Potassium trinickel(II) orthophosphate diphosphate, KNi3(PO4)P2O7

The structure of the title compound is characterized by the presence of two different anions, (PO4)3− and (P2O7)4− with an eclipsed conformation. The crystal structure consists of edge-sharing [NiO6] octahedra forming an [Ni3O14] chain running parallel to [001]. Adjacent chains are connected through edges and apices to PO4 and P2O7 groups in such a way as to build a three-dimensional host lattice. The resulting framework presents intersecting tunnels running along [010] and [101] in which the 11-coordinated potassium cation is located. The crystal structure of this new phosphate probably represents a new structural type.


Comment
Despite the large number of new structures of transition metal phosphates determined in recent years, only a limited number of phosphates show the existence of P 2 O 7 diphosphate and PO 4 monophosphate groups in their structures. Of these compounds, we report AgCr 2 (PO 4 )(P 2 O 7 ) discovered by Ayed (2012). The K 2 Ni 4 (PO 4 ) 2 (P 2 O 7 ), Na 4 Ni 5 (PO 4 ) 2 (P 2 O 7 ) 2 and Na 4 M II 3 (PO 4 ) 2 (P 2 O 7 ), (M II = Mn, Co, Ni) compounds were respectively developed by Palkina & Maksimova (1980), Nagornyi et al. (1996) and Sanz et al. (1996, 1999and 2001. In the present work, we report the synthesis and structural determination of the new mixed anion phosphate, KNi 3 (PO 4 )(P 2 O 7 ), from single-crystal X-ray diffraction data.
The partial three-dimensional plot in Fig.1 shows the connection ion-oxygen polyhedra in the crystal structure of the title compound. In the PO 4 tetrahedron, the P-O distances and O-P-O angles respectively between 1.5170-1.5916 (9) Å and 104.70-114.66 (5) Å are consistent with values found in the literature. In the P 2 O 7 group, the values of the P-O distances are in the range of 1.4924-1.5659 (9) Å while the shared oxygen is at P2-O8-P3 between 1.5971-1.6408 (9) Å and the angle P2-O8-P3 = 125.38 (6)°. P 2 O 7 adopts an eclipsed conformation as indicated by the dihedral angle of 7.42°, between the two plans through O5P2O8 and O8P3O9.
The chains are linked together by PO 4 and P 2 O 7 tetrahedra and by sharing two edges of two octahedra in the way to form a layer perpendicular to the b axis. The potassium atom K1, is coordinated to eleven oxygen atoms building a very distorted polyhedron.
The resulting 3-D framework presents intersecting tunnels running along the [010] and [001] directions, where the potassium cation is located (Fig.2). Probably, the structure of this phosphate represents a new structural type.

Experimental
KNi 3 (PO 4 )P 2 O 7 is obtained during the preparation of NaK 5 Ni 4 Co(P 2 O 7 ) 4 diphosphate. The powder of this phosphate was prepared by solid state reaction, by mixing the nominal proportions reagents NaNO 3 , KNO 3 , Ni(NO 3 ) 2 ,6H 2 O, Co(NO 3 ) 2 ,6H 2 O and (NH 4 )H 2 PO 4 . The mixture is put into a platinum crucible, and subjected to thermal treatment (473 K, 673 K and 873 K) interspersed with grinding until a temperature of 1073 K. The final powder is brown colour.
The prepared powder by the solid route is gradually increased to a temperature above its melting point (1273 K) for 2 h, followed by slow cooling to about 5 K per hour to 673 K. Then, the power supply of the furnace is cut, and cooling is continued to room temperature. Single crystals of brown colour are obtained.

Refinement
The highest peak and the deepest hole in the final Fourier map are at 0.70 Å and 0.84 Å, respectively, from O3 and Ni2.

Special details
Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'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 > 2σ(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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq