Crystal structure of a silver-, cobalt- and iron-based phosphate with an alluaudite-like structure: Ag_1.655Co_1.64Fe_1.36(PO₄)₃

The transition metal orthophosphate Ag_1.655Co_1.64Fe_1.36(PO₄)₃ crystallizes in an alluaudite-type structure. The chains characterizing the alluaudite structure are built up from edge-sharing [CoO₆] and [FeO₆] octa­hedra linked together by PO₄ tetra­hedra. The Ag⁺ cations are located in two types of channels in the resulting framework.

Abstract

The new silver-, cobalt- and iron-based phosphate, silver cobalt iron tris(ortho­phosphate), Ag_1.655Co_1.64Fe_1.36(PO₄)₃, was synthesized by solid-state reactions. Its structure is isotypic to that of Na₂Co₂Fe(PO₄)₃, and belongs to the alluaudite family, with a partial cationic disorder, the Ag^I atoms being located on an inversion centre and twofold rotation axis sites (Wyckoff positions 4a and 4e), with partial occupancies of 0.885 (2) and 0.7688 (19), respectively. One of the two P atoms in the asymmetric unit completely fills one 4e site while the Co and Fe atoms fill another 4e site, with partial occupancies of 0.86 (5) and 0.14 (5), respectively. The remaining Co²⁺ and Fe³⁺ cations are distributed on a general position, 8f, in a 0.39 (4):0.61 (4) ratio. All O atoms and the other P atoms are in general positions. The structure is built up from zigzag chains of edge-sharing [MO₆] (M = Fe/Co) octa­hedra stacked parallel to [101]. These chains are linked together through PO₄ tetra­hedra, forming polyhedral sheets perpendicular to [010]. The resulting framework displays two types of channels running along [001], in which the Ag^I atoms (coordination number eight) are located.

Chemical context  

Compounds belonging to the large alluaudite structural family (Moore, 1971 ▸; Moore & Ito, 1979 ▸; Hatert et al., 2000 ▸, 2004 ▸) have been of continuing inter­est owing to their open-framework architecture, with hexa­gonal-shaped channels, and their physical properties. This fact is amply justified by their practical applications, for example as corrosion inhibitors, passivators of metal surfaces, and catalysts (Korzenski et al., 1999 ▸). In addition, inter­est in alluaudite phosphates with monovalent cations has continued to grow in the electrochemical field, where they have applications as positive electrodes in lithium and sodium batteries (Trad et al., 2010 ▸). Accordingly, our attention is mostly focused on the elaboration and structural characterization of new alluaudite-type phosphates within the A ₂O–MO–P₂O₅ systems (A = monovalent cation M = divalent cation). For instance, most recently, the hydro­thermal investigation of the Na₂O–MO–P₂O₅ pseudo-ternary system has allowed the isolation of the sodium- and magnesium-based alluaudite phosphate NaMg₃(PO₄)(HPO₄)₂ (Ould Saleck et al., 2015 ▸). On the other hand, within the Na₂O–CoO–Fe₂O₃–P₂O₅ and Na₂O–ZnO–Fe₂O₃–P₂O₅ pseudo-quaternary systems, solid-state synthesis has allowed Na₂Co₂Fe(PO₄)₃ (Bouraima et al., 2015 ▸) and Na_1.67Zn_1.67Fe_1.33(PO₄)₃ (Khmiyas et al., 2015 ▸) to be obtained. With the same objective, a new silver-, cobalt- and iron-based alluaudite-type phosphate, namely Ag_1.655Co_1.64Fe_1.36(PO₄)₃, has been synthesized by means of solid-state reactions and characterized by single crystal X-ray diffraction.

Structural commentary  

In the new isolated compound, either cobalt or iron atoms are distributed in the two octa­hedral sites while the phosphorus atoms are tetra­hedrally coordinated, as shown in Fig. 1 ▸. The structure is built up from two edge-sharing [(Co1/Fe1)O₆] octa­hedra, leading to the formation of [(Co1/Fe1)₂O₁₀] dimers. Those dimers are connected by a common edge to [(Fe2/Co2)O₆] octa­hedra, forming an infinite chain (Fig. 2 ▸). The junction between these chains is ensured by sharing vertices with the PO₄ tetra­hedra so as to form an open layer perpendicular to [010] (Fig. 3 ▸). The three-dimensional framework resulting from the stacking of the sheets along the b-axis direction delimits channels parallel to [001] in which the Ag⁺ cations are accommodated, as shown in Fig. 4 ▸.

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, −y + 1, z + ; (ii) −x + 1, y, −z + ; (iii) −x + 1, −y + 1, −z + 1; (iv) −x + 2, y, −z + ; (v) −x + 2, −y + 1, −z + 1; (vi) x + , −y + , z + ; (vii) −x + , −y + , −z + 1; (viii) x, −y + 1, z − ; (ix) −x + , y − , −z + ; (x) x + , −y + , z + ; (xi) −x + , −y + , −z + 1; (xii) −x + , y + , −z + ; (xiii) x + , y + , z + 1.]

Figure 2.

Edge-sharing [(Fe/Co)O₆] octa­hedra forming a layer parallel to [101].

Figure 3.

A view along [010], showing a layer resulting from the connection of chains via vertices of PO₄ tetra­hedra and [FeO₆] octa­hedra.

Figure 4.

Polyhedral representation of Ag_1.655Co_1.64Fe_1.36(PO₄)₃, showing channels running along [001].

Comparison with a related structure  

It is worth mentioning that the distribution of metallic cations observed in the case of the silver–cobalt–iron-based phosphate is not encountered in the sodium homologue. Hence, in the title silver-based phosphate, the octa­hedral M1 site (Wyckoff position 8f) is occupied to 60% by Fe1 and to 40% by Co1. The octa­hedrally surrounded M2 site (Wyckoff position 4e) is essentially occupied by Fe2 atoms (43%) along with a small amount of Co2 (7%). However, in the Na₂Co₂Fe(PO₄)₃ phosphate, the M1 and M2 sites are entirely occupied by Fe and Co atoms, respectively. For the mixed sites, the occupancy rate was refined without any constraint. The results of the refinements are in good agreement with the electrical neutrality of the compound and calculations of the bond-valence sums of the atoms in the structure (Brown & Altermatt, 1985 ▸). Accordingly, in the silver-based phosphate, the cations at the M1 site form double octahedra [(Fe1/Co1)₂O₁₀] alternating with [(Fe2/Co2)O₆] octa­hedra, while in the sodium homologue phosphate, the obtained [Co₂O₁₀] double octahedra alternate with [FeO₆] octa­hedra (Fig. 4 ▸). Moreover, both the Ag1 and Ag2 atoms are located in channels, surrounded by eight oxygen atoms with Ag1—O bond lengths between 2.3320 (13) Å and 2.9176 (13) Å, whereas Ag2—O bond lengths are in the range 2.4733 (13)–2.9035 (12) Å. The structure of the title phosphate is isotypic to that of Na₂Co₂Fe(PO₄)₃ (Bouraima et al., 2015 ▸) and Na_1.67Zn_1.67Fe_1.33(PO₄)₃ (Khmiyas et al., 2015 ▸).

Synthesis and crystallization  

The title compound was isolated from solid-state reactions in air by mixing nitrates of silver, cobalt and iron with phospho­ric acid. The various precursors are taken in the molar ratio Ag:Co:Fe:P = 2:2:1:3. The mixture was stirred at room temperature overnight. After different heat treatments in a platinum crucible at up to 873 K, the reaction mixture was heated to the melting temperature of 1221 K. The molten product was then cooled to room temperature at a rate of 5 K h⁻¹. Brown homogeneous crystals corresponding to the title compound of a suitable size for X-ray diffraction were obtained.

Refinement  

Crystal data, data collection and structure refinement details are summarized in Table 1 ▸. The maximum and minimum residual electron densities in the final Fourier map are 0.68 and 0.55 Å from Ag1 and Ag2, respectively.

Table 1. Experimental details.

Crystal data
Chemical formula	Ag1.655Co1.64Fe1.36(PO4)3
M r	2544.10
Crystal system, space group	Monoclinic, C2/c
Temperature (K)	296
a, b, c (Å)	11.8680 (3), 12.5514 (3), 6.4386 (2)
β (°)	114.012 (1)
V (Å3)	876.09 (4)
Z	1
Radiation type	Mo Kα
μ (mm−1)	9.51
Crystal size (mm)	0.31 × 0.26 × 0.22
Data collection
Diffractometer	Bruker X8 APEX
Absorption correction	Multi-scan (SADABS; Krause et al., 2015 ▸)
T min, T max	0.066, 0.124
No. of measured, independent and observed [I > 2σ(I)] reflections	13097, 2137, 2079
R int	0.030
(sin θ/λ)max (Å−1)	0.833
Refinement
R[F 2 > 2σ(F 2)], wR(F 2), S	0.020, 0.047, 1.19
No. of reflections	2137
No. of parameters	99
Δρmax, Δρmin (e Å−3)	1.47, −0.92

Computer programs: APEX2 aand SAINT (Bruker, 2009 ▸), SHELXT2014 (Sheldrick, 2015a ▸), SHELXL2014 (Sheldrick, 2015b ▸), ORTEP-3 for Windows (Farrugia, 2012 ▸), DIAMOND (Brandenburg, 2006 ▸) and publCIF (Westrip, 2010 ▸).

Supplementary Material

CCDC reference: 1551181

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

supplementary crystallographic information

Crystal data

Ag1.655Co1.64Fe1.36(PO4)3	F(000) = 1194
Mr = 2544.10	Dx = 4.822 Mg m−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
a = 11.8680 (3) Å	Cell parameters from 2137 reflections
b = 12.5514 (3) Å	θ = 3.3–36.3°
c = 6.4386 (2) Å	µ = 9.51 mm−1
β = 114.012 (1)°	T = 296 K
V = 876.09 (4) Å3	Block, brown
Z = 1	0.31 × 0.26 × 0.22 mm

Data collection

Bruker X8 APEX diffractometer	2137 independent reflections
Radiation source: fine-focus sealed tube	2079 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.030
φ and ω scans	θmax = 36.3°, θmin = 3.3°
Absorption correction: multi-scan (SADABS; Krause et al., 2015)	h = −19→19
Tmin = 0.066, Tmax = 0.124	k = −20→20
13097 measured reflections	l = −10→10

Refinement

Refinement on F2	0 restraints
Least-squares matrix: full	w = 1/[σ2(Fo2) + (0.0121P)2 + 3.2851P] where P = (Fo2 + 2Fc2)/3
R[F2 > 2σ(F2)] = 0.020	(Δ/σ)max = 0.001
wR(F2) = 0.047	Δρmax = 1.47 e Å−3
S = 1.19	Δρmin = −0.92 e Å−3
2137 reflections	Extinction correction: SHELXL2014 (Sheldrick, 2015b), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
99 parameters	Extinction coefficient: 0.00102 (10)

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	Occ. (<1)
Ag1	0.5000	0.5000	0.5000	0.01952 (7)	0.885 (2)
Ag2	1.0000	0.48916 (3)	0.7500	0.02408 (10)	0.7688 (19)
Fe1	0.78227 (2)	0.34311 (2)	0.37115 (3)	0.00565 (6)	0.61 (4)
Co1	0.78227 (2)	0.34311 (2)	0.37115 (3)	0.00565 (6)	0.39 (4)
Fe2	1.0000	0.76503 (2)	0.7500	0.00714 (8)	0.14 (5)
Co2	1.0000	0.76503 (2)	0.7500	0.00714 (8)	0.86 (5)
P1	0.76272 (3)	0.61138 (3)	0.37428 (6)	0.00502 (8)
P2	0.5000	0.28909 (4)	0.2500	0.00535 (10)
O1	0.77807 (11)	0.67841 (10)	0.18620 (19)	0.00908 (19)
O2	0.81856 (12)	0.49999 (9)	0.3820 (2)	0.0112 (2)
O3	0.62598 (11)	0.60711 (11)	0.3280 (2)	0.0135 (2)
O4	0.83676 (12)	0.66524 (9)	0.60788 (19)	0.00917 (19)
O5	0.45841 (10)	0.21873 (10)	0.03381 (18)	0.00821 (19)
O6	0.60284 (12)	0.36401 (10)	0.2530 (2)	0.0122 (2)

Atomic displacement parameters (Ų)

	U11	U22	U33	U12	U13	U23
Ag1	0.02917 (14)	0.00879 (9)	0.01123 (10)	−0.00389 (7)	−0.00138 (8)	−0.00128 (6)
Ag2	0.01069 (12)	0.02795 (16)	0.02519 (15)	0.000	−0.00133 (10)	0.000
Fe1	0.00485 (9)	0.00657 (9)	0.00575 (9)	0.00037 (6)	0.00240 (6)	0.00058 (6)
Co1	0.00485 (9)	0.00657 (9)	0.00575 (9)	0.00037 (6)	0.00240 (6)	0.00058 (6)
Fe2	0.00620 (12)	0.00833 (13)	0.00796 (13)	0.000	0.00398 (10)	0.000
Co2	0.00620 (12)	0.00833 (13)	0.00796 (13)	0.000	0.00398 (10)	0.000
P1	0.00488 (15)	0.00490 (15)	0.00521 (15)	0.00006 (11)	0.00198 (11)	0.00022 (10)
P2	0.00397 (19)	0.0071 (2)	0.00466 (19)	0.000	0.00138 (16)	0.000
O1	0.0101 (5)	0.0112 (5)	0.0062 (4)	−0.0001 (4)	0.0036 (4)	0.0020 (3)
O2	0.0118 (5)	0.0070 (4)	0.0144 (5)	0.0024 (4)	0.0048 (4)	−0.0013 (4)
O3	0.0067 (4)	0.0125 (5)	0.0221 (6)	0.0007 (4)	0.0066 (4)	0.0031 (4)
O4	0.0127 (5)	0.0083 (4)	0.0062 (4)	−0.0011 (4)	0.0034 (4)	−0.0013 (3)
O5	0.0064 (4)	0.0124 (5)	0.0053 (4)	−0.0011 (4)	0.0018 (3)	−0.0017 (3)
O6	0.0085 (5)	0.0119 (5)	0.0166 (5)	−0.0036 (4)	0.0054 (4)	0.0007 (4)

Geometric parameters (Å, º)

Ag1—O6i	2.3320 (13)	Fe1—O4viii	2.0481 (12)
Ag1—O6ii	2.3320 (13)	Fe1—O1i	2.0669 (11)
Ag1—O3i	2.4356 (14)	Fe1—O5vi	2.0705 (12)
Ag1—O3ii	2.4356 (14)	Fe1—O1ix	2.1695 (12)
Ag1—O3iii	2.5724 (13)	Fe2—O3x	2.1099 (13)
Ag1—O3	2.5725 (13)	Fe2—O3xi	2.1099 (13)
Ag1—O6iii	2.9176 (13)	Fe2—O5xii	2.1575 (11)
Ag1—O6	2.9176 (13)	Fe2—O5xiii	2.1575 (11)
Ag2—O2iv	2.4733 (13)	Fe2—O4iv	2.1717 (12)
Ag2—O2	2.4733 (13)	Fe2—O4	2.1717 (12)
Ag2—O2i	2.6204 (13)	P1—O3	1.5270 (13)
Ag2—O2v	2.6204 (13)	P1—O2	1.5393 (12)
Ag2—O4	2.8341 (12)	P1—O1	1.5451 (12)
Ag2—O4iv	2.8341 (12)	P1—O4	1.5543 (12)
Ag2—O5vi	2.9035 (13)	P2—O6ii	1.5346 (13)
Ag2—O5vii	2.9035 (12)	P2—O6	1.5346 (13)
Fe1—O6	1.9656 (13)	P2—O5ii	1.5498 (12)
Fe1—O2	2.0108 (12)	P2—O5	1.5498 (12)
O6i—Ag1—O6ii	180.00 (4)	O2—Ag2—O1i	64.62 (4)
O6i—Ag1—O3i	80.58 (4)	O2i—Ag2—O1i	49.21 (3)
O6ii—Ag1—O3i	99.42 (4)	O2v—Ag2—O1i	136.08 (3)
O6i—Ag1—O3ii	99.42 (4)	O4—Ag2—O1i	92.94 (3)
O6ii—Ag1—O3ii	80.58 (4)	O4iv—Ag2—O1i	162.54 (3)
O3i—Ag1—O3ii	180.0	O5vi—Ag2—O1i	52.36 (3)
O6i—Ag1—O3iii	108.21 (5)	O5vii—Ag2—O1i	56.78 (3)
O6ii—Ag1—O3iii	71.79 (5)	O2iv—Ag2—O1v	64.62 (4)
O3i—Ag1—O3iii	66.27 (5)	O2—Ag2—O1v	119.96 (4)
O3ii—Ag1—O3iii	113.73 (5)	O2i—Ag2—O1v	136.08 (3)
O6i—Ag1—O3	71.79 (5)	O2v—Ag2—O1v	49.21 (3)
O6ii—Ag1—O3	108.21 (5)	O4—Ag2—O1v	162.54 (3)
O3i—Ag1—O3	113.73 (5)	O4iv—Ag2—O1v	92.94 (3)
O3ii—Ag1—O3	66.27 (5)	O5vi—Ag2—O1v	56.78 (3)
O3iii—Ag1—O3	180.0	O5vii—Ag2—O1v	52.36 (3)
O6i—Ag1—O6iii	53.64 (5)	O6—Fe1—O2	93.77 (5)
O6ii—Ag1—O6iii	126.36 (5)	O6—Fe1—O4viii	110.10 (5)
O3i—Ag1—O6iii	95.49 (4)	O2—Fe1—O4viii	86.76 (5)
O3ii—Ag1—O6iii	84.51 (4)	O6—Fe1—O1i	86.70 (5)
O3iii—Ag1—O6iii	68.02 (4)	O2—Fe1—O1i	100.62 (5)
O3—Ag1—O6iii	111.98 (4)	O4viii—Fe1—O1i	161.33 (5)
O6i—Ag1—O6	126.36 (5)	O6—Fe1—O5vi	163.25 (5)
O6ii—Ag1—O6	53.64 (5)	O2—Fe1—O5vi	101.04 (5)
O3i—Ag1—O6	84.51 (4)	O4viii—Fe1—O5vi	78.79 (5)
O3ii—Ag1—O6	95.49 (4)	O1i—Fe1—O5vi	82.95 (5)
O3iii—Ag1—O6	111.98 (4)	O6—Fe1—O1ix	80.26 (5)
O3—Ag1—O6	68.02 (4)	O2—Fe1—O1ix	171.95 (5)
O6iii—Ag1—O6	180.0	O4viii—Fe1—O1ix	90.22 (4)
O2iv—Ag2—O2	173.70 (6)	O1i—Fe1—O1ix	84.52 (5)
O2iv—Ag2—O2i	101.33 (4)	O5vi—Fe1—O1ix	85.66 (5)
O2—Ag2—O2i	78.34 (4)	O3x—Fe2—O3xi	80.97 (7)
O2iv—Ag2—O2v	78.34 (4)	O3x—Fe2—O5xii	91.27 (5)
O2—Ag2—O2v	101.33 (4)	O3xi—Fe2—O5xii	112.81 (5)
O2i—Ag2—O2v	174.04 (5)	O3x—Fe2—O5xiii	112.81 (5)
O2iv—Ag2—O4	118.73 (4)	O3xi—Fe2—O5xiii	91.27 (5)
O2—Ag2—O4	55.50 (4)	O5xii—Fe2—O5xiii	148.74 (7)
O2i—Ag2—O4	61.33 (4)	O3x—Fe2—O4iv	85.08 (5)
O2v—Ag2—O4	113.50 (4)	O3xi—Fe2—O4iv	164.39 (5)
O2iv—Ag2—O4iv	55.50 (4)	O5xii—Fe2—O4iv	74.29 (4)
O2—Ag2—O4iv	118.73 (4)	O5xiii—Fe2—O4iv	87.71 (4)
O2i—Ag2—O4iv	113.50 (4)	O3x—Fe2—O4	164.39 (5)
O2v—Ag2—O4iv	61.33 (4)	O3xi—Fe2—O4	85.08 (5)
O4—Ag2—O4iv	77.51 (5)	O5xii—Fe2—O4	87.71 (4)
O2iv—Ag2—O5vi	114.87 (4)	O5xiii—Fe2—O4	74.29 (4)
O2—Ag2—O5vi	71.23 (4)	O4iv—Fe2—O4	109.56 (7)
O2i—Ag2—O5vi	101.56 (4)	O3—P1—O2	112.57 (7)
O2v—Ag2—O5vi	83.86 (4)	O3—P1—O1	108.72 (7)
O4—Ag2—O5vi	125.84 (3)	O2—P1—O1	109.47 (7)
O4iv—Ag2—O5vi	144.70 (3)	O3—P1—O4	109.98 (7)
O2iv—Ag2—O5vii	71.23 (4)	O2—P1—O4	107.31 (7)
O2—Ag2—O5vii	114.87 (4)	O1—P1—O4	108.73 (7)
O2i—Ag2—O5vii	83.86 (4)	O6ii—P2—O6	104.43 (10)
O2v—Ag2—O5vii	101.56 (4)	O6ii—P2—O5ii	108.75 (7)
O4—Ag2—O5vii	144.70 (3)	O6—P2—O5ii	112.14 (7)
O4iv—Ag2—O5vii	125.84 (3)	O6ii—P2—O5	112.14 (7)
O5vi—Ag2—O5vii	52.03 (4)	O6—P2—O5	108.75 (7)
O2iv—Ag2—O1i	119.96 (4)	O5ii—P2—O5	110.52 (9)

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