Crystal structure of strontium dinickel iron orthophosphate

The transition metal orthophosphates SrM ₂Fe(PO₄)₃ (M = Co, Ni) crystallize in an α-CrPO₄-type structure. The chains characterizing this structure are then built up from [Ni₂O₁₀] units alternating with [PO₄] tetra­hedra and [FeO₆] octa­hedra. The structure is nearly the same as that observed in MMn^II ₂Mn^III(PO₄)₃ (M = Pb, Sr, Ba).

Abstract

The title compound, SrNi₂Fe(PO₄)₃, synthesized by solid-state reaction, crystallizes in an ordered variant of the α-CrPO₄ structure. In the asymmetric unit, two O atoms are in general positions, whereas all others atoms are in special positions of the space group Imma: the Sr cation and one P atom occupy the Wyckoff position 4e (mm2), Fe is on 4b (2/m), Ni and the other P atom are on 8g (2), one O atom is on 8h (m) and the other on 8i (m). The three-dimensional framework of the crystal structure is built up by [PO₄] tetra­hedra, [FeO₆] octa­hedra and [Ni₂O₁₀] dimers of edge-sharing octa­hedra, linked through common corners or edges. This structure comprises two types of layers stacked alternately along the [100] direction. The first layer is formed by edge-sharing octa­hedra ([Ni₂O₁₀] dimer) linked to [PO₄] tetra­hedra via common edges while the second layer is built up from a strontium row followed by infinite chains of alternating [PO₄] tetra­hedra and FeO₆ octa­hedra sharing apices. The layers are held together through vertices of [PO₄] tetra­hedra and [FeO₆] octa­hedra, leading to the appearance of two types of tunnels parallel to the a- and b-axis directions in which the Sr cations are located. Each Sr cation is surrounded by eight O atoms.

Chemical context  

Phosphates with the alluaudite (Moore, 1971 ▸) and α-CrPO₄ (Attfield et al., 1988 ▸) crystal structures have attracted great inter­est due to their potential applications as battery electrodes (Trad et al., 2010 ▸; Kim et al., 2014 ▸; Huang et al., 2015 ▸). In the last decade, our inter­est has focused on those two phosphate derivatives and we have succeeded in synthesizing and structurally characterizing 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 ▸) with the alluaudite structure type, and MMn^II ₂Mn^III(PO₄)₃ (M = Pb, Sr, Ba) (Alhakmi et al. (2013a ▸,b ▸; Assani et al., 2013 ▸) which belongs to the α-CrPO₄ structure type. In the same context, our solid-state chemistry investigations within the ternary system MO–M′O–NiO–P₂O₅ (M and M′ are divalent cations), have led to the synthesis of the title compound SrNi₂Fe(PO₄)₃ which has a related α-CrPO₄ structure.

Structural commentary  

The crystal structure of the title phosphate is formed by [PO₄] tetra­hedra linked to [NiO₆] and [FeO₆] octa­hedra, as shown in Fig. 1 ▸. The octa­hedral environment of iron is more distorted than that of nickel (see Table 1 ▸). In this model, bond-valence-sum calculations (Brown & Altermatt, 1985 ▸) for Sr²⁺, Ni²⁺, Fe³⁺, P1⁵⁺and P2⁵⁺ ions are as expected, viz. 1.88, 1.95, 2.91, 5.14 and 5.01 valence units, respectively. Atoms Sr1 and P1 occupy Wyckoff positions 4e (mm2), Fe1 is on 4b (2/m), Ni1 and P2 are on 8g (2), O1 is on 8h (m) and O2 is on 8i (m)·The three-dimensional network of the crystal structure is composed of two types of layers parallel to (100), as shown in Fig. 2 ▸. The first layer is built up from two adjacent edge-sharing octa­hedra ([Ni₂O₁₀] dimers) whose ends are connected to [PO₄] tetra­hedra by a common edge or vertex (Fig. 3 ▸). The second layer is formed by an Sr row followed by infinite chains of alternating [PO₄] tetra­hedra and [FeO₆] octa­hedra sharing apices. These two types of layers are linked together by common vertices of [PO₄] tetra­hedra, forming a three-dimensional framework which delimits two types of tunnels running along the a- and b-axis directions in which the Sr cations are located with eight neighbouring O atoms (Fig. 4 ▸). The structure of the title compound is isotypic to that of MMn^II ₂Mn^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 + 1, −y + , z − 1; (ii) x, y, z − 1; (iii) −x + 1, −y + , z; (iv) −x + , −y + 1, z − ; (v) x − , y − , z − ; (vi) −x + , y − , z − ; (vii) x − , −y + 1, z − ; (viii) −x + , y, −z + ; (ix) −x + , −y + , −z + ; (x) x, −y + 1, −z + 2; (xi) −x + 2, y, z; (xii) x, −y + 1, −z + 1; (xiii) −x + 2, −y + 1, −z + 1; (xiv) x + , y, −z + .]

Table 1. Selected bond lengths ().

Sr1O1i	2.6390(13)	Fe1O4	1.9703(8)
Sr1O2	2.6477(12)	Fe1O1ii	2.0751(12)
Sr1O3ii	2.6662(9)	P1O1	1.5239(12)
Ni1O4	2.0561(8)	P1O2	1.5514(12)
Ni1O2	2.0612(8)	P2O3	1.5223(9)
Ni1O3iii	2.0953(9)	P2O4	1.5722(9)

Symmetry codes: (i) ; (ii) ; (iii) .

Figure 2.

Stacking along [100] of layers building the crystal structure of SrNi₂Fe(PO₄)₃.

Figure 3.

View along the a axis of a layer resulting from the connection of [Ni₂O₁₀] dimers and [PO₄] tetra­hedra via common edges or vertices. Sr cations are omitted.

Figure 4.

Polyhedral representation of the crystal structure of SrNi₂Fe(PO₄)₃ showing tunnels running along [010].

Database Survey  

It is inter­esting to compare the crystal structure of α-CrPO₄ (Glaum et al., 1986 ▸) with that of the title compound. Both phosphates crystallize in the ortho­rhom­bic system in the space group Imma. Moreover, their unit-cell parameters are nearly the same despite the difference between their chemical formulas. In the structure of α-CrPO₄, the Cr³⁺ and P⁵⁺ cations occupy four special positions and the three-dimensional concatenation of [PO₄] tetra­hedra and [CrO₆] octa­hedra allows the formation of empty tunnels along the b-axis direction. We can write the formula of this phosphate as follows: LL′(Cr1)₂Cr2(PO₄)₃, and in the general case, AA′M ₂ M′(PO₄)₃ where L and L′ represent the two empty tunnels sites, while M and M′ correspond to the trivalent cation octa­hedral sites. This model is in accordance with that of the alluaudite structure which is represented by the general formula AA′M ₂ M′(XO₄)₃ and is closely related to the α-CrPO₄ structure (A and A′ represent the two tunnels sites which can be occupied by either mono- or divalent medium sized cations, while the M and M′ octa­hedral sites are generally occupied by transition metal cations). Accordingly, the substitution of Cr1 or Cr2 by a divalent cation requires charge compensation by a monovalent cation that will occupy the tunnel. Two very recently reported examples are Na₂Co₂Fe(PO₄)₃ and NaCr₂Zn(PO₄)₃, which were characterized by X-ray diffraction, IR spectroscopy and magnetic measurements (Souiwa et al., 2015 ▸). The replacement of Cr1 by a divalent cation involves an amendment of the charge by a divalent cation as in the present case, SrNi₂Fe(PO₄)₃, which is a continuation of our previous work, namely MMn^II ₂Mn^III(PO₄)₃ (M = Pb, Sr, Ba).

Synthesis and crystallization  

SrNi₂Fe(PO₄)₃ was synthesized by a solid state reaction in air. Stoichiometric qu­anti­ties of strontium, nickel, and iron nitrates and 85 wt% phospho­ric acid were dissolved in water. The resulting solution was stirred without heating for 20 h and was, subsequently, evaporated to dryness. The obtained dry residue was homogenized in an agate mortar and then progressively heated in a platinum crucible up to 873 K. The reaction mixture was maintained at this temperature during 24 h before being heated to the melting point of 1373 K. The molten product was then cooled down slowly to room temperature at a rate of 5 K h⁻¹ rate. Orange parallelepiped-shaped crystals of the title compound were thus obtained.

Refinement  

Crystal data, data collection and structure refinement details are summarized in Table 2 ▸. The highest peak and the deepest hole in the final Fourier map are at 0.72 and 0.80 Å from Sr1 and P1, respectively.

Table 2. Experimental details.

Crystal data
Chemical formula	SrNi2Fe(PO4)3
M r	545.80
Crystal system, space group	Orthorhombic, I m m a
Temperature (K)	296
a, b, c ()	10.3881(11), 13.1593(13), 6.5117(7)
V (3)	890.15(16)
Z	4
Radiation type	Mo K
(mm1)	12.34
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	8211, 1112, 1095
R int	0.024
(sin /)max (1)	0.820
Refinement
R[F 2 > 2(F 2)], wR(F 2), S	0.015, 0.041, 1.20
No. of reflections	1112
No. of parameters	54
max, min (e 3)	0.92, 0.57

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

Supplementary Material

CCDC reference: 1426730

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

supplementary crystallographic information

Crystal data

SrNi2Fe(PO4)3	Dx = 4.073 Mg m−3
Mr = 545.80	Mo Kα radiation, λ = 0.71073 Å
Orthorhombic, Imma	Cell parameters from 1112 reflections
a = 10.3881 (11) Å	θ = 3.1–35.6°
b = 13.1593 (13) Å	µ = 12.34 mm−1
c = 6.5117 (7) Å	T = 296 K
V = 890.15 (16) Å3	Parallelepiped, orange
Z = 4	0.31 × 0.25 × 0.19 mm
F(000) = 1044

Data collection

Bruker X8 APEX Diffractometer	1112 independent reflections
Radiation source: fine-focus sealed tube	1095 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.024
φ and ω scans	θmax = 35.6°, θmin = 3.1°
Absorption correction: multi-scan (SADABS; Bruker, 2009)	h = −17→17
Tmin = 0.504, Tmax = 0.748	k = −21→21
8211 measured reflections	l = −9→10

Refinement

Refinement on F2	0 restraints
Least-squares matrix: full	w = 1/[σ2(Fo2) + (0.0211P)2 + 1.0433P] where P = (Fo2 + 2Fc2)/3
R[F2 > 2σ(F2)] = 0.015	(Δ/σ)max = 0.001
wR(F2) = 0.041	Δρmax = 0.92 e Å−3
S = 1.20	Δρmin = −0.57 e Å−3
1112 reflections	Extinction correction: SHELXL, Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
54 parameters	Extinction coefficient: 0.0040 (3)

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
Sr1	0.5000	0.2500	0.40652 (3)	0.00832 (6)
Ni1	0.7500	0.36678 (2)	0.7500	0.00507 (6)
Fe1	1.0000	0.5000	0.5000	0.00365 (7)
P1	0.5000	0.2500	0.91246 (8)	0.00335 (9)
P2	0.7500	0.57166 (3)	0.7500	0.00391 (8)
O1	0.5000	0.34416 (9)	1.04869 (19)	0.00631 (18)
O2	0.61817 (11)	0.2500	0.76678 (18)	0.00566 (18)
O3	0.78842 (9)	0.63613 (6)	0.93417 (14)	0.00764 (14)
O4	0.86173 (8)	0.49396 (6)	0.70676 (14)	0.00586 (13)

Atomic displacement parameters (Ų)

	U11	U22	U33	U12	U13	U23
Sr1	0.00864 (10)	0.01114 (10)	0.00518 (9)	0.000	0.000	0.000
Ni1	0.00501 (9)	0.00407 (9)	0.00613 (10)	0.000	0.00049 (6)	0.000
Fe1	0.00281 (12)	0.00403 (12)	0.00410 (12)	0.000	0.000	0.00015 (9)
P1	0.0033 (2)	0.0031 (2)	0.0037 (2)	0.000	0.000	0.000
P2	0.00410 (15)	0.00389 (15)	0.00374 (15)	0.000	0.00042 (10)	0.000
O1	0.0074 (4)	0.0049 (4)	0.0067 (4)	0.000	0.000	−0.0014 (4)
O2	0.0043 (4)	0.0063 (4)	0.0064 (4)	0.000	0.0017 (3)	0.000
O3	0.0095 (3)	0.0080 (3)	0.0055 (3)	−0.0019 (3)	0.0002 (3)	−0.0020 (2)
O4	0.0045 (3)	0.0056 (3)	0.0074 (3)	0.0005 (2)	0.0019 (3)	0.0005 (2)

Geometric parameters (Å, º)

Sr1—O1i	2.6390 (13)	Fe1—O4xi	1.9703 (8)
Sr1—O1ii	2.6390 (13)	Fe1—O4xii	1.9703 (8)
Sr1—O2	2.6477 (12)	Fe1—O4xiii	1.9703 (8)
Sr1—O2iii	2.6477 (12)	Fe1—O4	1.9703 (8)
Sr1—O3iv	2.6662 (9)	Fe1—O1xiv	2.0751 (12)
Sr1—O3v	2.6662 (9)	Fe1—O1iv	2.0751 (12)
Sr1—O3vi	2.6662 (9)	P1—O1	1.5239 (12)
Sr1—O3vii	2.6662 (9)	P1—O1iii	1.5239 (12)
Ni1—O4viii	2.0561 (8)	P1—O2iii	1.5514 (12)
Ni1—O4	2.0561 (8)	P1—O2	1.5514 (12)
Ni1—O2	2.0612 (8)	P2—O3	1.5223 (9)
Ni1—O2ix	2.0612 (8)	P2—O3viii	1.5223 (9)
Ni1—O3x	2.0953 (9)	P2—O4	1.5722 (9)
Ni1—O3iv	2.0953 (9)	P2—O4viii	1.5722 (9)
O1i—Sr1—O1ii	56.01 (5)	O4—Ni1—O3x	92.39 (4)
O1i—Sr1—O2	141.47 (2)	O2—Ni1—O3x	93.49 (4)
O1ii—Sr1—O2	141.47 (2)	O2ix—Ni1—O3x	84.94 (4)
O1i—Sr1—O2iii	141.47 (2)	O4viii—Ni1—O3iv	92.39 (4)
O1ii—Sr1—O2iii	141.47 (2)	O4—Ni1—O3iv	89.31 (3)
O2—Sr1—O2iii	55.24 (5)	O2—Ni1—O3iv	84.94 (4)
O1i—Sr1—O3iv	108.88 (2)	O2ix—Ni1—O3iv	93.49 (4)
O1ii—Sr1—O3iv	78.22 (2)	O3x—Ni1—O3iv	177.91 (5)
O2—Sr1—O3iv	63.76 (3)	O4xi—Fe1—O4xii	180.0
O2iii—Sr1—O3iv	108.81 (3)	O4xi—Fe1—O4xiii	86.39 (5)
O1i—Sr1—O3v	78.22 (2)	O4xii—Fe1—O4xiii	93.61 (5)
O1ii—Sr1—O3v	108.88 (2)	O4xi—Fe1—O4	93.61 (5)
O2—Sr1—O3v	108.81 (3)	O4xii—Fe1—O4	86.39 (5)
O2iii—Sr1—O3v	63.76 (3)	O4xiii—Fe1—O4	180.00 (3)
O3iv—Sr1—O3v	172.25 (4)	O4xi—Fe1—O1xiv	93.70 (3)
O1i—Sr1—O3vi	78.22 (2)	O4xii—Fe1—O1xiv	86.30 (3)
O1ii—Sr1—O3vi	108.88 (2)	O4xiii—Fe1—O1xiv	86.30 (3)
O2—Sr1—O3vi	63.76 (3)	O4—Fe1—O1xiv	93.70 (3)
O2iii—Sr1—O3vi	108.81 (3)	O4xi—Fe1—O1iv	86.30 (3)
O3iv—Sr1—O3vi	68.39 (4)	O4xii—Fe1—O1iv	93.70 (3)
O3v—Sr1—O3vi	111.05 (4)	O4xiii—Fe1—O1iv	93.70 (3)
O1i—Sr1—O3vii	108.88 (2)	O4—Fe1—O1iv	86.30 (3)
O1ii—Sr1—O3vii	78.22 (2)	O1xiv—Fe1—O1iv	180.00 (7)
O2—Sr1—O3vii	108.81 (3)	O1—P1—O1iii	108.80 (10)
O2iii—Sr1—O3vii	63.76 (3)	O1—P1—O2iii	110.85 (3)
O3iv—Sr1—O3vii	111.05 (4)	O1iii—P1—O2iii	110.85 (3)
O3v—Sr1—O3vii	68.39 (4)	O1—P1—O2	110.85 (3)
O3vi—Sr1—O3vii	172.25 (4)	O1iii—P1—O2	110.85 (3)
O4viii—Ni1—O4	71.02 (5)	O2iii—P1—O2	104.61 (9)
O4viii—Ni1—O2	102.98 (3)	O3—P2—O3viii	112.25 (7)
O4—Ni1—O2	171.55 (4)	O3—P2—O4	108.06 (5)
O4viii—Ni1—O2ix	171.55 (4)	O3viii—P2—O4	114.51 (5)
O4—Ni1—O2ix	102.98 (3)	O3—P2—O4viii	114.51 (5)
O2—Ni1—O2ix	83.59 (5)	O3viii—P2—O4viii	108.06 (5)
O4viii—Ni1—O3x	89.31 (3)	O4—P2—O4viii	98.87 (6)

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