The solid solution K3.84Ni0.78Fe3.19(PO4)5

The title compound, tetrapotassium tetra[nickel(II)/iron(III)] pentakis(orthophosphate), K3.84Ni0.78Fe3.19(PO4)5, has been obtained from a flux. The structure is isotypic with that of K4MgFe3(PO4)5. The three-dimensional framework is built up from (Ni/Fe)O5 trigonal bipyramids with a mixed Fe:Ni occupancy of 0.799 (8):0.196 (10) and isolated PO4 tetrahedra, one of which is on a general position and one of which has -4.. site symmetry. Two K+ cations are statistically occupied and are distributed over two positions in hexagonally shaped channels that run parallel to [001]. One K+ cation [occupancy 0.73 (3)] is surrounded by nine O atoms, while the other K+ cation [occupancy 0.23 (3)] is surrounded by eight O atoms.

Supporting information for this paper is available from the IUCr electronic archives (Reference: WM5027).

Comment
Complex iron-containing phosphates have different applications, for example as ionic conductors (Fisher et al., 2008), cathode materials (Barpanda et al., 2012;Trad et al., 2010) and matrices for storage of nuclear waste (Huang et al., 2005;Shih, 2003). In the crystal structures of these compounds the iron cations can adopt different coordination numbers and hence different oxygen polyhedra: FeO 4 , FeO 5 (Hidouri et al., 2003) or FeO 6 .
The asymmetric unit of (I) consists of one mixed-occupied (Ni II /Fe III ) site, two P sites (one of which is located on a fourfold rotoinversion axis), five oxygen sites and two K + sites which are partly occupied and distributed over two positions (K1A and K1B) (Fig. 1) (Fig. 2). The main difference between the obtained solid solution and the phosphate K 4 MgFe 3 (PO 4 ) 5 (Hidouri et al., 2008) is the splitting of the K + site in two positions. The occupation of the K1B site (0.23 (3)

Experimental
The title compound was obtained during investigation of the melting system K 2 O-P 2 O 5 -Fe 2 O 3 -NiO-MoO 3 . A mixture of KPO 3 (14.16 g), NiO (2.70 g), Fe 2 O 3 (2.88 g) and K 2 Mo 2 O 7 (4 g) was ground in an agate mortar, placed in a platinum crucible and heated up to 1273 K. The melt was kept at this temperature for 3 h. After that, the temperature was cooled down to 873 K at a rate of 10 K/h. The crystals of (I) were separated from the remaining flux by boiling with water. The chemical composition of selected single-crystal was verified by EDX analysis. Analysis found (

Refinement
Because of the similarity of possible coordination by O atoms, Ni and Fe were placed on the same site. Their coordinates and anisotropic displacement parameters (ADP) were constrained to be equal. The corresponding occupancy factors were refined using free variables. After that procedure, an unidentified high electron density peak was found near the position of the K site. It was supposed that this site can be occupied only by another K + caion. ADPs of both split K sites were constrained to be equal, while the occupancies were refined using free variables. The calculated occupancy factors of all partially occupied positions were close to those reported in this paper. To fix the electroneutrality of the compound, SUMP restraints in SHELXL (Sheldrick, 2008) were applied to the occupancy factors of the refined atoms.
The highest and lowest electron densities were found 1.00 Å from O1 and 0.76 Å from NI1, respectively.

Figure 1
The asymmetric unit of (I), showing displacement ellipsoids at the 50% probability level.
supporting information

Figure 2
The main building blocks and their linkage into chains and the three-dimensional framework for (I) in polyhedral representation.

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. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.