solvent or counterion
• R factor = 0.023
• wR factor = 0.071
• Data-to-parameter ratio = 10.7

checkCIF results

No syntax errors found

ADDSYM reports no extra symmetry


 Alert Level C:



PLAT_302  Alert C Anion/Solvent Disorder .......................      40.00 Perc.


 0 Alert Level A = Potentially serious problem

 0 Alert Level B = Potential problem

 1 Alert Level C = Please check

---

S1. Comment

The phosphates with the alluaudite structure type (monoclinic C2/c, Z = 4) (Moore, 1971) are studied quite frequently. These materials present the X(2)X(1)M(1)M(2)₂(PO₄)₃ general formula and are known to be highly susceptible to substitution in the X(2), X(1), M(1) and M(2) sites, which can incorporate a variety of cations with different ionic radii and charges, leading to the formation of solid solutions in large composition domains (Hatert et al., 2000, 2002).

During an investigation of monophosphates belonging to the Na₃PO₄—Mg₃(PO₄)₂-FePO₄ system, in order to clear up the role of Mg²⁺ in the crystal chemistry of such materials, we have isolated the Na_{1 + x}Mg_{1 + x}Fe_{2 − x}(PO₄)₃ (x from 0.5 to 1) alluaudite-like solid solution. The structure of the x = 0.79 composition was determined by X-ray diffraction. This structure, as viewed along the [001] direction, is shown in Fig. 1. It consists of M2₂O₁₀ (M2 = 0.4 Mg2 + 0.6 Fe2) units of edge-sharing M2O₆ octahedra. These units share opposite edges with Mg1O₆ octahedra to form infinite –M2—M2—Mg1- chains running along the [101] direction (Fig. 2). These chains are linked to similar neighboring chains via common corners of P1O₄ and P2O₄ tetrahedra. The P1O₄ tetrahedron connects two chains by sharing each pair of its O atoms with one chain. The P2O₄ tetrahedron connects three adjacent chains by sharing two of its O atoms with one chain and the remaining two O atoms with two different chains.

The three-dimensional framework constructed in this way generates two crystallographically independant tunnels growing along the c axis. These tunnels, located at 0,0,z and at 1/2,0,z, respectively, are available for the Na⁺ cations.

Table 1 lists main bond distances and angles in Na_1.79Mg_1.79Fe_1.21(PO₄)₃. The M2O₆ octahedron is distorted as indicated by the M2—O bond lengths and O—M2—O bond angles. Such distortion can be correlated to the rigidity of phosphates groups which connect the chains. The Mg1O₆ octahedron is more distorted than M2O₆. However, the average Mg1—O distance is consistent with those observed in other phosphates containing Mg²⁺ in an octahedral environment (Alkemper & Fuess, 1998).

The interatomic distances in the phosphate tetrahedra are similar to those observed in phosphates without hydrogen bonding (Corbin et al., 1986; Korzenski et al., 1998; Warner et al., 1993). The P1O₄ tetrahedron is slightly more regular than P2O₄, which is in accordance with the fact that the P1 atom sits in a twofold axis, while atom P2 is located in a general position.

The coordination environment of each sodium ion was determined assuming Na—O distances below 3.0 Å. Na1 has a very distorted cubic environment. Each Na1O₈ polyhedron shares faces with equivalent polyhedra that form chains in the c direction. The Na2 site is partially filled with an occupation number of 0.79. This site has a strongly irregular environment which consists of four O atoms at short distances and four others at rather long distances.

The structure of Na_1.79Mg_1.79Fe_1.21(PO₄)₃ was compared to that of the natural alluaudite from the Buragna pegmatite of Central Africa (Moore, 1971). In spite of numerous similarities, the two compounds are not isostructural, since the cation distribution within the sites in the tunnels is not exactly the same. In fact, in the title compound, the X(2) site (at 0, ~0, 0) is partially occupied while the X(1) site (at 1/2, 0, 0) is totally filled with Na⁺ cations. By contrast, the natural alluaudite of Buragna features an X(1) site partially occupied by Na⁺ and Ca²⁺ cations and an empty X(2) site.

S2. Experimental

Crystals of Na_1.79Mg_1.79Fe_1.21(PO₄)₃ were grown in a flux of sodium dimolybdate with a 1:1 molar ratio. Amounts of starting products corresponding to the stoichiometry of the crystals studied [Fe(NO₃)₃·9H₂O, Mg(NO₃)₂·6H₂O, NaH₂PO₄, (NH₄)₂HPO₄ and MoO₃] were mixed and gradually heated up to 873 K to allow ammonia, water and carbon dioxide to evolve. After a final grinding, the sample was melted for 1 h at 1173 K and then cooled down to room temperature with a rate of 10 K h⁻¹. The crystals, obtained after washing with hot water to remove the flux, were yellow and presented elongated and parallelepipedic forms.

S3. Refinement

The Fe atoms were located by direct methods, and the remaining atoms were found by successive difference Fourier maps. The occupation number of the M2 site was constrained to 1.0 and the occupation number of the Na2 site was set equal to the partial occupation of Mg2 in the M2 site according to the Na_{1 + x}Mg_{1 + x}Fe_{2 − x}(PO₄)₃ formula.

Computing details

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, 1997); molecular graphics: ORTEP-3 for Windows (Farrugia, 1997); software used to prepare material for publication: WinGX (Farrugia, 1998).

Figures

	Fig. 1. The structure of Na1.79Mg1.79Fe1.21(PO4)3, viewed along the c direction. The Mg1O6 octahedra are illustrated by cross-hatched patterns. The remaining polyhedra are the M2O6 (M2 = Fe or Mg) octahedra and PO4 tetrahedra. The Na^+ cations are represented by solid circles.
	Fig. 2. A view of the infinite chains in the title compound. Displacement ellepsoids are drawn at the 50% probability level.

(I)

Crystal data

Fe1.21Mg1.79Na1.79O12P3	F(000) = 854.5
Mr = 437.5	Dx = 3.36 Mg m^−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2yc	Cell parameters from 25 reflections
a = 11.791 (3) Å	θ = 10–12°
b = 12.489 (3) Å	µ = 2.93 mm^−1
c = 6.4191 (10) Å	T = 293 K
β = 113.82 (2)°	Parallelepiped, yellow
V = 864.7 (4) Å^3	0.25 × 0.15 × 0.1 mm
Z = 4

Data collection

Enraf-Nonius TurboCAD4 diffractometer	Rint = 0.014
non–profiled ω/2θ scans	θmax = 28.0°, θmin = 2.5°
Absorption correction: analytical 'Alcock (1970)'	h = −15→14
Tmin = 0.59, Tmax = 0.73	k = −16→0
1138 measured reflections	l = 0→8
1048 independent reflections	2 standard reflections every 120 min
982 reflections with I > 2σ(I)	intensity decay: none

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)] = 0.023
wR(F^2) = 0.071	(Δ/σ)max = 0.033
S = 1.15	Δρmax = 0.48 e Å^−3
1048 reflections	Δρmin = −0.75 e Å^−3
98 parameters	Extinction correction: SHELXL97, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
2 constraints	Extinction coefficient: 0.0037 (6)

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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq	Occ. (<1)
Fe2	0.21714 (4)	0.15533 (3)	0.12852 (7)	0.00778 (16)	0.6041 (6)
Mg2	0.21714 (4)	0.15533 (3)	0.12852 (7)	0.00778 (16)	0.3960 (6)
Mg1	0	0.26564 (9)	0.25	0.0088 (2)
Na1	0.5	0	0	0.0234 (4)
Na2	0	−0.0115 (3)	0.25	0.0466 (7)	0.7917 (13)
P1	0.5	0.21062 (6)	0.25	0.00583 (19)
O11	0.45796 (15)	0.28120 (14)	0.0322 (3)	0.0087 (3)
O12	0.39708 (16)	0.13456 (14)	0.2493 (3)	0.0123 (4)
P2	0.23741 (5)	0.11008 (5)	0.62617 (10)	0.00601 (17)
O21	0.37541 (16)	0.10417 (14)	0.6758 (3)	0.0112 (4)
O22	0.17902 (17)	−0.00119 (14)	0.6170 (3)	0.0118 (4)
O23	0.16302 (16)	0.16543 (14)	0.3925 (3)	0.0098 (4)
O24	0.22191 (16)	0.17812 (14)	0.8158 (3)	0.0099 (3)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Fe2	0.0075 (2)	0.0071 (2)	0.0092 (2)	0.00082 (15)	0.00386 (18)	0.00084 (15)
Mg2	0.0075 (2)	0.0071 (2)	0.0092 (2)	0.00082 (15)	0.00386 (18)	0.00084 (15)
Mg1	0.0088 (5)	0.0081 (5)	0.0108 (5)	0	0.0053 (4)	0
Na1	0.0323 (9)	0.0092 (7)	0.0151 (8)	−0.0018 (6)	−0.0045 (7)	−0.0009 (6)
Na2	0.0290 (14)	0.0504 (18)	0.0477 (18)	0	0.0022 (13)	0
P1	0.0057 (4)	0.0051 (4)	0.0062 (4)	0	0.0020 (3)	0
O11	0.0078 (7)	0.0106 (8)	0.0073 (7)	0.0003 (6)	0.0025 (6)	0.0025 (6)
O12	0.0090 (8)	0.0095 (8)	0.0173 (9)	−0.0016 (6)	0.0042 (7)	0.0037 (7)
P2	0.0066 (3)	0.0048 (3)	0.0067 (3)	0.00032 (19)	0.0028 (2)	0.0002 (2)
O21	0.0087 (8)	0.0093 (8)	0.0166 (9)	0.0005 (6)	0.0060 (7)	−0.0002 (7)
O22	0.0134 (8)	0.0073 (7)	0.0151 (9)	−0.0018 (7)	0.0061 (7)	0.0013 (7)
O23	0.0116 (8)	0.0081 (8)	0.0090 (8)	0.0013 (6)	0.0036 (7)	0.0005 (6)
O24	0.0105 (8)	0.0104 (8)	0.0088 (8)	0.0001 (6)	0.0041 (6)	−0.0007 (6)

Geometric parameters (Å, º)

Fe2—O12	1.9599 (19)	Na1—P1^x	3.0813 (9)
Fe2—O22^i	1.9715 (19)	Na1—Na1^viii	3.2096 (5)
Fe2—O23	2.0415 (18)	Na2—O22	2.451 (2)
Fe2—O24^ii	2.0506 (18)	Na2—O22^i	2.5816 (19)
Fe2—O11^iii	2.0579 (18)	Na2—O23	2.827 (3)
Fe2—O24^iv	2.1825 (19)	Na2—O11^xi	2.889 (3)
Fe2—Mg2^iii	3.1539 (10)	Na2—Na2^xii	3.2223 (8)
Fe2—Na2^v	3.265 (2)	Na2—P2	3.2420 (18)
Fe2—Mg1	3.2670 (9)	P1—O12^viii	1.5398 (18)
Mg1—O21^iv	2.113 (2)	P1—O11^viii	1.5547 (17)
Mg1—O11^iii	2.1436 (17)	P1—Mg2^xiii	3.1995 (10)
Mg1—O23^vi	2.1628 (19)	P2—O21	1.5283 (18)
Mg1—Na1^vii	3.3380 (12)	P2—O22	1.5414 (18)
Na1—O12^viii	2.3041 (18)	P2—O24	1.5546 (18)
Na1—O21^ix	2.3898 (19)	P2—O23	1.5601 (18)
Na1—O21^i	2.5424 (18)	P2—Na1^xiv	3.3517 (11)
Na1—O12^x	2.9030 (19)
O12—Fe2—O22^i	94.56 (8)	O12^viii—Na1—O12^x	125.99 (7)
O12—Fe2—O23	109.33 (8)	O12^i—Na1—O12^x	54.01 (7)
O22^i—Fe2—O23	87.05 (7)	O21^ix—Na1—O12^x	85.30 (6)
O12—Fe2—O24^ii	87.27 (8)	O21^ii—Na1—O12^x	94.70 (6)
O22^i—Fe2—O24^ii	101.23 (8)	O21^i—Na1—O12^x	113.19 (5)
O23—Fe2—O24^ii	160.97 (7)	O21^viii—Na1—O12^x	66.81 (5)
O12—Fe2—O11^iii	162.93 (7)	O12^viii—Na1—O12	54.01 (7)
O22^i—Fe2—O11^iii	100.95 (7)	O22—Na2—O22^vi	174.00 (17)
O23—Fe2—O11^iii	78.80 (7)	O22—Na2—O22^i	79.21 (6)
O24^ii—Fe2—O11^iii	82.78 (7)	O22^vi—Na2—O22^i	100.41 (6)
O12—Fe2—O24^iv	80.16 (7)	O22^i—Na2—O22^xii	172.99 (16)
O22^i—Fe2—O24^iv	172.65 (7)	O22—Na2—O23	55.87 (7)
O23—Fe2—O24^iv	89.88 (7)	O22^vi—Na2—O23	118.64 (12)
O24^ii—Fe2—O24^iv	83.72 (7)	O22^i—Na2—O23	61.27 (7)
O11^iii—Fe2—O24^iv	84.97 (7)	O22^xii—Na2—O23	112.65 (10)
O21^iv—Mg1—O21^xv	79.36 (10)	O23—Na2—O23^vi	77.21 (11)
O21^iv—Mg1—O11^iii	91.34 (7)	O22—Na2—O11^xi	115.15 (11)
O21^xv—Mg1—O11^iii	113.37 (7)	O22^vi—Na2—O11^xi	70.66 (7)
O11^iii—Mg1—O11^xvi	148.33 (11)	O22^i—Na2—O11^xi	84.16 (7)
O21^iv—Mg1—O23^vi	163.53 (8)	O22^xii—Na2—O11^xi	102.21 (9)
O21^xv—Mg1—O23^vi	86.08 (7)	O23—Na2—O11^xi	144.92 (5)
O11^iii—Mg1—O23^vi	87.36 (7)	O23^vi—Na2—O11^xi	125.63 (5)
O11^xvi—Mg1—O23^vi	74.35 (7)	O11^xi—Na2—O11^xvii	52.63 (9)
O21^xv—Mg1—O23	163.53 (8)	O12^viii—P1—O12	103.82 (14)
O23^vi—Mg1—O23	109.29 (11)	O12^viii—P1—O11^viii	112.49 (9)
O12^viii—Na1—O12^i	180.00 (11)	O12—P1—O11^viii	108.48 (9)
O12^viii—Na1—O21^ix	79.85 (6)	O11^viii—P1—O11	110.92 (14)
O12^viii—Na1—O21^ii	100.15 (6)	O21—P2—O22	112.70 (10)
O12^viii—Na1—O21^i	107.40 (6)	O21—P2—O24	108.36 (10)
O12^i—Na1—O21^i	72.60 (6)	O22—P2—O24	109.31 (10)
O21^ix—Na1—O21^i	66.24 (7)	O21—P2—O23	111.09 (10)
O21^ii—Na1—O21^i	113.76 (7)	O22—P2—O23	107.08 (10)
O21^i—Na1—O21^viii	180	O24—P2—O23	108.21 (10)

Symmetry codes: (i) x, −y, z−1/2; (ii) x, y, z−1; (iii) −x+1/2, −y+1/2, −z; (iv) −x+1/2, −y+1/2, −z+1; (v) −x, −y, −z; (vi) −x, y, −z+1/2; (vii) −x+1/2, y+1/2, −z+1/2; (viii) −x+1, y, −z+1/2; (ix) −x+1, −y, −z+1; (x) −x+1, −y, −z; (xi) x−1/2, y−1/2, z; (xii) −x, −y, −z+1; (xiii) x+1/2, −y+1/2, z+1/2; (xiv) x, y, z+1; (xv) x−1/2, −y+1/2, z−1/2; (xvi) x−1/2, −y+1/2, z+1/2; (xvii) −x+1/2, y−1/2, −z+1/2.