Heptasodium tetraaluminium tetrakis(diphosphate) orthophosphate, Na7Al4(P2O7)4(PO4)

The asymmetric unit of title compound contains three Na+, one Al3+, three P5+ and eight O2− atoms, with one Na+ atom lying on a twofold rotation axis and one Na+ and one P5+ atom on fourfold rotoinversion axes. The fundamental building units of the title structure are isolated PO4 tetrahedra, AlO6 octahedra and P2O7 groups, which are further interlocked by corner-sharing O atoms, forming a three-dimensional framework structure. The Na+ atoms are located within the cavities of the framework, showing coordination numbers of 4, 6 and 7.

The asymmetric unit of title compound contains three Na + , one Al 3+ , three P 5+ and eight O 2À atoms, with one Na + atom lying on a twofold rotation axis and one Na + and one P 5+ atom on fourfold rotoinversion axes. The fundamental building units of the title structure are isolated PO 4 tetrahedra, AlO 6 octahedra and P 2 O 7 groups, which are further interlocked by corner-sharing O atoms, forming a three-dimensional framework structure. The Na + atoms are located within the cavities of the framework, showing coordination numbers of 4, 6 and 7.

Related literature
For isotypic structures, see: Rochè re et al. (1985); Stus et al.  et al., 2001) were synthesized, and their ion exchange and conductivity properties studied. However, for compound Na 7 (AlP 2 O 7 ) 4 PO 4 , a detailed crystal structure analysis has not been reported so far. In this work, the synthesis and results of the crystal structure refinement of this compound is reported. In comparison with the unit cell parameters reported by Rochère from X-ray powder data (a = 14.046 (3), c = 6.169 (2) Å; Rochère et al., 1985), the determined unit cell parameters from the single crystal X-ray study are slighty larger.
As shown in Figs 1 and 2, the structure of the title compound consists of a three-dimensional framework of isolated PO 4 tetrahedra, AlO 6 octahedra and P 2 O 7 groups, the conformation of the latter more eclipsed than staggered. The sodium cations are located in sites within cavities in the framework, exhibiting coordination numbers of 7 (Na1), 6 (Na2) and 4 (Na3). There are three crystallography distinct P atoms in the structure of the title compound. P1 and P2 atoms are located in general positions and their corresponding P1O 4 and P2O 4 tetrahedra are connected by the bridging O5 atom to form a P 2 O 7 group, which is further linked to four AlO 6 octahedra. P3 atoms are located on 4 axes, forming isolated P3O 4 tetrahedra which are further connected to four AlO 6 octahedra. The P3O 4 tetrahedra are regular with a P-O bond length of 1.5351 (18

Experimental
The finely ground reagents Na 2 CO 3 , Al 2 O 3 and NH 4 H 2 PO 4 were mixed in the molar ratio Na: Al: P = 2: 1: 8, were placed in a Pt crucible, and heated at 673 K for 4 h. The mixture was then re-ground and heated at 1173 K for 20 h, then cooled to 673 K at a rate of 3 K h -1 , and finally quenched to room temperature. A few colorless crystals of the title compound with prismatic shape were obtained.

Refinement
The highest peak in the difference electron density map equals to 0.22 e/Å 3 at the distance of 0.63 Å from O4 site while the deepest hole equals to -0.29 e/Å 3 at the distance of 0.68 Å from the P3 site.
Figures Fig. 1. The expanded asymmetric unit of Na 7 (AlP 2 O 7 ) 4 PO 4 showing the coordination environments of the P and Al atoms. The displacement ellipsoids are drawn at the 50% probability level. [Symmetry codes: (iii) -y + 1, x, -z + 1; (vii) -x + 1, -y + 1, z; (viii) -y + 1, x, -z + 2; (ix) x -1/2, -y + 3/2, -z + 3/2; (x) y, -x + 1, -z + 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 Rfactors(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.