Synthesis and crystal structure of calcium dizinc iron(III) tris-(orthophosphate), CaZn2Fe(PO4)3.

Single crystals of the title compound, CaZn2Fe(PO4)3, were synthesized by conventional solid-state reaction. In the asymmetric unit, all atoms are located in fully occupied general positions of the P21/c space group. The zinc atoms are located on two crystallographically independent sites with tetra-hedral and distorted triangular-based bipyramidal geometries. Two edge-sharing triangular bipyramidal ZnO5 units form a dimer, which is linked to slightly deformed FeO6 octa-hedra via a common edge. The resulting chains are inter-connected through PO4 tetra-hedra to form a layer perpendicular to the b axis. Moreover, the remaining PO4 and ZnO4 tetra-hedra are linked together through common vertices to form tapes parallel to the c axis and surrounding a chain of Ca2+ cations to build a sheet, also perpendicular to the b axis. The stacking of the two layers along the b axis leads to the resulting three-dimensional framework, which defines channels in which the Ca2+ cations are located, each cation being coordinated by seven oxygen atoms.

Chemical context

Microporous compounds with an open anionic framework containing transition metals have been widely studied during recent years, especially iron phosphates, because of their potential applications in several fields such as gas sensing (Abdurahman et al., 2014 ▸), catalysis (Ai, 1999 ▸), as cathode materials for rechargeable lithium batteries (Masquelier et al., 1998 ▸), biocompatibility of glass fibres for tissue engineering (Ahmed et al., 2004 ▸), and immobilization of spent nuclear fuel (Mesko & Day, 1999 ▸). Metal phosphates with an open framework can exhibit different architectures such as linear-chain, layered and three-dimensional structures with channels or cavities where a variety of cations with different sizes, ratio and charges are accommodated. The occupancy of the allowed sites by cations can provide different properties such as remarkable flexibility, fast ionic conduction and low thermal expansion, mainly observed in the compounds belonging to the NASICON family with the general formula MM′2P3O12 (where M = alkali metal, alkaline-earth metal or a vacant site and M′ = Zr, Ti, Hf, etc.; Senbhagaraman et al., 1993 ▸). In our previous hydro­thermal investigations, a variety of compounds have been synthesized and characterized with different ratios of alkaline earth metal:P, viz. Sr2Mn3(HPO4)2(PO4)2 (Khmiyas et al., 2013 ▸), BaMnII 2MnIII(PO4)3 (Assani et al., 2013 ▸), Mg7(PO4)2(HPO4)4 (Assani et al., 2011 ▸). In this context, our inter­est is focused on the synthesis of new iron orthophosphates with an open-framework structure. Accordingly, we have succeeded in synthesizing and structurally characterizing a new calcium, zinc and iron-based open-framework phosphate, namely CaZn2Fe(PO4)3.

Structural commentary

All atoms in asymmetric unit of the title compound occupy general positions of the P21/c space group. The refinement of this model was very easy and lead to an ordered structure in which the zinc cations occupy two sites with different environments. The coordination numbers of all cations were confirmed by bond-valence-sum calculations (Brown & Altermatt, 1985 ▸). The obtained values for CaII+, ZnII+, FeIII+ and PV+ are as expected, viz. Ca1 (1.93), Zn1 (2.00), Zn2 (1.91), Fe1 (3.04), P1 (5.11), P2 (4.97) and P3 (4.94). The crystal structure is build up from PO4 and Zn1O4 tetra­hedra, distorted triangular-based bipyramidal Zn2O5 and FeO6 octa­hedra, as shown in Fig. 1 ▸. The FeO6 octa­hedra are slightly deformed with Fe—O distances varying from 1.8908 (8) to 2.1318 (8) Å and share a common edge with the highly distorted [(Zn2)2O8] dimer resulting from the edge-sharing of two triangular-based bipyramidal Zn2O5 units. Sequences of these polyhedra build chains inter­connected by PO4 tetra­hedra, forming a layer perpendicular to the b axis, as shown in Fig. 2 ▸. The remaining Zn1O4 tetra­hedra are linked to irregular PO4 groups via common corners, forming tapes parallel to the c axis, which are linked together by Ca2+ cations in sheets perpendicular to the b axis (see Fig. 3 ▸). The obtained three-dimensional framework shows one type of channel running along the [001] direction in which the Ca2+ cations are located, each being coordinated by seven oxygen atoms (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 + , z − ; (ii) x, −y + , z + ; (iii) x + 1, y, z; (iv) −x + 1, −y + 1, −z + 1; (v) −x + 2, −y + 1, −z + 1; (vi) −x + 1, −y + 1, −z + 2; (vii) x, y, z + 1; (viii) x − 1, y, z; (ix) x − 1, −y + , z + .]

Figure 2

Edge-sharing triangular bipyramidal ZnO5 units linked to FeO6 octa­hedra and to PO4 tetra­hedra, forming a layer perpendicular to the b axis.

Figure 3

A layer perpendicular to the b axis, resulting from the chains connected via vertices of the ZnO4 and PO4 tetra­hedra.

Figure 4

Polyhedral representation of CaZn2Fe(PO4)3, showing the channels running along the [001] direction.

Database Survey

The formula of the title compound, CaZn2Fe(PO4)3, is similar to some compounds with alluaudite structures, space group C2/c or the α-CrPO4 structure, space group Imma. However, its structure is different and to our knowledge there is no known isotypic structure. Crystals of CaM 2Fe(PO4)3 (M = Mg, Co, Ni, Cu) compounds, which are predicted to have the same structures or isotypes are in preparation, while the structures of SrM 2Fe(PO4)3 (M = Co, Ni) compounds are isotypic with α-CrPO4 (Bouraima et al., 2016 ▸; Ouaatta et al., 2015 ▸). Mention may also be made of other similar compounds, for example the phosphates Na2Co2Fe(PO4)3, NaCr2Zn(PO4)3 and Na1.66Zn1.66Fe1.34(PO4)3 (Bouraima et al., 2015 ▸; Souiwa et al., 2015 ▸; Khmiyas et al., 2015 ▸) adopting the alluaudite structure type. In conclusion, we can say that the structure of this phosphate is similar to the alluaudite structure but with lower symmetry.

Synthesis and crystallization

Single crystals of CaZn2Fe(PO4)3 were synthesized by a conventional solid-state method. Appropriate amounts of metal nitrate reagents, in the presence of H3PO4 85 wt%, were first dissolved in deionized water in the molar ratio Ca:Zn:Fe:P = 2:2:1:3 for 24 h. Then, the resulting solution was evaporated to dryness. The powder residue was ground in an agate mortar and progressively heated in a platinum crucible at a heating rate of 141 K h−1 until melting occurred at 1283 K. The melted product was cooled down at a rate of 5 K h−1. As result of the reaction, we obtained transparent crystals corresponding to the title compound CaZn2Fe(PO4)3.

Refinement

Crystal data, data collection and structure refinement details are summarized in Table 1 ▸. The reflections (202) and (330) probably affected by the beam-stop were omitted from the refinement. The maximum and minimum electron densities in the final Fourier map are at 0.56 and 0.44 Å from Ca1 and Zn2, respectively.

Table 1

Experimental details

Crystal data
Chemical formula	CaZn2Fe(PO4)3
M r	511.58
Crystal system, space group	Monoclinic, P21/c
Temperature (K)	296
a, b, c (Å)	8.5619 (3), 15.2699 (5), 8.1190 (3)
β (°)	117.788 (2)
V (Å3)	939.06 (6)
Z	4
Radiation type	Mo Kα
μ (mm−1)	7.72
Crystal size (mm)	0.30 × 0.26 × 0.18
Data collection
Diffractometer	Bruker X8 APEX
Absorption correction	Multi-scan (SADABS; Krause et al., 2015 ▸)
T min, T max	0.600, 0.747
No. of measured, independent and observed [I > 2σ(I)] reflections	54053, 4985, 4493
R int	0.033
(sin θ/λ)max (Å−1)	0.859
Refinement
R[F 2 > 2σ(F 2)], wR(F 2), S	0.017, 0.041, 1.04
No. of reflections	4985
No. of parameters	172
Δρmax, Δρmin (e Å−3)	1.07, −0.78

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

Crystal structure: contains datablock(s) I. DOI: 10.1107/S2056989016012421/pj2033sup1.cif

Structure factors: contains datablock(s) I. DOI: 10.1107/S2056989016012421/pj2033Isup2.hkl

CCDC reference: 1497218

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

CaZn2Fe(PO4)3	F(000) = 988
Mr = 511.58	Dx = 3.618 Mg m−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
a = 8.5619 (3) Å	Cell parameters from 4985 reflections
b = 15.2699 (5) Å	θ = 2.7–37.6°
c = 8.1190 (3) Å	µ = 7.72 mm−1
β = 117.788 (2)°	T = 296 K
V = 939.06 (6) Å3	Block, black
Z = 4	0.30 × 0.26 × 0.18 mm

Bruker X8 APEX diffractometer	4985 independent reflections
Radiation source: fine-focus sealed tube	4493 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.033
φ and ω scans	θmax = 37.6°, θmin = 2.7°
Absorption correction: multi-scan (SADABS; Krause et al., 2015)	h = −14→14
Tmin = 0.600, Tmax = 0.747	k = −26→26
54053 measured reflections	l = −10→13

Refinement on F2	172 parameters
Least-squares matrix: full	0 restraints
R[F2 > 2σ(F2)] = 0.017	w = 1/[σ2(Fo2) + (0.0174P)2 + 0.7655P] where P = (Fo2 + 2Fc2)/3
wR(F2) = 0.041	(Δ/σ)max = 0.003
S = 1.04	Δρmax = 1.07 e Å−3
4985 reflections	Δρmin = −0.78 e Å−3

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.

	x	y	z	Uiso*/Ueq
Zn1	0.81685 (2)	0.73588 (2)	0.30142 (2)	0.00740 (3)
Zn2	0.88412 (2)	0.52265 (2)	0.59417 (2)	0.01026 (3)
Fe1	0.67075 (2)	0.49009 (2)	0.83003 (2)	0.00500 (3)
Ca1	0.27619 (3)	0.75762 (2)	0.47919 (3)	0.01070 (4)
P1	0.29665 (3)	0.58370 (2)	0.77261 (4)	0.00503 (4)
P2	0.96807 (3)	0.62244 (2)	0.09438 (4)	0.00499 (4)
P3	0.60325 (3)	0.64096 (2)	0.49014 (4)	0.00491 (4)
O1	0.29942 (13)	0.67932 (6)	0.72349 (13)	0.01271 (15)
O2	0.30273 (12)	0.58596 (6)	0.96463 (12)	0.01006 (14)
O3	0.12207 (10)	0.54161 (6)	0.62684 (12)	0.01114 (15)
O4	0.43948 (11)	0.52893 (6)	0.76475 (14)	0.01312 (16)
O5	0.77926 (10)	0.58832 (5)	0.00379 (12)	0.00895 (13)
O6	1.00136 (11)	0.67493 (6)	−0.04745 (13)	0.01045 (14)
O7	1.00262 (11)	0.68143 (6)	0.26134 (13)	0.01055 (14)
O8	1.09932 (10)	0.54481 (5)	0.17408 (12)	0.00724 (13)
O9	0.75426 (11)	0.65280 (6)	0.43941 (13)	0.01160 (15)
O10	0.59377 (10)	0.71947 (6)	0.60355 (12)	0.00974 (14)
O11	0.66602 (11)	0.55677 (5)	0.61102 (12)	0.00822 (13)
O12	0.41993 (10)	0.62799 (6)	0.32620 (12)	0.01012 (14)

	U11	U22	U33	U12	U13	U23
Zn1	0.00645 (5)	0.00817 (5)	0.00801 (6)	0.00013 (4)	0.00374 (4)	0.00104 (4)
Zn2	0.00760 (5)	0.01471 (6)	0.01073 (6)	0.00042 (4)	0.00616 (4)	−0.00080 (5)
Fe1	0.00454 (5)	0.00602 (6)	0.00488 (6)	0.00044 (4)	0.00256 (4)	0.00061 (4)
Ca1	0.01029 (8)	0.01095 (9)	0.01178 (10)	0.00175 (6)	0.00591 (7)	0.00601 (7)
P1	0.00467 (9)	0.00607 (10)	0.00452 (10)	0.00011 (7)	0.00229 (8)	−0.00022 (8)
P2	0.00441 (9)	0.00482 (10)	0.00581 (11)	0.00038 (7)	0.00243 (8)	−0.00009 (8)
P3	0.00448 (9)	0.00500 (10)	0.00483 (10)	0.00004 (7)	0.00182 (8)	0.00031 (8)
O1	0.0222 (4)	0.0076 (3)	0.0109 (4)	−0.0001 (3)	0.0098 (3)	0.0019 (3)
O2	0.0169 (3)	0.0093 (3)	0.0063 (3)	0.0009 (3)	0.0073 (3)	0.0011 (3)
O3	0.0051 (3)	0.0176 (4)	0.0102 (4)	−0.0030 (3)	0.0030 (3)	−0.0063 (3)
O4	0.0065 (3)	0.0172 (4)	0.0157 (4)	0.0026 (3)	0.0052 (3)	−0.0045 (3)
O5	0.0056 (3)	0.0100 (3)	0.0101 (3)	−0.0016 (2)	0.0028 (2)	−0.0034 (3)
O6	0.0097 (3)	0.0107 (3)	0.0132 (4)	0.0029 (3)	0.0072 (3)	0.0061 (3)
O7	0.0079 (3)	0.0122 (3)	0.0115 (4)	−0.0005 (2)	0.0045 (3)	−0.0062 (3)
O8	0.0065 (3)	0.0073 (3)	0.0088 (3)	0.0026 (2)	0.0043 (2)	0.0020 (2)
O9	0.0111 (3)	0.0118 (3)	0.0161 (4)	0.0008 (3)	0.0098 (3)	0.0042 (3)
O10	0.0076 (3)	0.0091 (3)	0.0103 (3)	0.0006 (2)	0.0024 (3)	−0.0040 (3)
O11	0.0100 (3)	0.0080 (3)	0.0082 (3)	0.0028 (2)	0.0055 (3)	0.0039 (3)
O12	0.0068 (3)	0.0086 (3)	0.0097 (3)	−0.0002 (2)	−0.0006 (3)	−0.0020 (3)

Zn1—O9	1.9266 (9)	Ca1—O2i	2.4075 (9)
Zn1—O7	1.9518 (8)	Ca1—O7viii	2.4709 (9)
Zn1—O10i	1.9578 (8)	Ca1—O6ix	2.4840 (9)
Zn1—O6ii	2.0120 (8)	Ca1—O10	2.4885 (8)
Zn2—O3iii	1.9496 (8)	Ca1—O12	2.8984 (9)
Zn2—O11	2.0038 (8)	P1—O4	1.5073 (9)
Zn2—O3iv	2.0241 (9)	P1—O1	1.5166 (9)
Zn2—O8v	2.0911 (8)	P1—O2	1.5358 (9)
Zn2—O9	2.3371 (9)	P1—O3	1.5487 (8)
Fe1—O4	1.8908 (8)	P2—O5	1.5222 (8)
Fe1—O2vi	1.9561 (9)	P2—O6	1.5348 (9)
Fe1—O5vii	1.9700 (8)	P2—O7	1.5371 (9)
Fe1—O11	2.0330 (8)	P2—O8	1.5519 (8)
Fe1—O8v	2.0547 (8)	P3—O12	1.5253 (8)
Fe1—O12iv	2.1318 (8)	P3—O10	1.5365 (9)
Ca1—O1	2.2439 (9)	P3—O9	1.5396 (9)
Ca1—O1i	2.3795 (10)	P3—O11	1.5534 (8)
O9—Zn1—O7	106.60 (4)	O2i—Ca1—O6ix	72.04 (3)
O9—Zn1—O10i	106.09 (4)	O7viii—Ca1—O6ix	65.76 (3)
O7—Zn1—O10i	124.80 (4)	O1—Ca1—O10	83.50 (3)
O9—Zn1—O6ii	116.31 (4)	O1i—Ca1—O10	85.92 (3)
O7—Zn1—O6ii	85.46 (3)	O2i—Ca1—O10	98.16 (3)
O10i—Zn1—O6ii	116.93 (4)	O7viii—Ca1—O10	132.22 (3)
O3iii—Zn2—O11	154.16 (4)	O6ix—Ca1—O10	160.73 (3)
O3iii—Zn2—O3iv	77.67 (4)	O1—Ca1—O12	97.73 (3)
O11—Zn2—O3iv	123.13 (3)	O1i—Ca1—O12	71.19 (3)
O3iii—Zn2—O8v	108.82 (4)	O2i—Ca1—O12	126.00 (3)
O11—Zn2—O8v	75.00 (3)	O7viii—Ca1—O12	79.74 (3)
O3iv—Zn2—O8v	121.44 (4)	O6ix—Ca1—O12	145.10 (3)
O3iii—Zn2—O9	98.73 (4)	O10—Ca1—O12	53.99 (3)
O11—Zn2—O9	65.84 (3)	O1—Ca1—O12ii	69.93 (3)
O3iv—Zn2—O9	97.26 (4)	O1i—Ca1—O12ii	114.39 (3)
O8v—Zn2—O9	135.92 (3)	O2i—Ca1—O12ii	57.96 (3)
O4—Fe1—O2vi	96.63 (4)	O7viii—Ca1—O12ii	140.52 (3)
O4—Fe1—O5vii	92.55 (4)	O6ix—Ca1—O12ii	78.49 (3)
O2vi—Fe1—O5vii	90.75 (4)	O10—Ca1—O12ii	82.24 (3)
O4—Fe1—O11	90.37 (4)	O12—Ca1—O12ii	135.98 (3)
O2vi—Fe1—O11	171.82 (3)	O4—P1—O1	114.24 (6)
O5vii—Fe1—O11	93.18 (4)	O4—P1—O2	114.28 (5)
O4—Fe1—O8v	164.34 (4)	O1—P1—O2	104.36 (5)
O2vi—Fe1—O8v	97.38 (3)	O4—P1—O3	104.50 (5)
O5vii—Fe1—O8v	94.22 (3)	O1—P1—O3	109.05 (5)
O11—Fe1—O8v	75.18 (3)	O2—P1—O3	110.42 (5)
O4—Fe1—O12iv	93.16 (4)	O5—P2—O6	110.07 (5)
O2vi—Fe1—O12iv	82.55 (4)	O5—P2—O7	110.55 (5)
O5vii—Fe1—O12iv	171.65 (3)	O6—P2—O7	109.20 (5)
O11—Fe1—O12iv	92.86 (4)	O5—P2—O8	109.84 (5)
O8v—Fe1—O12iv	81.77 (3)	O6—P2—O8	111.11 (5)
O1—Ca1—O1i	167.91 (4)	O7—P2—O8	106.00 (5)
O1—Ca1—O2i	126.91 (3)	O12—P3—O10	107.69 (5)
O1i—Ca1—O2i	60.49 (3)	O12—P3—O9	115.63 (5)
O1—Ca1—O7viii	92.62 (3)	O10—P3—O9	110.75 (5)
O1i—Ca1—O7viii	90.17 (3)	O12—P3—O11	110.86 (5)
O2i—Ca1—O7viii	120.77 (3)	O10—P3—O11	111.50 (5)
O1—Ca1—O6ix	89.31 (3)	O9—P3—O11	100.36 (5)
O1i—Ca1—O6ix	102.55 (3)