Crystal structure of a new tripotassium hexanickel iron hexaphosphate

K3Ni6Fe(PO4)6 has been synthesized by solid-state reaction and structurally characterized by single-crystal X-ray diffraction. Its structure is built up by [PO4] tetrahedra and [FeO6] and [NiO6] octahedra linked to each other by edge or corner sharing, leading to a three-dimensional framework delimiting tunnels along the [100] direction in which the K+ cations are localized.


Chemical context
Iron-based phosphates are widely studied materials today. They present a promising field for various applications such as electronics (Saw et al., 2014), ferroelectrics (Lazoryak et al., 2004), magnetic materials (Hatert et al., 2004;Essehli et al., 2015) and catalytic processes (Moffat, 1978). The introduction of alkali metals into these phosphates materials can be of great interest to improve the ion-conduction properties for applications in rechargeable alkaline batteries (La Parola et al., 2018;Orikasa et al., 2016). The present work is part of our activity devoted particularly to the investigation of new materials based on phosphates belonging to the A 2 O-MO-Fe 2 O 3 -P 2 O 5 (A = an alkali metal; M = divalent cation) quaternary system, which could have interesting ionic conductivity or magnetic proprieties. We report herein on the synthesis and structural characterization by single crystal X-ray diffraction of a new potassium nickel iron phosphate with formula K 3 Ni 6 Fe(PO 4 ) 6 .

Structural commentary
The asymmetric unit of the title compound, K 3 Ni 6 Fe(PO 4 ) 6 , consists of two [NiO 6 ] octahedra, one [FeO 6 ] octahedron, two [PO 4 ] tetrahedra, and three K atoms, as shown in Fig. 1. One Ni 2+ , Fe 3+ , P 5+ , two K + cations and two of the seven oxygen atoms lie on special positions. The Ni2 atom occupies Wyckoff position 4g (2), the Fe atom is localized on the 2a (2/m) Wyckoff position, P2, K1, K3, O6 and O7 are positioned on 4i (m) sites. The octahedral coordination sphere of the nickel(II) cation is more distorted than that of the iron(III) atom, with average <Ni-O> distances of 2.066 and 2.119 Å for Ni1 and Ni2, respectively. The mean <P-O> distance in the two PO 4 tetrahedra is equal to 1.547 Å for P1 and 1.543 Å for P2.  (Fig. 2). In addition, the association between the [P1O 4 ] tetrahedra and the [Ni1O 6 ] octahedra by means of edge-sharing allows the formation of a zigzag chain running parallel to the [100] direction. Each of the P1O 4 tetrahedra and Ni1O 6 octahedra, both belonging to the same layer, share vertices with Ni1O 6 and P1O 4 , respectively, of the adjacent one (Fig. 3). The two types of chain linkages lead to the formation of layers parallel to the ab plane (Fig. 4). One vertex of an Ni1O 6 octahedron belonging to one layer is shared with a P1O 4 vertex of the neighbouring layer. This configuration leads to a three-dimensional centrosymmetric framework, delimiting hexagonal tunnels along the [100] direction, in which the K + cations are located (Fig. 5). The potassium cations are distributed over three independent crystallographic positions with partial occupancies

Database survey
The investigated compound is a new member of the -xenophyllite family that includes Na 4 Ni 7 (PO 4 ) 6 (Moring & Kostiner, 1986), Na 4 Co 7 (PO 4 ) 6 (Kobashi et al., 1998)   View along the c axis of corner-and edge-sharing [PO 4 ] tetrahedra and [NiO 6 ] octahedra forming a layer parallel to the ab plane.

Figure 5
Polyhedral representation of the crystal structure of K 3 Ni 6 Fe(PO 4 ) 6 showing large tunnels running along the [100] direction that contain the K + cations.

Synthesis and crystallization
Single crystals of K 3 Ni 6 Fe(PO 4 ) 6 were prepared by solid-state reaction in air. A mixture of K 2 CO 3 , Ni(NO 3 ) 2 Á6H 2 O, Fe(NO 3 ) 3 Á9H 2 O and H 3 PO 4 (85 wt.%) reagents with a K:Ni:Fe:P molar ratio of 2:2:1:3 was dissolved in 50 mL of distilled water. The resulting solution was stirred without heating for 24 h and was subsequently evaporated to dryness at 343 K. The obtained dry residue was progressively heated in a platinum crucible up to 673 K in order to eliminate volatile products. In a second step, the powder was homogenized in an agate mortar and then progressively heated to 1303 K. Kept at this temperature for 2 h, the reaction mixture then underwent slow cooling at a rate of 5 Kh À1 to 1103 K and then to room temperature with the furnace inertia. After washing with distilled water, the obtained crystals were brown with block-type shape. A qualitative EDX analysis (energy dispersive X-ray spectroscopy) detected the presence of the expected chemical elements corresponding to K, Ni, Fe, P and O atoms (see Fig. 6).

Refinement
Crystal data, data collection and structure refinement details of K 3 Ni 6 Fe(PO 4 ) 6 are summarized in Table 1 (Farrugia, 2012) and DIAMOND (Brandenburg, 2006); software used to prepare material for publication: publCIF (Westrip, 2010). 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 (Å 2 )
x y z U iso */U eq Occ. (