Dipotassium trisodium triphosphate, K2Na3P3O10

The structure of the title compound, K2Na3P3O10, is characterized by open chains of three PO4 tetrahedra linked by single oxygen bridges. The P3O10 groups have crystallographic twofold symmetry, with the central P atom being located on the twofold rotation axis. One of the sodium ions lies on a centre of inversion, whereas all the remaining atoms are in general positions. The structure is isotypic with that of the high-temperature form of Na5P3O10 phase I.

The structure of the title compound, K 2 Na 3 P 3 O 10 , is characterized by open chains of three PO 4 tetrahedra linked by single oxygen bridges. The P 3 O 10 groups have crystallographic twofold symmetry, with the central P atom being located on the twofold rotation axis. One of the sodium ions lies on a centre of inversion, whereas all the remaining atoms are in general positions. The structure is isotypic with that of the high-temperature form of Na 5 P 3 O 10 phase I.

Comment
The present triphosphate was obtained by chance, during the preparation of a mixed pyrophosphate. A bibliographic study of alkali triphosphates like M 5 P 3 0 10 shows that there are few known structures. Thus, in the case of sodium triphosphate, the crystal structures of two anhydrous forms noted Phase I and II were determined by Cruickshank (1964) and Davies & Corbridge (1958) and that of the hexahydrate was performed by Dyroff (1965)  with the central phosphorus P2 atom being located on a binary axis. Moreover, the Na2 sodium ion lies on the symmetry center whereas all the remaining atoms are in general positions of the C2/c space group. The Na2 sodium atom located at Wyckoff position 4c (1/4, 3/4, 1/2) could be surrounded by a roughly octahedral arrangement of six oxygen atoms and the other sodium and potassium (Na1, K1) atoms are coordinated to six and eight oxygen atoms respectively. The Na2O 6 octahedra, Na1O 6 and K1O 8 polyhedra are connected through the apices to triphosphate groups and form a threedimensional host lattice (Fig.1). The resulting 3-D framework presents intersecting tunnels running along the [010] and [110] directions, where the six-coordinated Na1 + cations are located (Fig.2). The structure of this compound is isotype to that of the high-temperature form of Na 5 P 3 0 10 phase I (Dymon and King, 1951 andCorbridge, 1960).

Experimental
The present triphosphate is obtained by chance, during the preparation of a mixture of pyrophosphate. Indeed, the powder phase NaKNiP 2 O 7 synthesized by wet process is introduced into a platinum crucible, and then gradually heated to a temperature above its melting point (1173 K) for 2 h, followed by slow cooling of the order of 6 K per hour up to 673 K.
Then the furnace is shuts down and the cooling is continued until room temperature. Small colourless single crystals of K 2 Na 3 P 3 O 10 were isolated from the mixtures of phases.

Dipotassium trisodium triphosphate
Crystal data

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.  (4)