Crystal structure of strontium dicobalt iron(III) tris­­(orthophosphate): SrCo₂Fe(PO₄)₃

The title compound, SrCo₂Fe(PO₄)₃, has been synthesized by a solid-state reaction. It crystallizes with the α-CrPO₄ type structure. In this structure, all atoms are on special positions of the Imma space group, except for two O atoms which are located on general positions. The three-dimensional network in the crystal structure is made up of two types of layers stacked normal to (100). The first layer is built from two edge-sharing CoO₆ octa­hedra, leading to the formation of Co₂O₁₀ dimers that are connected to two PO₄ tetra­hedra by a common edge and corners. The second layer results from apex-sharing FeO₆ octa­hedra and PO₄ tetra­hedra, which form linear chains alternating with a zigzag chain of Sr^II cations. These layers are linked together by common vertices of PO₄ tetra­hedra and FeO₆ octa­hedra to form an open three-dimensional framework that delimits two types of channels parallel to [100] and [010] where the Sr^II cations are located. Each Sr^II cation is surrounded by eight O atoms.

1. Chemical context

The phosphate literature includes important works on the structural study of transition metal phosphates. The basic framework is built from tetra­hedrally coordinated phospho­rus linked to transition metals M in various environments, such as MO_n (n = 4, 5 or 6). The manner in which polyhedra are inter­connected generates important structure types with porous or lamellar set-ups that can exhibit inter­esting physical properties. Accordingly, widespread studies have been devoted to this family of compounds, stimulated by the wide range of potential and commercial applications of these materials. Examples include applications in catalysis, as ion exchangers and in the manufacture of lithium and sodium rechargeable batteries. One particular scientific area in our laboratory is focused on investigating new functional phosphates belonging to the alluaudite (Moore, 1971) or α-CrPO₄ (Attfield et al., 1988) structure types, owing to their potential use as new cathode materials for battery devices (Trad et al., 2010; Kim et al., 2014; Huang et al., 2015).

Our earlier hydro­thermal investigations were undertaken with the A₂O–MO–P₂O₅ and M′O–MO–P₂O₅ systems (A = monovalent cations, M and M′ = divalent cations) with approximate molar ratios A:M:P = 2:3:3 and M′:M:P = 1:3:3, which characterize the alluaudite or α-CrPO₄ phases. Those studies involved the synthesis and structural characterization of new phosphates such as Na₂Co₂Fe(PO₄)₃ (Bouraima et al., 2015) and Na_1.67Zn_1.67Fe_1.33(PO₄)₃ (Khmiyas et al., 2015) belonging to the alluaudite-type structure group. In addition, divalent and trivalent transition-metal-based phosphates, such as SrNi₂Fe(PO₄)₃ (Ouaatta et al., 2015) and MMn^II₂Mn^III(PO₄)₃ (M = Pb, Sr, Ba) (Alhakmi et al., 2013a,b; Assani et al., 2013) have been shown to adopt the α-CrPO₄ structure type.

In search of a new promising phosphate, a solid-state chemistry investigation of A₂O–MO–M′₂O₃–P₂O₅ systems was undertaken. The present work reports on synthesis and crystal structure of the new strontium cobalt iron phosphate, SrCo₂Fe(PO₄)₃, which has the α-CrPO₄ type structure.

2. Structural commentary

In the title phosphate, SrCo₂Fe(PO₄)₃, all atoms are on special positions, except two oxygen atoms (O3, O4) which are on general positions of the Imma space group. Its three-dimensional structure is constructed on the basis of PO₄ tetra­hedra, FeO₆ and CoO₆ octa­hedra, as shown in Fig. 1. The connection between these polyhedra produces two types of layers stacked normal to (100). The first layer is built from two edge-sharing CoO₆ octa­hedra, leading to the formation of Co₂O₁₀ dimers, which are connected to two PO₄ tetra­hedra by a common edge and vertex, as shown in Fig. 2. The second layer is formed by alternating FeO₆ octa­hedra and PO₄ tetra­hedra, which share corners, building linear chains that surround a zigzag chain of Sr^II cations (see Fig. 3). The layers are joined by the apices of PO₄ tetra­hedra and FeO₆ octa­hedra, giving rise to an open three-dimensional framework that delimits two types of channels parallel to [100] and [010] where the Sr^II cations are located, as shown in Fig. 4 and Fig. 5. This structure is characterized by a stoichiometric composition in which the Sr atom is surrounded by eight oxygen atoms with Sr—O bond lengths that vary between 2.6561 (13) and 2.6690 (9)Å. The same Sr environment is observed in the manganese homologue phosphates, namely MMn^II₂Mn^III(PO₄)₃ (M = Pb, Sr, Ba).

Figure 1 The principal building units in the structure of the title compound. Displacement ellipsoids are drawn at the 50% probability level. [Symmetry codes: (i) −x + 2, −y + , z + 1; (ii) x, y, z + 1; (iii) −x + 2, −y + , z; (iv) −x + , −y + 1, z + ; (v) x + , y + , z + ; (vi) −x + , y + , z + ; (ix) −x + , y, −z + ; (x) x, −y + 1, −z; (xi) −x + 1, y, z; (xii) x, −y + 1, −z + 1; (xiii) −x + 1, −y + 1, −z + 1;(xiv) x − , y, −z + .]

Figure 2 Edge-sharing [CoO6] octa­hedra forming a layer parallel to (100).

Figure 3 A view along the a axis, showing a layer resulting from chains connected via vertices of PO4 tetra­hedra and FeO6 octa­hedra, alternating with a zigzag chain of Sr atoms.

Figure 4 Polyhedral representation of SrCo2Fe(PO4)3, showing channels running along [100].

Figure 5 Polyhedral representation of SrCo2Fe(PO4)3, showing channels running along [010].

3. Synthesis and crystallization

The title phosphate, SrCo₂Fe(PO₄)₃, was synthesized in a solid-state reaction by mixing nitrates of strontium, cobalt and iron along with NH₄H₂PO₄, taken in the molar proportions Sr:Co:Fe:P = 1:2:1:3. After a series of heat treatments up to 873 K in a platinum crucible, inter­spersed with grinding, the reaction mixture was heated to the melt (1343 K). The molten product was then cooled to room temperature at 5 K/h. The resulting solid contained brown crystals of a suitable size for X-ray diffraction.

4. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 1. The highest peak and the deepest hole in the final Fourier map are at 0.63 and 0.68 Å from Sr1 and P2, respectively.

Table 1
Experimental details

Crystal data
Chemical formula	SrCo2Fe(PO4)3
Mr	546.24
Crystal system, space group	Orthorhombic, Imma
Temperature (K)	296
a, b, c (Å)	10.4097 (2), 13.2714 (3), 6.5481 (2)
V (Å^3)	904.63 (4)
Z	4
Radiation type	Mo Kα
μ (mm^−1)	11.64
Crystal size (mm)	0.30 × 0.27 × 0.21
Data collection
Diffractometer	Bruker X8 APEX
Absorption correction	Multi-scan (SADABS; Krause et al., 2015)
Tmin, Tmax	0.595, 0.747
No. of measured, independent and observed [I > 2σ(I)] reflections	10008, 1297, 1243
Rint	0.030
(sin θ/λ)max (Å^−1)	0.858
Refinement
R[F^2 > 2σ(F^2)], wR(F^2), S	0.017, 0.046, 1.16
No. of reflections	1297
No. of parameters	54
Δρmax, Δρmin (e Å^−3)	1.00, −0.74
Computer programs: APEX2 and SAINT (Bruker, 2009), SHELXT2014 (Sheldrick, 2015a), SHELXL2014 (Sheldrick, 2015b), ORTEP-3 for Windows (Farrugia, 2012), DIAMOND (Brandenburg, 2006) and publCIF (Westrip, 2010).

The distinction between cobalt and iron by X-ray diffraction is nearly impossible. Therefore we have examined several crystallographic models during the crystal structure refinements of the title compound. Based on the stoichiometric ratio of 1:2 for iron and cobalt in the starting materials, we assumed the same ratio in the crystal structures with oxidation states of +II for cobalt and and +III for iron. The best model is obtained with Fe1 and Co1 atoms in the Wyckoff positions 4a (2/m) and 8g (2), respectively. This cationic distribution in this model corresponds to the stoichiometry of the expected compound, in addition to the electric neutrality in the structure in reasonable agreement with the final model.

Supporting information

Computing details

Data collection: APEX2 (Bruker, 2009); cell refinement: SAINT (Bruker, 2009); data reduction: SAINT (Bruker, 2009); program(s) used to solve structure: SHELXT2014 (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015b); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012) and DIAMOND (Brandenburg, 2006); software used to prepare material for publication: publCIF (Westrip, 2010).

Strontium dicobalt iron(III) tris(orthophosphate)

Crystal data

SrCo2Fe(PO4)3	Dx = 4.011 Mg m^−3
Mr = 546.24	Mo Kα radiation, λ = 0.71073 Å
Orthorhombic, Imma	Cell parameters from 1297 reflections
a = 10.4097 (2) Å	θ = 3.1–37.6°
b = 13.2714 (3) Å	µ = 11.64 mm^−1
c = 6.5481 (2) Å	T = 296 K
V = 904.63 (4) Å^3	Block, brown
Z = 4	0.30 × 0.27 × 0.21 mm
F(000) = 1036

Data collection

Bruker X8 APEX diffractometer	1297 independent reflections
Radiation source: fine-focus sealed tube	1243 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.030
φ and ω scans	θmax = 37.6°, θmin = 3.1°
Absorption correction: multi-scan (SADABS; Krause et al., 2015)	h = −17→17
Tmin = 0.595, Tmax = 0.747	k = −22→19
10008 measured reflections	l = −11→11

Refinement

Refinement on F^2	0 restraints
Least-squares matrix: full	w = 1/[σ^2(Fo^2) + (0.0245P)^2 + 0.761P] where P = (Fo^2 + 2Fc^2)/3
R[F^2 > 2σ(F^2)] = 0.017	(Δ/σ)max < 0.001
wR(F^2) = 0.046	Δρmax = 1.00 e Å^−3
S = 1.16	Δρmin = −0.74 e Å^−3
1297 reflections	Extinction correction: SHELXL2014 (Sheldrick, 2014b), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
54 parameters	Extinction coefficient: 0.0131 (4)

Special details

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 (Ų)

	x	y	z	Uiso*/Ueq
Sr1	1.0000	0.7500	0.59715 (3)	0.00785 (6)
Co1	0.7500	0.63284 (2)	0.2500	0.00537 (6)
Fe1	0.5000	0.5000	0.5000	0.00392 (7)
P1	1.0000	0.7500	0.09098 (8)	0.00336 (9)
P2	0.7500	0.42747 (3)	0.2500	0.00388 (7)
O1	1.0000	0.65633 (9)	−0.04439 (19)	0.00660 (18)
O2	0.88277 (11)	0.7500	0.23618 (18)	0.00607 (18)
O3	0.71075 (8)	0.36360 (6)	0.06735 (14)	0.00776 (14)
O4	0.63833 (7)	0.50376 (6)	0.29533 (14)	0.00600 (13)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Sr1	0.00819 (9)	0.01003 (10)	0.00534 (9)	0.000	0.000	0.000
Co1	0.00533 (9)	0.00376 (10)	0.00704 (10)	0.000	0.00073 (6)	0.000
Fe1	0.00292 (11)	0.00439 (13)	0.00443 (12)	0.000	0.000	0.00015 (9)
P1	0.00344 (18)	0.0029 (2)	0.0038 (2)	0.000	0.000	0.000
P2	0.00410 (14)	0.00365 (17)	0.00388 (14)	0.000	0.00051 (10)	0.000
O1	0.0081 (4)	0.0045 (5)	0.0073 (4)	0.000	0.000	−0.0017 (4)
O2	0.0046 (4)	0.0073 (5)	0.0063 (4)	0.000	0.0020 (3)	0.000
O3	0.0094 (3)	0.0075 (3)	0.0064 (3)	−0.0017 (3)	0.0003 (3)	−0.0023 (2)
O4	0.0050 (3)	0.0057 (3)	0.0074 (3)	0.0013 (2)	0.0021 (2)	0.0006 (2)

Geometric parameters (Å, º)

Sr1—O1^i	2.6561 (13)	Fe1—O4	1.9678 (8)
Sr1—O1^ii	2.6561 (13)	Fe1—O4^xi	1.9678 (8)
Sr1—O2^iii	2.6600 (12)	Fe1—O4^xii	1.9678 (8)
Sr1—O2	2.6600 (12)	Fe1—O4^xiii	1.9678 (8)
Sr1—O3^iv	2.6690 (9)	Fe1—O1^iv	2.0950 (12)
Sr1—O3^v	2.6690 (9)	Fe1—O1^xiv	2.0950 (12)
Sr1—O3^vi	2.6690 (9)	P1—O1^iii	1.5268 (12)
Sr1—O3^vii	2.6690 (9)	P1—O1	1.5268 (12)
Co1—O2	2.0824 (8)	P1—O2^iii	1.5470 (12)
Co1—O2^viii	2.0824 (8)	P1—O2	1.5470 (12)
Co1—O4^ix	2.0913 (8)	P2—O3	1.5219 (9)
Co1—O4	2.0914 (8)	P2—O3^ix	1.5219 (9)
Co1—O3^x	2.1183 (9)	P2—O4	1.5698 (8)
Co1—O3^iv	2.1183 (9)	P2—O4^ix	1.5698 (8)
O1^i—Sr1—O1^ii	55.81 (5)	O2^viii—Co1—O3^x	84.14 (4)
O1^i—Sr1—O2^iii	141.74 (2)	O4^ix—Co1—O3^x	89.21 (3)
O1^ii—Sr1—O2^iii	141.74 (2)	O4—Co1—O3^x	92.88 (3)
O1^i—Sr1—O2	141.74 (2)	O2—Co1—O3^iv	84.14 (4)
O1^ii—Sr1—O2	141.74 (2)	O2^viii—Co1—O3^iv	93.94 (4)
O2^iii—Sr1—O2	54.61 (5)	O4^ix—Co1—O3^iv	92.89 (3)
O1^i—Sr1—O3^iv	109.21 (2)	O4—Co1—O3^iv	89.21 (3)
O1^ii—Sr1—O3^iv	78.48 (2)	O3^x—Co1—O3^iv	177.44 (5)
O2^iii—Sr1—O3^iv	108.19 (3)	O4—Fe1—O4^xi	94.07 (5)
O2—Sr1—O3^iv	63.77 (3)	O4—Fe1—O4^xii	85.93 (5)
O1^i—Sr1—O3^v	78.48 (2)	O4^xi—Fe1—O4^xii	180.0
O1^ii—Sr1—O3^v	109.21 (2)	O4—Fe1—O4^xiii	180.0
O2^iii—Sr1—O3^v	63.77 (3)	O4^xi—Fe1—O4^xiii	85.93 (5)
O2—Sr1—O3^v	108.19 (3)	O4^xii—Fe1—O4^xiii	94.07 (5)
O3^iv—Sr1—O3^v	171.61 (4)	O4—Fe1—O1^iv	86.02 (3)
O1^i—Sr1—O3^vi	78.48 (2)	O4^xi—Fe1—O1^iv	86.02 (3)
O1^ii—Sr1—O3^vi	109.21 (2)	O4^xii—Fe1—O1^iv	93.98 (3)
O2^iii—Sr1—O3^vi	108.19 (3)	O4^xiii—Fe1—O1^iv	93.98 (3)
O2—Sr1—O3^vi	63.77 (3)	O4—Fe1—O1^xiv	93.98 (3)
O3^iv—Sr1—O3^vi	68.78 (4)	O4^xi—Fe1—O1^xiv	93.98 (3)
O3^v—Sr1—O3^vi	110.56 (4)	O4^xii—Fe1—O1^xiv	86.02 (3)
O1^i—Sr1—O3^vii	109.21 (2)	O4^xiii—Fe1—O1^xiv	86.02 (3)
O1^ii—Sr1—O3^vii	78.48 (2)	O1^iv—Fe1—O1^xiv	180.0
O2^iii—Sr1—O3^vii	63.77 (3)	O1^iii—P1—O1	109.01 (10)
O2—Sr1—O3^vii	108.19 (3)	O1^iii—P1—O2^iii	110.91 (3)
O3^iv—Sr1—O3^vii	110.56 (4)	O1—P1—O2^iii	110.91 (3)
O3^v—Sr1—O3^vii	68.78 (4)	O1^iii—P1—O2	110.91 (3)
O3^vi—Sr1—O3^vii	171.61 (4)	O1—P1—O2	110.91 (3)
O2—Co1—O2^viii	83.39 (5)	O2^iii—P1—O2	104.15 (9)
O2—Co1—O4^ix	103.68 (3)	O3—P2—O3^ix	112.30 (7)
O2^viii—Co1—O4^ix	170.65 (4)	O3—P2—O4	108.00 (5)
O2—Co1—O4	170.65 (4)	O3^ix—P2—O4	114.17 (5)
O2^viii—Co1—O4	103.68 (3)	O3—P2—O4^ix	114.17 (5)
O4^ix—Co1—O4	70.01 (4)	O3^ix—P2—O4^ix	108.00 (5)
O2—Co1—O3^x	93.94 (4)	O4—P2—O4^ix	99.68 (6)

Symmetry codes: (i) −x+2, −y+3/2, z+1; (ii) x, y, z+1; (iii) −x+2, −y+3/2, z; (iv) −x+3/2, −y+1, z+1/2; (v) x+1/2, y+1/2, z+1/2; (vi) −x+3/2, y+1/2, z+1/2; (vii) x+1/2, −y+1, z+1/2; (viii) −x+3/2, −y+3/2, −z+1/2; (ix) −x+3/2, y, −z+1/2; (x) x, −y+1, −z; (xi) −x+1, y, z; (xii) x, −y+1, −z+1; (xiii) −x+1, −y+1, −z+1; (xiv) x−1/2, y, −z+1/2.