Sodium terbium(III) polyphosphate

Single crystals of the title compound, NaTb(PO3)4, were obtained by solid-state reaction. This compound belongs to type II of long-chain polyphosphates with the general formula A I B III(PO3)4. It is isotypic with the NaNd(PO3)4 and NaEr(PO3)4 homologues. The crystal structure is built up of infinite crenelated chains of corner-sharing PO4 tetrahedra with a repeating unit of four tetrahedra. These chains, extending parallel to [100], are linked by isolated TbO8 square antiprisms, forming a three-dimensional framework. The Na+ ions are located in channels running along [010] and are surrounded by six oxygen atoms in a distorted octahedral environment within a cut-off distance <2.9 Å.


Comment
It is now well established that long-chain polyphosphates with general formula A I B III (PO 3 ) 4 can be divided in seven structural types (Jaoudi et al., 2003). All long-chain polyphosphates of formula NaLnP 4 O 12 (Ln = rare earth element) reported up to now ( El Masloumi et al. 2005;Zhu et al. 2006;Zhu et al. 2008;Horchani-Naifer et al. 2009;Koizumi et al. 1976;Amami et al. 2005;El Masloumi et al. 2008;Amami et al. 2004;Maksimova et al. 1988) belong to the structural type II which has been first described on basis of the KCe(PO 3 ) 4 structure (Linde et al., 1983). A few other A I B III cationic combinations such as AgGd (Naïli et al. 2006) andNaBi (Jaoudi et al. 2003;Belam et al. 2007) also lead to polyphosphates which belong to the structural type II. The structure of the title compound also fits in this isotypic series. The underlying structure has many times been described as built up of (PO 3 ) ∞ chains running along the [100] direction and further linked by isolated LnO 8 polyhedra. The resulting three dimensional framework delimits tunnels where the Na + ions are located. Instead of using this description, we will focus on the connectivity between the (PO 3 ) ∞ chains and the TbO 8 square antiprisms for our account.
Each TbO 8 square antiprism is linked to four (PO 3 ) ∞ chains by corner-sharing involving the non-bridging oxygen atoms of the PO 4 groups that exhibit the shorter P-O distances within the chain. Their P-O distances range from 1.4792 (13) Å to 1.4918 (13) Å. The chains are crenelated with a repeating unit of four corner-sharing tetrahedra, as displayed in Fig.   1. The repeating unit is built up of PO 4 tetrahedra corresponding to the four crystallographically independent phosphorus atoms labelled from P1 to P4. If the origin of the chain is taken at the O2 position for instance, then the P2 and P4 tetrahedra are the end-groupings of the repeating unit while P1 and P3 tetrahedra are involved in the internal diphosphate group. Each (PO 3 ) ∞ chain is linked to four rows of isolated TbO 8 square antiprisms parallel to the direction of the chain (Fig. 2). With the aforementioned origin convention both terminal P(2)O 4 and P(4)O 4 tetrahedra are connected in a bidentate fashion on one side of the square face of the archimedean antiprisms of the first row while the internal P(1)O 4 -P(3)O 4 diphosphate group is also connected in a bidentate fashion on one side of the square face of the antiprisms of the second row ( Fig. 3a and 3 b). Therefore the two rows of TbO 8 polyhedra are translated with a half-period of the (PO 3 ) ∞ chain relative to one another (Fig. 3c). Thus the tetrahedra involved in the internal P 2 O 7 groups share their non-bridging oxygen atoms with two TbO 8 polyhedra belonging to each of the first and second rows, respectively. Then the (PO 3 ) ∞ chain is connected to the third and fourth rows in a similar way but the role played by the couples P(1)O 4 -P(3)O 4 and P(2)O 4 -P(4)O 4 are inverted, this last becoming the internal diphosphate group.
For a general review on the crystal chemistry of polyphosphates, see: Durif (1995).

Experimental
Crystals of the title compound were synthesized by reacting Tb 4 O 7 with (NH 4 )H 2 PO 4 and Na 2 CO 3 in a platinum crucible.
A mixture of these reagents in the molar ratio 5 : 85 : 10 was used for the synthesis. The mixture has first been heated at 473 K for 12 h, then at 573 K for 12 h and finally at 773 K for 24 h. The muffle furnace was then cooled down first to supplementary materials sup-2 723 K at the rate of 2 K . h -1 and then to room temperature at the rate of 15 K . h -1 . Single crystals were extracted from the batch by washing with hot water.

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.
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.