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 MMnII 2MnIII(PO4)3 (M = Pb, Sr, Ba).

The title compound, SrNi 2 Fe(PO 4 ) 3 , synthesized by solid-state reaction, crystallizes in an ordered variant of the -CrPO 4 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 threedimensional framework of the crystal structure is built up by [PO 4 ] tetrahedra, [FeO 6 ] octahedra and [Ni 2 O 10 ] 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 edgesharing octahedra ([Ni 2 O 10 ] dimer) linked to [PO 4 ] tetrahedra via common edges while the second layer is built up from a strontium row followed by infinite chains of alternating [PO 4 ] tetrahedra and FeO 6 octahedra sharing apices. The layers are held together through vertices of [PO 4 ] tetrahedra and [FeO 6 ] octahedra, leading to the appearance of two types of tunnels parallel to the aand 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 -CrPO 4 (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 Na 2 Co 2-Fe(PO 4 ) 3 (Bouraima et al., 2015) and Na 1.67 Zn 1.67 Fe 1.33 (PO 4 ) 3 (Khmiyas et al., 2015) with the alluaudite structure type, and MMn II 2 Mn III (PO 4 ) 3 (M = Pb, Sr, Ba) (Alhakmi et al. (2013a,b;Assani et al., 2013) which belongs to the -CrPO 4 structure type. In the same context, our solid-state chemistry investigations within the ternary system MO-M 0 O-NiO-P 2 O 5 (M and M 0 are divalent cations), have led to the synthesis of the title compound SrNi 2 Fe(PO 4 ) 3 which has a related -CrPO 4 structure.

Structural commentary
The crystal structure of the title phosphate is formed by [PO 4 ] tetrahedra linked to [NiO 6 ] and [FeO 6 ] 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-valencesum calculations (Brown & Altermatt, 1985) for Sr 2+ , Ni 2+ , Fe 3+ , P1 5+ and P2 5+ 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 ISSN 2056-9890 and P2 are on 8g (2), O1 is on 8h (m) and O2 is on 8i (m)ÁThe 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 edgesharing octahedra ([Ni 2 O 10 ] dimers) whose ends are connected to [PO 4 ] tetrahedra by a common edge or vertex (Fig. 3). The second layer is formed by an Sr row followed by infinite chains of alternating [PO 4 ] tetrahedra and [FeO 6 ] octahedra sharing apices. These two types of layers are linked together by common vertices of [PO 4 ] 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 The principal building units in the structure of the title compound. Displacement ellipsoids are drawn at the 50% probability level.

Figure 3
View along the a axis of a layer resulting from the connection of [Ni 2 O 10 ] dimers and [PO 4 ] tetrahedra via common edges or vertices. Sr cations are omitted.

Database Survey
It is interesting to compare the crystal structure of -CrPO 4 (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 -CrPO 4 , the Cr 3+ and P 5+ cations occupy four special positions and the three-dimensional concatenation of [PO 4 ] tetrahedra and [CrO 6 ] octahedra allows the formation of empty tunnels along the b-axis direction. We can write the formula of this phosphate as follows: LL 0 (Cr1) 2 Cr2(PO 4 ) 3 , and in the general case, AA 0 M 2 M 0 (PO 4 ) 3 where L and L 0 represent the two empty tunnels sites, while M and M 0 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 0 M 2 M 0 (XO 4 ) 3 and is closely related to the -CrPO 4 structure (A and A 0 represent the two tunnels sites which can be occupied by either mono-or divalent medium sized cations, while the M and M 0 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 Na 2 Co 2 Fe-(PO 4 ) 3 and NaCr 2 Zn(PO 4 ) 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, SrNi 2 Fe(PO 4 ) 3 , which is a continuation of our previous work, namely MMn II 2 Mn III (PO 4 ) 3 (M = Pb, Sr, Ba).

Synthesis and crystallization
SrNi 2 Fe(PO 4 ) 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 À1 rate. Orange parallelepipedshaped crystals of the title compound were thus obtained.