Crystal structure of a sodium, zinc and iron(III)-based non-stoichiometric phosphate with an alluaudite-like structure: Na1.67Zn1.67Fe1.33(PO4)3.

The new title compound, disodium dizinc iron(III) tris-(phosphate), Na1.67Zn1.67Fe1.33(PO4)3, which belongs to the alluaudite family, has been synthesized by solid-state reactions. In this structure, all atoms are in general positions except for four, which are located on special positions of the C2/c space group. This structure is characterized by cation substitutional disorder at two sites, one situated on the special position 4e (2) and the other on the general position 8f. The 4e site is partially occupied by Na(+) [0.332 (3)], whereas the 8f site is entirely filled by a mixture of Fe and Zn. The full-occupancy sodium and zinc atoms are located at the Wyckoff positions on the inversion center 4a (-1) and on the twofold rotation axis 4e, respectively. Refinement of the occupancy ratios, bond-valence analysis and the electrical neutrality requirement of the structure lead to the given composition for the title compound. The three-dimensional framework of this structure consists of kinked chains of edge-sharing octa-hedra stacked parallel to [10-1]. The chains are formed by a succession of trimers based on [ZnO6] octa-hedra and the mixed-cation Fe(III)/Zn(II) [(Fe/Zn)O6] octa-hedra [Fe(III):Zn(III) ratio 0.668 (3)/0.332 (3)]. Continuous chains are held together by PO4 phosphate groups, forming polyhedral sheets perpendicular to [010]. The stacked sheets delimit two types of tunnels parallel to the c axis in which the sodium cations are located. Each Na(+) cation is coordinated by eight O atoms. The disorder of Na in the tunnel might presage ionic mobility for this material.

Chemical context

Alkali transition-metal phosphates belonging to the alluaudite family constitute one of the most diverse and rich classes of minerals, and have been studied intensively over the last few years. Owing to their outstanding physico-chemical properties, these compounds have many potential applications in various fields, such as catalytic activity (Kacimi et al., 2005 ▸) and as promising cathodes for sodium-ion batteries through the presence of mobile cations located in the tunnels of the open three-dimensional framework (Huang et al., 2015 ▸). In their recent study, Huang et al. (2015 ▸) point out that the electrochemical performance is not only associated with morphology, but also with the electronic and crystalline structure.

Accordingly, a large number of alluaudite phases with alkali cations in the tunnels have been reported. Nevertheless, the presence of alkali metals in the tunnels of synthetic alluaudite phases is frequently accompanied by cationic distributions that lead to non-stoichiometric compositions, such as: (Na0.38,Ca0.31)MgFe2(PO4)3 (Zid et al., 2005 ▸); NaFe3.67(PO4)3 (Korzenski et al., 1998 ▸); Cu1.35Fe3(PO4)3 (Warner et al., 1993 ▸); K0.53Mn2.37Fe1.24(PO4)3 (Hidouri & Ben Amara, 2011 ▸); Na1.79Mg1.79Fe1.21(PO4)3 (Hidouri et al., 2003 ▸); Na1.50Mn2.48Al0.85(PO4)3 (Hatert, 2006 ▸); Na1−LiMnFe2(PO4)3 where x = 0, 0.25, 0.50, and 0.75 (Hermann et al., 2002 ▸). As part of our study on alluaudite-related phosphates (Bouraima et al., 2015 ▸; Assani et al., 2011 ▸), we report the synthesis and the crystal structure of a new sodium, zinc and iron-based non-stoichiometric phosphate, namely Na1.67Zn1.67Fe1.33(PO4)3.

Structural commentary

The alluaudite structure of the title compound crystallizes in the monoclinic space group C2/c, with Z = 4. The principal building units of the crystal structure are represented in Fig. 1 ▸. Refinement of the occupancy fractions, bond-valence analysis based on the formula proposed by Brown & Altermatt (1985 ▸) and the required electrical neutrality of the structure lead to the formula Na1.67Zn1.67Fe1.33(PO4)3 for the title compound. The mixed Fe1 and Zn1 atoms are located at the general position 8f with Fe3+/Zn2+ occupancy fractions of 0.668 (3)/0.332 (3), and form a highly distorted [(Fe1/Zn1)O6] octa­hedral group, with Fe3+/Zn2+—O bond lengths ranging from 1.951 (1) to 2.209 (1)Å. The Zn2 atom is surrounded by six oxygen atoms, building a slightly distorted octa­hedron with an average Zn2—O bond length of 2.153 (1) Å.

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

The crystal structure of this phosphate compound consists of infinite kinked chains of two edge-sharing [Fe1/Zn1O6] octa­hedra leading to the formation of [(Fe1,Zn1)2O10] dimers that are connected by a common edge to [Zn2O6] octa­hedra, as shown in Fig. 2 ▸. These chains are linked by PO4 tetra­hedral groups, forming a stack of sheets perpendicular to [010] and alternating with sodium layers, as shown in Fig. 3 ▸, which reveal small tunnels along the [201] direction. The three-dimensional framework also encloses two types of large tunnels, in which the Na+ cations reside, as shown in Fig. 4 ▸. The site 4e centred on the first tunnel is partially occupied by Na1 [0.332 (3)], whereas Na2 occupies site 4a centred on the second tunnel. Each sodium atom is surrounded by eight oxygen atoms with Na1—O and Na2—O bond lengths in the ranges 2.448 (1)–2.908 (2) Å, and 2.324 (1)–2.901 (1) Å, respectively. The displacement ellipsoids of the partially occupied atom Na1 are rather larger than those of the rest of the atoms. Most probably this is due to the size of the channels, which allows atom Na1 to have more freedom. The disorder of Na in the tunnel may presage ionic mobility for this material.

Figure 2

A view along the b axis of a sheet resulting from chains connected by vertices of PO4 tetra­hedra.

Figure 3

A stack of layers perpendicular to the b axis, showing small tunnels along the [201] direction.

Figure 4

Polyhedral representation of Na1.67Zn1.67Fe1.33(PO4)3 showing tunnels running along the [001] direction.

Synthesis and crystallization

Single crystals of Na1.67Zn1.67Fe1.33(PO4)3 were synthesised by conventional solid-state reaction (Girolami et al., 1999 ▸). The nitrate-based sodium, zinc and iron precursors, in addition to the 85 wt% H3PO4 were taken in proportions corresponding to the molar ratio Na:Zn:Fe:P = 2:2:1:3. The resulting reaction mixture was ground in an agate mortar and progressively heated in a platinum crucible to the melting temperature of 1135 K. The melted product was cooled at a rate of 5 K/h. The product was obtained as transparent brown crystals corresponding to the title phosphate.

Refinement

Crystal data, data collection and structure refinement details are summarized in Table 1 ▸. Refinements of the site-occupancy factors of the metal site 8f revealed the ratio of Fe1:Zn1 = 0.668 (3):0.332 (3), whereas the the occupancy fraction of Na1 was constrained to that of Zn1 in order to maintain electrical neutrality. The highest peak and the deepest hole in the final difference Fourier map are at 0.72 and 0.40 Å from O1 and Zn2, respectively.

Table 1

Experimental details

Crystal data
Chemical formula	Na1.67Zn1.67Fe1.33(PO4)3
M r	506.59
Crystal system, space group	Monoclinic, C2/c
Temperature (K)	296
a, b, c ()	11.7545(4), 12.5080(4), 6.4014(2)
()	113.507(1)
V (3)	863.06(5)
Z	4
Radiation type	Mo K
(mm1)	7.52
Crystal size (mm)	0.31 0.25 0.19
Data collection
Diffractometer	Bruker X8 APEX
Absorption correction	Multi-scan (SADABS; Bruker, 2009 ▸)
T min, T max	0.504, 0.748
No. of measured, independent and observed [I > 2(I)] reflections	18880, 2101, 1997
R int	0.031
(sin /)max (1)	0.833
Refinement
R[F 2 > 2(F 2)], wR(F 2), S	0.017, 0.046, 1.19
No. of reflections	2101
No. of parameters	96
max, min (e 3)	0.65, 1.21

Computer programs: APEX2 and SAINT (Bruker, 2009 ▸), SHELXS97 and SHELXL97 (Sheldrick, 2008 ▸), ORTEPIII (Burnett Johnson, 1996 ▸), ORTEP-3 for Windows Farrugia, 2012 ▸), DIAMOND (Brandenburg, 2006 ▸) and publCIF (Westrip, 2010 ▸).

Crystal structure: contains datablock(s) I. DOI: 10.1107/S2056989015009767/pk2551sup1.cif

Structure factors: contains datablock(s) I. DOI: 10.1107/S2056989015009767/pk2551Isup2.hkl

CCDC reference: 1402019

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

Na1.67Zn1.67Fe1.33(PO4)3	F(000) = 977
Mr = 506.59	Dx = 3.904 Mg m−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -c 2yc	Cell parameters from 2101 reflections
a = 11.7545 (4) Å	θ = 2.5–36.3°
b = 12.5080 (4) Å	µ = 7.52 mm−1
c = 6.4014 (2) Å	T = 296 K
β = 113.507 (1)°	Block, brown
V = 863.06 (5) Å3	0.31 × 0.25 × 0.19 mm
Z = 4

Bruker X8 APEX diffractometer	2101 independent reflections
Radiation source: fine-focus sealed tube	1997 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.031
φ and ω scans	θmax = 36.3°, θmin = 2.5°
Absorption correction: multi-scan (SADABS; Bruker, 2009)	h = −19→16
Tmin = 0.504, Tmax = 0.748	k = −20→20
18880 measured reflections	l = −10→10

Refinement on F2	0 restraints
Least-squares matrix: full	w = 1/[σ2(Fo2) + (0.016P)2 + 1.8532P] where P = (Fo2 + 2Fc2)/3
R[F2 > 2σ(F2)] = 0.017	(Δ/σ)max = 0.001
wR(F2) = 0.046	Δρmax = 0.65 e Å−3
S = 1.19	Δρmin = −1.20 e Å−3
2101 reflections	Extinction correction: SHELXL97 (Sheldrick, 2008), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
96 parameters	Extinction coefficient: 0.0027 (2)

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.

Refinement. Reflections were merged by SHELXL according to the crystal class for the calculation of statistics and refinement._reflns_Friedel_fraction is defined as the number of unique Friedel pairs measured divided by the number that would be possible theoretically, ignoring centric projections and systematic absences.

	x	y	z	Uiso*/Ueq	Occ. (<1)
Fe1	0.71738 (2)	0.84648 (2)	0.12925 (3)	0.00552 (5)	0.668 (3)
Zn1	0.71738 (2)	0.84648 (2)	0.12925 (3)	0.00552 (5)	0.332 (3)
Zn2	0.5000	0.73133 (2)	0.2500	0.00966 (5)
P1	0.76212 (3)	0.60983 (2)	0.37448 (5)	0.00388 (6)
P2	0.5000	0.28835 (3)	0.2500	0.00330 (7)
Na1	1.0000	0.49141 (19)	0.7500	0.0356 (7)	0.664 (6)
Na2	0.5000	0.5000	0.0000	0.01511 (18)
O1	0.83510 (9)	0.66524 (7)	0.60760 (15)	0.00695 (15)
O2	0.77771 (9)	0.67779 (8)	0.18481 (15)	0.00760 (15)
O6	0.45837 (8)	0.21761 (8)	0.03327 (15)	0.00622 (15)
O3	0.62448 (9)	0.60224 (8)	0.32496 (16)	0.00806 (16)
O5	0.39712 (9)	0.36396 (8)	0.24820 (16)	0.00747 (16)
O4	0.82109 (10)	0.49950 (8)	0.38406 (18)	0.01105 (17)

	U11	U22	U33	U12	U13	U23
Fe1	0.00523 (7)	0.00632 (7)	0.00583 (7)	−0.00082 (5)	0.00307 (5)	−0.00067 (5)
Zn1	0.00523 (7)	0.00632 (7)	0.00583 (7)	−0.00082 (5)	0.00307 (5)	−0.00067 (5)
Zn2	0.01124 (10)	0.00917 (10)	0.01083 (10)	0.000	0.00679 (8)	0.000
P1	0.00555 (12)	0.00355 (11)	0.00280 (11)	−0.00051 (9)	0.00192 (9)	−0.00023 (8)
P2	0.00319 (15)	0.00365 (15)	0.00256 (15)	0.000	0.00063 (12)	0.000
Na1	0.0167 (8)	0.0452 (13)	0.0345 (11)	0.000	−0.0008 (7)	0.000
Na2	0.0221 (4)	0.0079 (3)	0.0091 (3)	0.0024 (3)	−0.0004 (3)	0.0006 (3)
O1	0.0098 (4)	0.0072 (4)	0.0035 (3)	−0.0020 (3)	0.0022 (3)	−0.0015 (3)
O2	0.0090 (4)	0.0101 (4)	0.0043 (3)	−0.0022 (3)	0.0032 (3)	0.0013 (3)
O6	0.0057 (3)	0.0081 (4)	0.0044 (3)	−0.0004 (3)	0.0015 (3)	−0.0024 (3)
O3	0.0068 (4)	0.0083 (4)	0.0101 (4)	−0.0015 (3)	0.0044 (3)	−0.0006 (3)
O5	0.0055 (3)	0.0064 (3)	0.0092 (4)	0.0012 (3)	0.0016 (3)	−0.0029 (3)
O4	0.0147 (4)	0.0062 (4)	0.0125 (4)	0.0026 (3)	0.0057 (3)	−0.0019 (3)

Fe1—O5i	1.9514 (10)	P2—O6	1.5510 (9)
Fe1—O4ii	1.9607 (10)	P2—O6vi	1.5510 (9)
Fe1—O1iii	2.0170 (10)	Na1—O4ix	2.4476 (11)
Fe1—O2iv	2.0567 (10)	Na1—O4	2.4476 (11)
Fe1—O6v	2.0684 (9)	Na1—O4x	2.5713 (12)
Fe1—O2	2.2091 (10)	Na1—O4vii	2.5713 (12)
Zn2—O3vi	2.1019 (10)	Na1—O1	2.812 (2)
Zn2—O3	2.1019 (10)	Na1—O1ix	2.812 (2)
Zn2—O6vii	2.1549 (10)	Na1—O6xi	2.908 (2)
Zn2—O6v	2.1549 (10)	Na1—O6xii	2.908 (2)
Zn2—O1iii	2.2028 (9)	Na2—O5xiii	2.3239 (9)
Zn2—O1viii	2.2028 (9)	Na2—O5vi	2.3239 (9)
P1—O3	1.5225 (10)	Na2—O3	2.3823 (9)
P1—O4	1.5345 (10)	Na2—O3v	2.3824 (9)
P1—O2	1.5518 (10)	Na2—O3xiii	2.5179 (10)
P1—O1	1.5563 (9)	Na2—O3vi	2.5179 (10)
P2—O5	1.5317 (10)	Na2—O5v	2.9008 (10)
P2—O5vi	1.5317 (10)	Na2—O5	2.9008 (10)
O5i—Fe1—O4ii	95.89 (4)	O4—Na1—O1	55.92 (4)
O5i—Fe1—O1iii	109.05 (4)	O4x—Na1—O1	114.04 (7)
O4ii—Fe1—O1iii	88.00 (4)	O4vii—Na1—O1	61.59 (4)
O5i—Fe1—O2iv	87.13 (4)	O4ix—Na1—O1ix	55.92 (4)
O4ii—Fe1—O2iv	101.39 (4)	O4—Na1—O1ix	119.76 (8)
O1iii—Fe1—O2iv	160.53 (4)	O4x—Na1—O1ix	61.58 (4)
O5i—Fe1—O6v	161.52 (4)	O4vii—Na1—O1ix	114.04 (7)
O4ii—Fe1—O6v	100.93 (4)	O1—Na1—O1ix	78.70 (7)
O1iii—Fe1—O6v	79.35 (4)	O4ix—Na1—O6xi	114.24 (8)
O2iv—Fe1—O6v	82.12 (4)	O4—Na1—O6xi	70.35 (5)
O5i—Fe1—O2	79.41 (4)	O4x—Na1—O6xi	83.40 (5)
O4ii—Fe1—O2	173.23 (4)	O4vii—Na1—O6xi	101.22 (6)
O1iii—Fe1—O2	88.95 (4)	O1—Na1—O6xi	125.27 (3)
O2iv—Fe1—O2	83.34 (4)	O1ix—Na1—O6xi	144.38 (3)
O6v—Fe1—O2	84.43 (4)	O4ix—Na1—O6xii	70.35 (5)
O3vi—Zn2—O3	79.62 (5)	O4—Na1—O6xii	114.24 (8)
O3vi—Zn2—O6vii	92.79 (4)	O4x—Na1—O6xii	101.22 (6)
O3—Zn2—O6vii	113.99 (4)	O4vii—Na1—O6xii	83.40 (5)
O3vi—Zn2—O6v	113.99 (4)	O1—Na1—O6xii	144.38 (3)
O3—Zn2—O6v	92.79 (4)	O1ix—Na1—O6xii	125.27 (3)
O6vii—Zn2—O6v	145.51 (5)	O6xi—Na1—O6xii	51.96 (5)
O3vi—Zn2—O1iii	164.40 (4)	O5xiii—Na2—O5vi	180.00 (3)
O3—Zn2—O1iii	86.51 (4)	O5xiii—Na2—O3	100.43 (3)
O6vii—Zn2—O1iii	86.30 (4)	O5vi—Na2—O3	79.57 (3)
O6v—Zn2—O1iii	73.53 (3)	O5xiii—Na2—O3v	79.57 (3)
O3vi—Zn2—O1viii	86.51 (4)	O5vi—Na2—O3v	100.43 (3)
O3—Zn2—O1viii	164.40 (4)	O3—Na2—O3v	180.0
O6vii—Zn2—O1viii	73.53 (3)	O5xiii—Na2—O3xiii	107.29 (3)
O6v—Zn2—O1viii	86.30 (4)	O5vi—Na2—O3xiii	72.71 (3)
O1iii—Zn2—O1viii	108.06 (5)	O3—Na2—O3xiii	113.43 (4)
O3—P1—O4	112.20 (6)	O3v—Na2—O3xiii	66.57 (4)
O3—P1—O2	108.58 (5)	O5xiii—Na2—O3vi	72.71 (3)
O4—P1—O2	109.36 (6)	O5vi—Na2—O3vi	107.29 (3)
O3—P1—O1	111.16 (6)	O3—Na2—O3vi	66.57 (4)
O4—P1—O1	107.14 (6)	O3v—Na2—O3vi	113.43 (4)
O2—P1—O1	108.32 (5)	O3xiii—Na2—O3vi	180.00 (3)
O5—P2—O5vi	103.73 (8)	O5xiii—Na2—O5v	53.55 (4)
O5—P2—O6	112.27 (5)	O5vi—Na2—O5v	126.45 (4)
O5vi—P2—O6	109.00 (5)	O3—Na2—O5v	85.32 (3)
O5—P2—O6vi	109.00 (5)	O3v—Na2—O5v	94.68 (3)
O5vi—P2—O6vi	112.28 (5)	O3xiii—Na2—O5v	67.11 (3)
O6—P2—O6vi	110.43 (7)	O3vi—Na2—O5v	112.89 (3)
O4ix—Na1—O4	175.26 (12)	O5xiii—Na2—O5	126.45 (4)
O4ix—Na1—O4x	79.21 (3)	O5vi—Na2—O5	53.55 (4)
O4—Na1—O4x	100.58 (3)	O3—Na2—O5	94.68 (3)
O4ix—Na1—O4vii	100.58 (3)	O3v—Na2—O5	85.32 (3)
O4—Na1—O4vii	79.20 (3)	O3xiii—Na2—O5	112.89 (3)
O4x—Na1—O4vii	174.93 (11)	O3vi—Na2—O5	67.11 (3)
O4ix—Na1—O1	119.76 (8)	O5v—Na2—O5	180.00 (3)